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We hypothesized that L-arginine and conjugated linoleic acid (CLA) would have additive effects in decreasing adiposity. Sprague Dawley rats were assigned to the following dietary groups (n = 6/group; 5 wk total): 1) control (2.55% L-alanine plus 1.5% canola oil); 2) arginine (1.25% L-arginine plus 1.5% canola oil); 3) CLA (2.55% L-alanine plus 1.5% CLA); and 4) arginine plus CLA (1.25% L-arginine plus 1.5% CLA). Supplemental amino acids were provided in drinking water and CLA was incorporated into the food pellets. Daily weight gain, food intake, arginine intake, and final body and eviscerated body weights were greater in rats fed supplemental CLA then in rats fed canola oil. The retroperitoneal adipose tissue:body weight ratio was less in rats fed supplemental CLA than in rats fed canola oil, but epididymal adipose tissue, liver, and soleus and extensor digitorum Iongus muscle weights were unaffected by arginine or CLA. CLA decreased epididymal adipose tissue concentrations of palmitoleic, oleic, and cis-vaccenic acid. CLA and arginine increased palmitate oxidation to C[O.sub.2] in epididymal adipose tissue in vitro relative to control rats. Glucose and palmitate incorporation into total lipids in epididymal adipose tissue was lower in rats fed supplemental arginine than in alanine-fed rats. Arginine increased plasma glycerol relative to alanine-fed rats and CLA and arginine independently decreased most serum essential amino acids and alanine, glutamate, glutamine, and ornithine. We conclude that CLA and arginine modulated adipose tissue metabolism by separate, but not additive, effects. Also, CLA and arginine may have depressed muscle protein turnover.

L-Arginine administration may be useful for the treatment of intrauterine growth restriction, but concerns remain about effective precursors for administration into pregnant dams. Therefore, we used an ovine model to test the hypothesis that infusion of h-citrulline into the maternal circulation increases L-arginine availability to the fetus. On d 135 [+ or -] 1 of gestation, ewes received an i.v. bolus dose of L-citrulline (155 [micro]mol/kg body weight) or the same dose of L-arginine-HCI. Maternal and fetal arterial blood samples were obtained simultaneously at -120, -60, 0, 5, 15, 30, 60, 120, 180, and 240 min relative to the time of amino acid administration. Concentrations of arginine in maternal plasma increased to peak values within 5 min after its injection in ewes and declined rapidly thereafter, whereas concentrations of arginine in fetal plasma increased between 15 and 30 min and returned to baseline values by 60 min. In contrast, administration of citrulline increased concentrations of citrulline and arginine in maternal and fetal plasma between 5 and 60 min and values remained elevated thereafter. The differential pharmacokinetics for arginine compared with citrulline infusion was consistent with the observation that the half-life of citrulline was twice that of arginine in ewes. We conclude that i.v. administration of citrulline is more effective than arginine in sustaining high concentrations of arginine in the maternal and fetal circulations of pregnant ewes. These novel findings provide support for studies of the clinical use of arginine and citrulline as therapeutic means to prevent or ameliorate fetal growth retardation in mammals.

Dietary arginine supplementation increases growth of neonatal pigs, but the underlying mechanisms are unknown. This study was conducted to test the hypothesis that the arginine treatment activates translation initiation factors and protein synthesis in skeletal muscle. Piglets were fed milk-based diets supplemented with 0 or 0.6% L-arginine between 7 and 14 d of age. Following a 7-d period of arginine supplementation, at 1 h after the last meal, jugular venous blood samples were obtained for metabolite analysis, whereas longissimus muscle and liver were collected to determine the abundance and phosphorylation state of the mammalian target of the rapamycin (mTOR), ribosomal protein S6 kinase 1 (S6K1), eukaryotic initiation factor (elF) 4E-binding protein-1 (4E-BP1), elF4E, and elF4G. Fractional rates of protein synthesis were measured in muscle and liver using the [[sup.3]H]phenylalanine flooding-dose technique. Arginine supplementation increased (P < 0.05) daily gain, the plasma insulin concentration, and protein synthesis in skeletal muscle but not in liver. The arginine treatment enhanced the formation of the active elF4E.elF4G complex but reduced the amount of the inactive 4E-BP1 * elF4E complex in muscle. These changes were associated with elevated levels of phosphorylated mTOR and 4E-BP1 in muscle of arginine-supplemented piglets (P < 0.05). Neither the total amounts nor phosphorylation levels of the translation initiation factors in the liver differed between control and arginine-supplemented piglets. Collectively, these results suggest that dietary arginine supplementation increases mTOR signaling activity in skeletal muscle, but not in liver, of milk-fed neonatal pigs. The findings provide a molecular mechanism for explaining the previous observation that increased circulating arginine stimulated muscle protein synthesis and promoted weight gain in neonatal pigs.

The arginases catalyze the divalent cation dependent hydrolysis of L-arginine to produce L-ornithine and urea. Although traditionally considered in terms of its role as the final enzyme of the urea cycle, the enzyme is found in a variety of nonhepatic tissues. These findings suggest that the enzyme may have other functions in addition to its role in nitrogen metabolism. High-resolution crystal structures have been determined for recombinant rat liver (type I) arginase and for recombinant human kidney (type II) arginase, their variants, and complexes with products and inhibitors. Each identical subunit of the trimeric enzyme contains an active site that lies at the bottom of a 15 [Angstrom] deep cleft. The 2 essential Mn(II) ions are located at the bottom of this cleft, separated by ~3.3 [Angstrom] and bridged by oxygens derived from 2 aspartic acid residues and a solvent-derived hydroxide. This metal bridging hydroxide is proposed to be the nucleophile that attacks the guanidinium carbon of substrate arginine. On the basis of this proposed mechanism, boronic acid inhibitors of the enzyme have been synthesized and characterized kinetically and structurally. These inhibitors display slow-onset inhibition at the pH optimum of the enzyme, and are found as tetrahedral species at the active site, as determined by X-ray diffraction. The potent inhibition of arginases I and II by these compounds has not only delineated key enzyme-substrate interactions, but has also led to a greater understanding of the role of arginase in nonhepatic tissues. J. Nutr. 134: 2760S-2764S, 2004. KEY WORDS: * arginase * manganese * protein structure * enzyme inhibitors * boronic acids

