Search

Pyridine nucleotide coenzymes are involved in >500 enzyme reactions and are biosynthesized from the amino acid L-tryptophan (L-Trp) as well as the vitamin niacin. Hence, "true" niacin-deficient animals cannot be "created" using nutritional techniques. We wanted to establish a truly niacin-deficient model animal using a protocol that did not involve manipulating dietary L-Trp. We generated mice that are missing the quinolinic acid (QA) phosphoribosyltransferase (QPRT) gene. QPRT activity was not detected in qprt(-/-)mice. The qprt(+/+), qprt(+/-), or qprt(-/-) mice (8 wk old) were fed a complete diet containing 30 mg nicotinic acid (NiA) and 2.3 g L-Trp/kg diet or an NiA-free diet containing 2.3 g L-Trp/kg diet for 23 d. When qprt(-/-)mice were fed a complete diet, food intake and body weight gain did not differ from those of the qprt(+/+) and qprt(+/-) mice. On the contrary, in the qprt(-/-) mice fed the NiA-free diet, food intake and body weight were reduced to 60% (P < 0.01) and 70% (P < 0.05) of the corresponding values for the qprt(-/-) mice fed the complete diet at d 23, respectively. The nutritional levels of niacin, such as blood and liver NAD concentrations, were also lower in the qprt(-/-) mice than in the qprt(+/+) and the qprt(+/-) mice. Urinary excretion of QA was greater in the qprt(-/-) mice than in the qprt(+/+) and qprt(+/-) mice (P < 0.01). These data suggest that we generated truly niacin-deficient mice.

Pyridine nucleotide coenzymes are involved in500 enzyme reactions and are biosynthesized from the amino acid L-tryptophan (L-Trp) as well as the vitamin niacin. Hence, "true" niacin-deficient animals cannot be "created" using nutritional techniques. We wanted to establish a truly niacin-deficient model animal using a protocol that did not involve manipulating dietary L-Trp. We generated mice that are missing the quinolinic acid (QA) phosphoribosyltransferase (QPRT) gene. QPRT activity was not detected in qprt(-/-)mice. The qprt(+/+), qprt(+/-), or qprt(-/-) mice (8 wk old) were fed a complete diet containing 30 mg nicotinic acid (NiA) and 2.3 g L-Trp/kg diet or an NiA-free diet containing 2.3 g L-Trp/kg diet for 23 d. When qprt(-/-)mice were fed a complete diet, food intake and body weight gain did not differ from those of the qprt(+/+) and qprt(+/-) mice. On the contrary, in the qprt(-/-) mice fed the NiA-free diet, food intake and body weight were reduced to 60% (P0.01) and 70% (P0.05) of the corresponding values for the qprt(-/-) mice fed the complete diet at d 23, respectively. The nutritional levels of niacin, such as blood and liver NAD concentrations, were also lower in the qprt(-/-) mice than in the qprt(+/+) and the qprt(+/-) mice. Urinary excretion of QA was greater in the qprt(-/-) mice than in the qprt(+/+) and qprt(+/-) mice (P0.01). These data suggest that we generated truly niacin-deficient mice.

In the tryptophan-niacin conversion, 2-amino-