Omithine [alpha]-ketoglutarate (OKG) is a salt formed of 2 molecules of omithine and 1 [alpha]-ketoglutarate. Its administration improves nutritional status in chronically malnourished (e.g., elderly) and acutely malnourished patients (especially burn and trauma patients). There is evidence that OKG activity is not the simple addition of the effects of ornithine (Orn) and [alpha]-ketoglutarate ([alpha]KG), because the presence of both moieties is required to induce the generation of key metabolites such as glutamine, proline, and arginine (Arg), whereas this does not occur when one or the other is given separately. This observation is related to the fact that the main feature of Orn at the whole-body level is to be metabolized through the Orn aminotransferase-dependent pathway, whereas the simultaneous administration of Orn and [alpha]KG saturates this pathway, diverting Orn toward metabolism into Arg. For years, OKG activity has been associated with its ability to induce the secretion of anabolic hormones, such as insulin and growth hormone, and to increase glutamine and polyamine synthesis. Recent studies using chemical inhibitors of nitric oxide synthase (NOS) suggest that nitric oxide derived from Arg could be partly involved in OKG activity. The use of genetically modified animals (i.e., knockout for NOS expression) is required to confirm this hypothesis. Key-words: * ornithine [alpha]-ketoglutarate * arginine * nitric oxide * burn injury

The vascular endothelium is a crucial regulator of vascular function and homeostasis. Nitric oxide (NO) is an important paracrine substance released by the endothelium to regulate vasomotor tone. Risk factors for atherosclerosis, as well as atherosclerosis per se, are associated with endothelial dysfunction and decreased bioavailablilty of NO. Indeed, endothelial dysfunction is integral to the pathogenesis of atherosclerosis and other cardiovascular diseases. Moreover, endothelial dysfunction relates to an increased risk of adverse cardiovascular outcomes. L-Arginine is an essential amino acid required by the constitutive enzyme, endothelial NO oxide synthase (eNOS), to produce NO. Administration of L-arginine improves endothelial function in animal models and in humans with hypercholesterolemia and with atherosclerosis. Clinical trials to date support potential clinical applications of L-arginine in the treatment of coronary artery disease and peripheral arterial disease, as well as in the prevention of in-stent restenosis. The mechanism of benefit of L-arginine on endothelial function is unclear, because intracellular concentrations of L-arginine far exceed that required by eNOS. One potential explanation of this 'arginine paradox' is that L-arginine restores endothelial function in atherosclerotic patients, in whom there are elevated levels of asymmetric dimethylarginine, an endogenous inhibitor of eNOS. Given the promising findings of early studies of L-arginine as a potential therapy for cardiovascular disorders, large-scale clinical trials are warranted. KEY WORDS: * endothelium * arginine * nitric oxide * cardiovascular disease

The rationale for the use of nutritional supplements to enhance exercise capacity is based on the assumption that they will confer an ergogenic effect above and beyond that afforded by regular food ingestion alone. The proposed or advertised ergogenic effect of many supplements is based on a presumptive metabolic pathway and may not necessarily translate to quantifiable changes in a variable as broadly defined as exercise performance. L-arginine is a conditionally essential amino acid that has received considerable attention due to potential effects on growth hormone secretion and nitric oxide production. In some clinical circumstances (e.g., burn injury, sepsis) in which the demand for arginine cannot be fully met by de novo synthesis and normal dietary intake, exogenous arginine has been shown to facilitate the maintenance of lean body mass and functional capacity. However, the evidence that supplemental arginine may also confer an ergogenic effect in normal healthy individuals is less compelling. In contrast to arginine, numerous studies have reported that supplementation with the arginine metabolite creatine facilitates an increase in anaerobic work capacity and muscle mass when accompanied by resistance training programs in both normal and patient populations. Whereas improvement in the rate of phosphocreatine resynthesis is largely responsible for improvements in acute work capacity, the direct effect of creatine supplementation on skeletal muscle protein synthesis is less clear. The purpose of this review is to summarize the role of arginine and its metabolite creatine in the context of a nutrition supplement for use in conjunction with an exercise stimulus in both healthy and patient populations. KEY WORDS: * diet * anabolism * muscle

Immune-enhancing diets contain nutrients that have putative benefits, including arginine, (n-3) fats, glutamine, nucleotides, and structured lipids. Although under most circumstances the systemic inflammatory response is beneficial to the host, improving the eventual outcome of injury, infection, or inflammation, excessive proinflammation (leading to cardiac, hepatic, and mitochondrial dysfunction) or excessive counterinflammation (leading to immune depression) can worsen outcome. In critically ill septic patients, the synthesis of arginine can be exceeded by its catabolism to nitric oxide (NO) and urea, rendering arginine conditionally essential. In patients with sepsis, increased production of NO increases serum nitrite and nitrate levels, whereas levels in patients with trauma and trauma with sepsis are lower than in controls. In septic patients, supplemental arginine might further increase NO levels and be potentially harmful through excessive proinflammation. However, administration of increased amounts of arginine might improve immune function in surgical and trauma patients by increasing NO production in macrophages. When the diet provides at least 1 g of the (n-3) fatty acids eicosapentaenoic and docosahexaenoic acid combined, 2-series eicosanoids (prostaglandins, prostacyclins, thromboxanes) are replaced partially by 3-series eicosanoids, and 4-series leukotrienes are replaced partially by 5-series leukotrienes that are less proinflammatory. Thus, the effects of arginine and (n-3)-fat supplementation might be expected to be complementary--arginine might improve cytokine and NO production in patients with immunodepression, whereas (n-3) fats might be beneficial when there is excessive proinflammation, particularly when supplemental arginine is supplied, by reducing cytokine-induced eicosanoid production. KEY WORDS: * immune-enhancing diets * arginine * (n-3) fats * clinical outcome

Asymmetric dimethylarginine (ADMA) is a recently recognized endogenous inhibitor of nitric oxide production. Its role in cardiovascular disease is emerging, and ADMA appears to be an important causal factor in dysfunction of the vascular system. Several studies show that ADMA accumulates during renal failure, and ADMA has been identified as causing the cardiovascular complications accompanying renal failure. In addition to the kidney, we recently suggested an important role for the liver as an ADMA-eliminating organ. In a population of critically ill patients, hepatic failure was the most prominent determinant of ADMA concentration, and, notably, high ADMA concentration proved to be a strong and independent risk factor for intensive care unit mortality in these patients. We here summarize the role of both the kidney and the liver in the regulation of ADMA levels. In addition, the potential central role of ADMA as a causative factor in the development of multiple organ failure is discussed. KEY WORDS: * asymmetric dimethylarginine * dimethylarginine dimethylaminohydrolase * nitric oxide * liver * kidney * multiple organ failure

L-Arginine is the biological precursor of nitric oxide (NO), which serves as an important signal and effector molecule in animals. This review summarizes some structure-function aspects of the mammalian nitric oxide synthases, which are enzymes that catalyze the oxidation of L-arginine to NO and L-citrulline. These include aspects related to: 1) the chemical transformations of L-arginine during enzyme catalysis, 2) binding of L-arginine or its structural analogs to the nitric oxide synthases, and 3) how L-arginine levels may affect product formation by the nitric oxide synthases and how this can be modulated by structural analogs of L-arginine. KEY WORDS: * L-arginine, nitric oxide * flavoprotein * reactive oxygen species * hemeprotein

The amino acid L-arginine has been administered as a single supplement to humans in an effort to improve the outcome of seriously ill patients. In normal individuals, markers of collagen biosynthesis have increased with daily oral doses ranging from 14 to 24.8 g of free arginine for 14 d. No clinical evidence of improved wound healing has been reported in the few patient studies performed to date. Administration of enteral, but not intravenous, arginine has been associated with markers of improved immune function in normal individuals and in some, but not all, patient groups studied. A single study in premature infants suggested that supplementation of L-arginine (261 mg x [kg.sup.-1] x [d.sup.-1]) administered by both the parenteral and enteral routes decreased the incidence of necrotizing enterocolitis. A single study demonstrated that oral arginine administration in conjunction with conventional chemotherapy for active tuberculosis to HIV--but not HIV+ individuals enhanced treatment responses. In both these area, larger multicenter investigations are needed. For a difference to be a difference it has to make a difference. Supplementation of only L-arginine does not to date universally show benefit, nor does it show harm. At this time there is no rationale for the routine supplementation of arginine alone to enhance recovery from serious illness. Because of the potential for harm, this amino acid should only be administered to critically ill patients in large doses under carefully monitored study conditions. KEY WORDS: * arginine * parenteral nutrition

Arginine metabolism involves various organs such as the kidney, the intestines, and the liver, which act together in an interorgan axis. Major pathways for arginine production are protein breakdown and de novo arginine production from citrulline; disposal of arginine is mainly used for protein synthesis or used by the enzymes arginase and nitric oxide synthase (NOS). To assess in vivo organ arginine metabolism under normal conditions and during endotoxemia we used a mouse model, and analyzed for gender and strain differences. Male and female inbred FVB and C57BL6/J mice were anesthetized and catheterized to study whole body, gut, liver, renal and muscle metabolism, using a stable isotope infusion protocol. Animals were treated with saline or lipopolysaccharide. Plasma arginine levels tended to be higher in female mice, although levels were not significantly different from male mice (P = 0.09). Although not all significantly different, whole body arginine production and arginine clearance tended to be higher in C57BL6/J mice (P < 0.1), while citrulline (P = 0.05), NO (P = 0.08), and de novo arginine (P < 0.01) production were higher in FVB mice. During endotoxemia, NO production increased in general (P < 0.05), while whole body arginine clearance increased in FVB mice, but decreased in C57BL6/J mice (P < 0.01). At the organ level, portal-drained viscera (PDV) arginine metabolism was higher in FVB than in C57BL6/J mice (P < 0.05). During endotoxemia, liver arginine metabolism decreased in general (P < 0.05), while strain differences existed for PDV, muscle, and renal arginine metabolism. In conclusion, stable isotope techniques in multicatheterized mice allow measurements of arginine metabolism on whole body and organ level. Strain and gender differences are present in arginine metabolism under physiological conditions and during endotoxemia. J. Nutr. 134: 2768S-2774S, 2004. KEY WORDS: * arginine * gut * liver * muscle * kidney

There is abundant evidence that the endothelium plays a crucial role in the maintenance of vascular tone and structure. One of the major endothelium-derived vasoactive mediators is nitric oxide (NO). Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of NO synthase. ADMA inhibits vascular NO production in concentrations found in pathophysiological conditions; ADMA also causes local vasoconstriction when it is infused intraarterially. Thus, elevated ADMA levels may explain the 'L-arginine paradox,' i.e., the observation that supplementation with exogenous L-arginine improves NO-mediated vascular functions in vivo, although its baseline plasma concentration is about 25-fold higher than the Michaelis-Menten constant [K.sub.m] of the isolated, purified endothelial NO synthase in vitro. The biochemical and physiological pathways related to ADMA are well understood: Dimethylarginines are the result of degradation of methylated proteins; the methyl group is derived from S-adenosylmethionine. Both ADMA and its regioisomer, symmetric dimethylarginine, are eliminated from the body by renal excretion, whereas only ADMA is metabolized via hydrolytic degradation to citrulline and dimethylamine by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). DDAH activity and/or expression may therefore contribute to the pathogenesis of endothelial dysfunction in various diseases. Plasma ADMA levels are increased in humans with hypercholesterolemia, atherosclerosis, hypertension, chronic renal failure, and chronic heart failure. Increased ADMA levels are associated with reduced NO synthesis as assessed by impaired endothelium-dependent vasodilation. In several prospective and cross-sectional studies, ADMA evolved as a marker of cardiovascular risk. With increasing knowledge of the role of ADMA in the pathogenesis of cardiovascular disease, ADMA is becoming a goal for pharmacotherapeutic interventions. Among other potential strategies that are currently being tested, administration of L-arginine has been shown to improve endothelium-dependent vascular functions in subjects with high ADMA levels. Finally, ADMA has gained clinical importance recently because several studies have shown that ADMA is an independent cardiovascular risk factor. KEY WORDS: * endothelium * nitric oxide, atherosclerosis * coronary heart disease * prognosis * oxidative stress

Arginine is a dibasic, cationic, semiessential amino acid with numerous roles in cellular metabolism. It serves as an intermediate in the urea cycle and as a precursor for protein, polyamine, creatine and nitric oxide (NO) biosynthesis. Arginine is conditionally essential since it becomes necessary under periods of growth and after recovery after injury. Arginine also promotes wound healing and functions as a secretagogue stimulating the release of growth hormone, insulin-like growth factor 1, insulin, and prolactin. Furthermore, arginine has several immunomodulatory effects such as stimulating T- and natural killer cell activity and influencing pro-inflammatory cytokine levels. The discover that L-arginine is the sole precursor for the multifunctional messenger molecule nitric oxide (NO) led to investigation into the role of arginine in numerous physiologic and pathophysiologic phenomena including cancer. Although NO was first identified in endothelial cells, it is now recognized to be generated by a variety of cell types, including several tumor cell lines and solid human tumors. Unfortunately, the precise role of NO in cancer is poorly understood but it may influence tumor initiation, promotion, and progression, tumor-cell adhesion, apoptosis angiogenesis, differentiation, chemosensitivity, radiosensitivity, and tumor-induced immunosuppression. The biological effects of NO are complex and dependent upon numerous regulatory factors. Further research is necessary to enhance our understanding of the complex mechanisms that regulate NO's role in tumor biology. A better understanding of the role of arginine-derived NO in cancer may lead to novel antineoplastic and chemopreventative strategies. KEY WORDS: * arginine * nitric oxide * apoptosis * angiogenesis * cancer

Arginine appears to be a semiessential amino acid in humans during critical illness. Catabolic disease states such as sepsis, injury, and cancer cause an increase in arginine utilization, which exceeds body production, leading to arginine depletion. This is aggravated by the reduced nutrient intake that is associated with critical illness. Arginine depletion may have negative consequences on tissue function under these circumstances. Nutritional regimens containing arginine have been shown to improve nitrogen balance and lymphocyte function, and stimulate arginine transport in the liver. We have studied the effects of stress mediators on arginine transport in vascular endothelium, liver, and gut epithelium. In vascular endothelium, endotoxin stimulates arginine uptake, an effect that is mediated by the cytokine tumor necrosis factor-[alpha] (TNF-[alpha]) and by the cyclo-oxygenase pathway. This TNF-[alpha] stimulation involves the activation of intracellular protein kinase C (PKC). A significant increase in hepatic arginine transport activity also occurs following burn injury and in rats with progressive malignant disease. Surgical removal of the growing tumor results in a normalization of the accelerated hepatic arginine transport within days. Chronic metabolic acidosis and sepsis individually augment intestinal arginine transport in rats and Caco-2 cell culture. PKC and mitogen-activated protein kinases are involved in mediating the sepsis/acidosis stimulation of arginine transport. Understanding the regulation of plasma membrane arginine transport will enhance our knowledge of nutrition and metabolism in seriously iii patients and may lead to the design of improved nutritional support formulas. KEY WORDS: * arginine * membrane transport * catabolism

In this review the current knowledge about the arginine-degrading enzyme arginase and its unexpected roles in survival and regeneration in the central nervous system will be discussed. Recent data suggest the neuroprotective effects of extracellularly applied arginase can be attributed to an activation of the endoplasmic reticulum stress response with a consequent change of the pro-survival gene expression profile. However, the activation of neural regeneration pathways caused by an upregulation of endogenous arginase I is mediated by polyamines, a group of arginase downstream products with widespread biological effects. In light of these new discoveries, there is heightened interest in the regulation of arginase I gene expression within the central nervous system. A number of transcription factors such as Sp1, C/EBP (CCATT/enhancer-binding protein), and CREB seem to be involved in the transcriptional control of arginase I and may contribute to the complex expression pattern of arginase I in distinct brain regions and during development. Beyond molecular mechanisms, this review will also include relevant clinical findings in patients with neurodegenerative diseases. J. Nutr. 134: 2812S-2817S, 2004. KEY WORDS: * arginase * arginine * neurodegeneration

L-arginine is a semi essential amino acid and also a substrate for the synthesis of nitric oxide (NO), polyamines, and agmatine. These L-arginine metabolites may participate in the pathogenesis of renal disease and constitute the rationale for manipulating L-arginine metabolism as a strategy to ameliorate kidney disease. Modification of dietary L-arginine intake in experimental models of kidney diseases has been shown to have both beneficial as well as deleterious effects depending on the specific model studied. L-arginine supplementation in animal models of glomerulonephritis has been shown to be detrimental, probably by increasing the production of NO from increased local expression of inducible NO synthase (iNOS). L-arginine supplementation does not modify the course of renal disease in humans with chronic glomerular diseases. However, beneficial effects of L-arginine supplementation have been reported in several models of chronic kidney disease including renal ablation, ureteral obstruction, nephropathy secondary to diabetes, and salt-sensitive hypertension. L-arginine is reduced in preeclampsia and recent experimental studies indicate that L-arginine supplementation may be beneficial in attenuating the symptoms of preeclampsia. Administration of exogenous L-arginine has been shown to be protective in ischemic acute renal failure. In summary, the role of L-arginine in the pathogenesis and treatment of renal disease is not completely understood and remains to be established. J. Nutr. 134: 2801S-2806S, 2004. KEY WORDS: * L-arginine * kidney failure * glomerulonephritis * hypertension * preeclampsia * nitric oxide

Nitric oxide (NO) is synthesized from arginine by NO synthase (NOS), and the availability of arginine is one of the rate-limiting factors in cellular NO production. Citrulline that is formed as a by-product of the NOS reaction can be recycled to arginine by successive actions of argininosuccinate synthetase (AS) and argininosuccinate lyase (AL), forming the citrulline-NO cycle. AS and sometimes AL have been shown to be coinduced with inducible NOS (iNOS) in various ceil types including activated macrophages, microglia, vascular smooth muscle cells, glial cells, neuronal PC12 cells, retinal pigment epithelial cells, and pancreatic [beta]-cells. Coinduction of endothelial NOS (eNOS), AS, and AL are observed in human umbilical vein endothelial cells. In contrast, arginase can downregulate NO production by decreasing intracellular arginine concentrations, iNOS and arginase activities are regulated reciprocally in macrophages by cytokines, and this may guarantee the efficient production of NO. In contrast, iNOS and arginase isoforms (type I and/or II) are coinduced in immunostimulated macrophages, but not in PC12 cells and glial cells. These results indicate that NO production is modulated by the recycling and degradation of arginine. Arginase also plays an important role in regulation of polyamine and proline synthesis. KEY WORDS: * arginine * arginase * argininosuccinate synthetase * nitric oxide * nitric oxide synthase

Asthma, a complex chronic inflammatory pulmonary disorder, is on the rise despite intense ongoing research underscoring the need for new scientific inquiry. Using global microarray analysis, we recently discovered that asthmatic responses involve metabolism of arginine by arginase. We found that the cationic amino acid transporter (CAT)2, arginase I, and arginase II were particularly prominent among the allergen-induced gene transcripts. These genes are key regulators of critical processes associated with asthma, including airway tone, cell hyperplasia, and collagen deposition, respectively. Recent data suggest that arginase induction is not just a marker of allergic airway responses, but that arginase is involved in the pathogenesis of multiple aspects of disease. This review focuses on the current body of knowledge on L-arginine metabolism in asthma. KEY WORDS: * arginine * arginase * asthma * allergy * lung

The kidney plays a major role in arginine metabolism in 3 principal ways: arginine synthesis, creatine synthesis, and arginine reabsorption. Appreciable quantities of arginine are synthesized in the kidney from citrulline produced by the intestine. The renal enzymes of arginine synthesis, argininosuccinate synthetase and argininosuccinate lyase, occur in the cells of the proximal tubule. The rate of arginine synthesis depends on citrulline delivery and does not appear to be regulated by dietary arginine availability. Renal arginine synthesis in humans produces ~2 g arginine/d, which may be compared to an intake, from a Western diet, of ~4 to 5 g/d. Spontaneous, nonenzymatic breakdown of creatine and creatine phosphate to creatinine causes the excretion of 1 to 2 g creatinine/d and requires the replacement of an equivalent amount of creatine from the diet and by endogenous synthesis. The first enzyme of creatine biosynthesis, L-arginine:glycine amidinotransferase, occurs in the kidney and produces guanidinoacetate, which is released into the renal vein. The renal output of guanidinoacetate, however, is rather low, and we propose that the entire pathway of creatine synthesis may also occur in the liver. Renal arginine reabsorption salvages ~3 g arginine/d. At the apical membrane of proximal tubular cells, arginine shares a transporter with lysine, ornithine, and cystine. Defects in this heteromeric transporter cause cystinuria, which is also characterized by urinary loss of arginine, lysine, and ornithine. Arginine is transported out of the proximal tubular cells at the basolateral membrane by another heteromeric transporter, which also transports lysine and ornithine. Defects in this transporter cause lysinuric protein intolerance. J. Nutr. 134: 2791S-2795S, 2004. KEY WORDS: * arginine synthesis * citrulline * kidney * creatine synthesis * arginine reabsorption

Hypertension is a major healthcare problem afflicting nearly 50 million individuals in the United States. Despite its strong causal association with cardiovascular disease complications including myocardial infarction, heart failure, and stroke, the majority of patients with hypertension do not achieve optimal blood pressure control. The prevalence of hypertension is expected to increase with the aging population, growing obesity epidemic, and rising incidence of metabolic syndrome. Endothelial dysfunction and reduced nitric oxide (NO) bioactivity represent prominent pathophysiological abnormalities associated with hypertensive cardiovascular disease. Individuals with hypertension exhibit blunted epicardial and resistance vascular dilation to endothelium-derived nitric oxide (EDNO) agonists in the peripheral and coronary circulation that likely contributes to mechanisms of altered vascular tone in hypertension. The amino acid L-arginine serves as the principal substrate for vascular NO production. Numerous studies, though not uniformly, demonstrate a beneficial effect of acute and chronic L-arginine supplementation on EDNO production and endothelial function, and L-arginine has been shown to reduce systemic blood pressure in some forms of experimental hypertension. This brief review discusses the potential role of L-arginine in hypertension, and reviews possible mechanisms of L-arginine action including modulation of EDNO production, alteration of asymmetrical dimethylarginine (ADMA):L-arginine balance, and possible improvement of insulin sensitivity. In view of the rising prevalence of hypertension, randomized human clinical studies investigating the potential therapeutic role of L-arginine may be warranted. J. Nutr. 134: 2807S-2811S, 2004. KEY WORDS: * L-arginine * hypertension * endothelium * nitric oxide

The concentration of arginine (an essential amino acid for neonates) in sow's milk is remarkably low, and thus endogenous synthesis of arginine plays a crucial role in maintaining arginine homeostasis in milk-fed piglets. Paradoxically, intestinal synthesis of citrulline from glutamine/glutamate and proline (the endogenous source of arginine) declines markedly in 7- to 21 -d-old suckling pigs, compared with 1 - to 3-d-old pigs. Therefore, plasma concentrations of arginine and its immediate precursors (ornithine and citrulline) decrease progressively by 20-41%, whereas plasma ammonia levels increase progressively by 18-46%, between d 3 and 14 of life. Dietary supplementation of 0.2 and 0.4% arginine to 7- to 21-d-old pigs (artificially reared on a milk feeding system) dose dependently enhances the plasma arginine concentration (30 and 61%), reduces the plasma ammonia level (20 and 35%), and increases weight gain (28 and 66%). These compelling metabolic and growth data demonstrate unequivocally that arginine is insufficient for supporting the maximal growth in milk-fed young pigs and that this arginine deficiency represents a major obstacle to realizing the growth potential in piglets. A low concentration of mitochondrial N-acetylglutamate (an activator of both pyrroline-5-carboxylate synthase and carbamoylphosphate synthase-I) is responsible for the striking decline in the intestinal synthesis of citrulline and arginine during the suckling period. Accordingly, oral administration of N-carbamoylglutamate [a metabolically stable analogue of N-acetylglutamate; 2 x 50 mg/(kg body wt. d)] enhances plasma arginine level (68%) and weight gain (61%) of 4to 14-d-old sow-reared pigs. Thus, the metabolic activation of intestinal citrulline and arginine synthesis provides a novel, effective means to increase endogenous arginine provision and therefore piglet growth (a major goal of animal agriculture). Our findings not only generate new fundamental knowledge about amino acid utilization by neonatal pigs, but they also have important practical implications for improving the efficiency of pork production. J. Nutr. 134: 2783S-2790S, 2004. KEY WORDS: * amino acids * intestine * metabolism * milk * swine

Urea cycle disorders (UCD) are human conditions caused by the dysregulation of nitrogen transfer from ammonia nitrogen into urea. The biochemistry and the genetics of these disorders were well elucidated. Earlier diagnosis and improved treatments led to an emerging, longer-lived cohort of patients. The natural history of some of these disorders began to point to pathophysiological processes that may be unrelated to the primary cause of acute morbidity and mortality, i.e., hyperammonemia. Carbamyl phosphate synthetase I single nucleotide polymorphisms may be associated with altered vascular resistance that becomes clinically relevant when specific environmental stressors are present. Patients with argininosuccinic aciduria due to a deficiency of argininosuccinic acid lyase are uniquely prone to chronic hepatitis, potentially leading to cirrhosis. Moreover, our recent observations suggest that there may be an increased prevalence of essential hypertension. In contrast, hyperargininemia found in patients with arginase 1 deficiency is associated with pyramidal tract findings and spasticity, without significant hyperammonemia. An intriguing potential pathophysiological link is the dysregulation of intracellular arginine availability and its potential effect on nitric oxide (NO) metabolism. By combining detailed natural history studies with the development of tissue-specific null mouse models for urea cycle enzymes and measurement of nitrogen flux through the cycle to urea and NO in UCD patients, we may begin to dissect the contribution of different sources of arginine to NO production and the consequences on both rare genetic and common multifactorial diseases. J. Nutr. 134: 2775S-2782S, 2004. KEY WORDS: * urea cycle * arginine * nitric oxide

The supply of arginine may become rate limiting for enzymatic reactions that use this semiessential amino acid as a substrate (e.g., nitric oxide, agmatine, creatine, and urea synthesis), particularly under conditions of high demand such as growth, sepsis, or wound healing. In addition, arginine acts as a signaling molecule that regulates essential cellular functions such as protein synthesis, apoptosis, and growth. In the past decade, a number of carrier proteins for amino acids have been identified on the molecular level. They belong to different gene families, exhibit overlapping but distinctive substrate specificities, and can further be distinguished by their requirement for the cotransport or countertransport of inorganic ions. A number of these transporters function as exchangers rather than uniporters. Uptake of amino acids by these transporters therefore depends largely on the intracellular substrate composition. Hence, there is a complex crosstalk between transporters for cationic and neutral amino acids as well as for peptides. This article briefly reviews current knowledge regarding mammalian plasma membrane transporters that accept arginine as a substrate. J. Nutr. 134: 2752S-2759S, 2004. KEY WORDS: * cationic amino acid transporter * hetero(di)meric amino acid transporter * acid transporter [B.sup.0., +] * peptide transporter

In mammals, L-arginine is classified as a semiessential or conditionally essential amino acid, depending on the developmental stage and health status of the individual. It can be derived from proline or glutamate, with the ultimate synthetic step catalyzed by argininosuccinate lyase. L-arginine is catabolized by arginases, nitric oxide synthases, arginine:glycine amidinotransferase, and possibly also by arginine decarboxylase, resulting ultimately in the production of urea, proline, glutamate, polyamines, nitric oxide, creatine, or agmatine. There is considerable diversity in tissue-specific and stimulus-dependent regulation of expression within this group of enzymes, and the expression of several of them can be regulated at transcriptional and translational levels by changes in the concentration of L-arginine itself. Consequently, the interplay among these enzymes in the regulation of specific aspects of arginine metabolism can be quite complex. For example, nitric oxide production can be affected by the interplay between nitric oxide synthases, arginases, and argininosuccinate synthetase. This metabolic complexity can pose challenges for analyses of arginine metabolism not only because L-arginine is a substrate for several different enzymes but also because ornithine and citrulline, key products of arginine metabolism, can each be produced by multiple enzymes. This overview highlights key features of the arginine metabolic enzymes and their interactions. KEY WORDS: * arginine * ornithine * citrulline * nitric oxide * polyamines

L-Arginine (Arg) is the substrate for the synthesis of nitric oxide (NO), the endothelium-derived relaxing factor essential for regulating vascular tone and hemodynamics, NO stimulates angiogenesis, but inhibits endothelin-1 release, leukocyte adhesion, platelet aggregation, superoxide generation, the expression of vascular cell adhesion molecules and monocyte chemotactic peptides, and smooth muscle cell proliferation, Arg exerts its vascular actions also through NO-independent effects, including membrane depolarization, syntheses of creatine, proline and polyamines, secretion of insulin, growth hormone, glucagon and prolactin, plasmin generation and fibrinogenolysis, superoxide scavenging and inhibition of leukocyte adhesion to nonendothelial matrix, Compelling evidence shows that enteral or parenteral administration of Arg reverses endothelial dysfunction associated with major cardiovascular risk factors (hypercholesterolemia, smoking, hypertension, diabetes, obesity/insulin resistance and aging) and ameliorates many common cardiovascular disorders (coronary and peripheral arterial disease, ischemia/ reperfusion injury, and heart failure), Dietary Arg supplementation may represent a potentially novel nutritional strategy for preventing and treating cardiovascular disease. J. Nutr. 130: 2626-2629, 2000. KEY WORDS: * arginine * cardiovascular disease

Many key metabolic and physiologic functions involve arginine and arginine-derived metabolites. Requirements for arginine, a 'conditionally essential' amino acid for most mammalian species, are met in variable proportions by dietary intake and endogenous synthesis, the latter being sufficient to fulfill arginine needs in adult humans and mice under nonpathologic conditions. However, dietary arginine restriction causes orotic aciduria and abnormal function of the urea cycle. Furthermore, the importance of dietary arginine in the maintenance of homeostasis of arginine-derived metabolites in the body has not yet been analyzed in detail. We therefore examined whether the deprivation or supplementation of dietary arginine affects tissue and circulating levels of arginine-derived polyamines. We pair-fed male Swiss albino mice (30 g) for 15 or 30 d synthetic diets containing 0, 1.12 or 2.24 g/100 g L-arginine. Tissue and blood levels of the main free polyarnines, putrescine, spermidine and spermine, were measured by HPLC. In general, neither the deprivation nor the supplementation of arginine dramatically affected the levels of any of the polyamines analyzed. Variations were organ, time and polyamine specific, and most differences were in the levels of putrescine at 15 d and of spermidine at 30 d. Thus, in contrast to effects on urea cycle function, dietary arginine does not appear to be essential for the maintenance of the homeostasis of free polyamine levels in adult mice, emphasizing the importance of endogenous arginine synthesis in preserving the polyamine body pool. J. Nutr. 132: 3715-3720, 2002. KEY WORDS: * arginine * polyamines * putrescine * spermidine * spermine * mice

Arginine metabolism is enhanced in the small intestine of weanling pigs, but the molecular mechanism(s) involved is not known. The objectives of this study were to determine the following: 1) whether glucocorticoids play a role in induction of intestinal arginine metabolic enzymes during weaning; 2) whether the induction of enzyme activities was due to increases in corresponding mRNA levels; and 3) the identity of the arginase isoform(s) expressed in the small intestine. Jejunum was obtained from 29-d-old weaned pigs that were or were not treated with 17-[Beta]-hydroxy-11[Beta]-(4-dimethylaminophenyl)17[Alpha]-(prop-1-ynyl)estra-4,9-dien-3-one (RU486, an antagonist of glucocorticoid receptors), or from age-matched suckling pigs. Activities and mRNA levels for type I and type II arginases, argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL) were determined. Activities of arginase, ASL and ASS increased by 635, 56 and 106%, respectively, in weanling pigs, compared with suckling pigs. RU486 treatment attenuated the increase in arginase activity by 74% and completely prevented the ASL induction in weanling pigs, but had no effect on ASS activity. Pig intestine expresses both type I and type II arginases. On the basis of immunoblot analyses, there was no significant difference in levels of intestinal type I arginase among these three groups of pigs, indicating that changes in arginase activity were due only to type II arginase. The mRNA levels for type II arginase and ASL increased by 135 and 198%, respectively, in weanling pigs compared with suckling pigs, and this induction was completely prevented by RU486. In contrast, ASS mRNA levels did not differ between suckling and weanling pigs. These results suggest that intestinal type II arginase, ASS and ASL are regulated differentially at transcriptional and post-translational levels and that glucocorticoids play a major role in the induction of type II arginase and ASL mRNAs in the small intestine of weanling pigs. KEY WORDS: arginase; intestine; weaning; pigs

This study was conducted to determine whether endogenous synthesis of arginine plays a role in regulating arginine homeostasis in postweaning pigs. Pigs were fed a sorghum-based diet containing 0.98% arginine and were used for studies at 75 d of age (28.4 kg body weight). Mitochondria were prepared from the jejunum and other major tissues for measuring the activities of [[Delta].sup.1]-pyrroline-5-carboxylate (P5C) synthase and proline oxidase (enzymes catalyzing P5C synthesis from glutamate and proline, respectively) and of ornithine aminotransferase (OAT) (the enzyme catalyzing the interconversion of P5C into ornithine). For metabolic studies, jejunal enterocytes were incubated at 37 [degrees] C for 30 min in Krebs-Henseleit bicarbonate buffer containing 2 mmol/L L-glutamine, 2 mmol/L L-[U-14C]proline, and 0-200 [[micro]mol]/L gabaculine (an inhibitor of OAT). The activities of P5C synthase, proline oxidase and OAT were greatest in enterocytes among all of the tissues studied. Incubation of enterocytes with gabaculine resulted in decreases (P < 0.05) in the synthesis of ornithine and citrulline from glutamine and proline. When gabaculine was orally administered to pigs (0.83 mg/kg body weight) to inhibit intestinal synthesis of citrulline from glutamine and proline, plasma concentrations of citrulline (-26%) and arginine (-22%) decreased (P < 0.05), whereas those of alanine (+21%), ornithine (+17%), proline (+107%), taurine (+56%) and branched-chain amino acids (+21-40%) increased (P < 0.05). On the basis of dietary arginine intake and estimated arginine utilization, the endogenous synthesis of arginine in the 28-kg pig provided [greater than or equal to]50.2% of total daily arginine requirement. Taken together, our results suggest an important role for endogenous synthesis of arginine in regulating arginine homeostasis in postweaning growing pigs, as previously shown in neonatal pigs. KEY WORDS: arginine glutamine proline ornithine aminotransferase pigs

We studied the effects of increasing dietary concentrations of either L-arginine or L-threonine on growth, feed intake, feed conversion ratio and composition of gain in rainbow trout. Semipurified diets containing 20.1 MJ digestible energy/kg dry matter, with wheat gluten and crystalline amino acids as sole sources of amino acids, were fed to rainbow trout (initial mean body weight 47 [+ or -] 0.7 g). In one series of 12 diets, arginine concentration ranged from 5.0 to 23.8 g/kg dry matter; in a second series of 12 diets, threonine concentration ranged from 3.7 to 21.0 g/kg dry matter. Each diet was fed to a group of 20 fish. During the experiment of 51 feeding days, dry matter intake, weight gain, feed conversion ratio, protein concentration of gain and total protein deposition followed exponential functions. For achieving 95% of the potential maximum protein deposition, dietary concentrations of 11.6 g arginine and 10.4 g threonine/kg dry matter were required. Arginine and threonine were both utilized most efficiently at dietary concentrations of [approximately] 6 g/kg dry matter. At low dietary concentrations of arginine, deposition of this amino acid exceeded the quantity fed. Recommended dietary concentrations of arginine and threonine will depend on the trait desired in the trout. INDEXING KEY WORDS: requirement; rainbow trout; arginine; threonine

Arteriovenous (A-V) differences in the plasma concentrations of amino acids across the jejunum were studied in preweaning (14- to 21-d-old) and post-weaning (29 to 58-d-old) pigs in the postabsorptive state. Glutamine was the only amino acid that was extracted by the small intestine in both pre- and post-weaning pigs. The production of citrulline by the jejunum was low in preweaning pigs, but was threefold greater in the post-weaning pigs than in the preweaning pigs. The output of proline by the intestine was observed in the post-weaning pigs but not in the preweaning pigs. Arginine and alanine were the predominant amino acids released by the jejunum of the pre- and post-weaning pigs, respectively. Whereas glutamate was the major amino acid formed from glutamine in pig enterocytes in vitro, the jejunum of the post-weaning pigs released comparable amounts of citrulline, proline and glutamate in vivo. There were no significant A-V differences in the concentrations of urea, ornithine, taurine, tyrosine, serine, glycine and nutritionally essential amino acids in either the preweaning or the post-weaning pigs. Thus, we demonstrated for the first time the uptake in vivo of glutamine and the release of arginine, alanine, citrulline, glutamate and proline by the small intestine of developing pigs. Our findings on the release in vivo of citrulline (the precursor of arginine) and proline by the small intestine of the pre- and post-weaning pigs provide a basis for explaining why arginine and proline are nutritionally essential amino acids for young suckling piglets but not for adult pigs. INDEXING KEY WORDS: arginine; intestine; pigs; glutamine; citrulline

Two experiments were conducted to determine the dietary arginine requirement of juvenile hybrid striped bass (Morone saxatilis X M. chrysops); a third experiment evaluated the interaction of lysine and arginine. Diets in Experiments 1 and 2 were supplemented with graded concentrations of L-arginine-HCl, resulting in eight dietary treatments. Dietary arginine concentrations ranged from 1.0 to 2.4 g/100 g diet in Experiment 1 and from 0.6 to 2.0 g/100 g diet in Experiment 2. Weight gain was not affected by dietary treatments in Experiment 1. Feed efficiency was significantly affected by dietary arginine concentrations, and the data, when subjected to broken-line analysis, resulted in a requirement estimate of 1.53 + or - 0.20 g/100 g diet. Weight gain and feed efficiency were both significantly affected by dietary arginine concentrations in Experiment 2. Broken-line analyses of weight gain and feed efficiency data indicated the dietary arginine requirement to be 1.55 + or - 0.10 and 1.45 + or - 0.12 g/100 g diet, respectively. Diets in Experiment 3 contained lysine and arginine in ratios of 1:1, 1:1.5, 1:2 and 1:2.5 for the previously estimated requirements for both lysine:arginine and arginine:lysine. No differences were observed in weight gain or feed efficiency for fish fed various lysine:arginine ratios, but serum lysine was significantly different among treatment groups.

The purpose of the present study was to determine whether the regulation of urea synthesis was mediated through the activation of N-acetylglutamate synthesis by arginine and whether the concentration of glutamate or activity of N-acetylglutamate synthetase might control N-acetylglutamate concentration when the thyroid status was manipulated. Three groups of rats were given 6-propyl-2-thiouracil (PTU, thyroid inhibitor) without triiodothyronine T3 treatment, treated with PTU + T3 or treated with neither PTU nor T3 (control). Urinary excretion of urea, liver concentration of N-acetylglutamate and plasma concentration of arginine in rats given PTU + T3 were significantly lower than in rats given PTU alone. Liver concentration of N-acetylglutamate was correlated with plasma concentration of arginine (r = 0.842, P < 0.001). The activity of N-acetylglutamate synthetase and glutamate concentration in liver of the PTU + T3-treated group were significantly higher than in the group treated with PTU alone. Arginine administration (0.5 mmol/100 g body wt) elevated the liver concentration of N-acetylglutamate in all three groups. The results suggest that the greater concentration of arginine in the hypothyroid (PTU alone) rats is likely to increase the N-acetylglutamate concentration and stimulate urea synthesis.

A study was done on the effects of dietary arginine deficiency on glutamine and urea cycle across the portal-drained viscera and liver of rats. Results show that production of citrulline by the rat intestine was not altered significantly by feeding, the absence of arginine in the diet or the presence of additional glutamate. The liver takes up 13% of the cirulline produced by the intestines and only citrulline released from, or bypasses the liver is available for net arginine synthesis. These results suggest that the liver does not contribute to the circulating citrulline pool.

L-arginine, the principal substrate for endothelial nitric oxide synthase, is oxidized to L-citruliine and nitric oxide. Endothelial dysfunction is associated with decreased bioactive nitric oxide production, an abnormality observed in atherothrombosis. Acute or chronic administration of supplemental L-arginine enhances endothelial nitric oxide production and improves endothelial function in the setting of atherothrombosis. The mechanisms by which L-arginine improves endothelial nitric oxide bioactivity include increased intracellular uptake via the high-affinity cationic transporter; substrate competition with asymmetric dimethylarginine, a naturally occurring inhibitor of nitric oxide synthase; direct antioxidant activity; stimulated release of histamine from mast cells, which produces a vasodilator response; decreased activity of norepinephrine, which promotes the effect of endogenous vasodilators including nitric oxide; and increased insulin secretion, which causes vasodilation. By virtue of its link to methyl group metabolism, supplemental L-arginine can, however, also increase the production of S-adenosylhomocysteine from S-adenosylmethionine through the methylation-dependent generation of creatine from guanidinoacetate. This reaction can theoretically lead to increased homocysteine synthesis from its S-adenosyl derivative, which itself can have adverse effects on endothelial function. The interrelationships among these effects of L-arginine are reviewed here, and the potential benefits and risks of L-arginine supplementation are discussed. J. Nutr. 134: 2798S-2800S, 2004. KEY WORDS: * arginine * atherothrombosis * vasodilation * nitric oxide

Arginine supplementation enhances in vitro lymphocyte proliferation in healthy adult humans and in rodent models. Studies examining the effect of arginine supplementation on in vivo immune responses are lacking. The purpose of this study was to determine whether arginine supplementation could enhance in vivo immune responses in adult mice and reverse known age-associated alterations in immune function of young and aged mice. Mice (1, 10 and 33 mo old) were fed a 2% arginine or an isonitrogenous diet for 2 wk. Delayed-type hypersensitivity to 2,4-dinitrofluorobenzene--challenged ears and changes in popliteal lymph node weights to injected sheep red blood cells were measured. The mean percentage of increase in ear thickness in challenged vs. unchallenged ears was 27, 35 and 24% with arginine supplementation and 7, 12 and 0% with the isonitrogenous diet in the 1-, 10- and 33-mo-old mice, respectively (P [is less than or equal to] 0.05 for each age). Across all ages, the mean differences in popliteal lymph node weights were 1.9 [+ or -] 0.3 vs. 1.0 [+ or -] 0.3 mg with the arginine and isonitrogenous diets, respectively (P [is less than or equal to] 0.05). Only the 33-mo-old mice fed the isonitrogenous diet did not respond to this immune challenge. These findings suggest that arginine supplementation may enhance in vivo immune responses and/or reverse age-associated changes. J. Nutr. 130: 1827-1830, 2000. KEY WORDS: * mice * arginine * aged * immune function * delayed-type hypersensitivity

Increasing aquacultural production of red drum (Sciaenops ocellatus) has prompted the determination of many of this species' nutritional requirements. However, limited information is available concerning its amino acid requirements, especially for arginine. Therefore, a feeding trial was conducted with juvenile red drum to determine their quantitative dietary requirement for arginine. Experimental diets contained 35 g crude protein/100 g from red drum muscle and crystalline amino acids. Incremental levels of arginine were added to the diets in place of a mixture of glycine and aspartic acid to maintain all diets isonitrogenous. All diets were fed in triplicate to juvenile red drum for 7 wk. Graded levels of arginine significantly (P [is less than] 0.05) affected weight gain, feed efficiency, protein efficiency ratio (PER) and plasma arginine levels of the fish. Based on least-squares regression of feed efficiency and PER data, the minimum requirement ([+ or -] SEM) of red drum for arginine was estimated at 1.44 ([+ or -] 0.15) and 1.48 ([+ or -] 0.12) g/100 g diet (4.11 and 4.23 g/100 g dietary protein), respectively. The arginine requirements estimated from weight gain data were 1.75 ([+ or -] 0.18) g/100 g diet or 5.0 g/100 g dietary protein. These values are similar to those reported for other carnivorous fish species. J. Nutr. 130: 1796-1799, 2000. KEY WORDS: * arginine requirement * amino acids * red drum

Arginine is an intermediate of the ornithine cycle and serves as a precursor for the synthesis of nitric oxide, creatine, agmatine and proteins. It is considered to be a conditionally essential amino acid because endogenous synthesis only barely meets daily requirements. In rapidly growing suckling neonates, endogenous arginine biosynthesis is crucial to compensate for the insufficient supply of arginine via the milk. Evidence is accumulating that the intestine rather than the kidney plays a major role in arginine synthesis in this period. Accordingly, ectopic expression of hepatic arginase in murine enterocytes by genetic modification induces a selective arginine deficiency. The ensuing phenotype, whose severity correlates with the level of transgene expression in the enterocytes, could be reversed with arginine supplementation. We analyzed the effect of arginine deficiency on guanidine metabolism and neuromotor behavior. Arginine-deficient transgenic mice continued to suffer from an arginine deficiency after the arginine biosynthetic enzymes had disappeared from the enterocytes. Postweaning catch-up growth in arginine-deficient mice was characterized by increased levels of all measured amino acids except arginine. Furthermore, plasma total amino acid concentration, including arginine, was significantly lower in adult male than in adult female transgenic mice. Decreases in the concentration of plasma and tissue arginine led to significant decreases in most metabolites of arginine. However, the accumulation of the toxic guanidino compounds, guanidinosuccinic acid and methylguanidine, corresponded inversely with circulating arginine concentration, possibly reflecting a higher oxidative stress under hypoargininemic conditions. In addition, hypoargininemia was associated with disturbed neuromotor behavior, although brain levels of toxic guanidino compounds and ammonia were normal. KEY WORDS: * hypoargininemia * transgenes * intestine * guanidinosuccinic acid * methyguanidine * mice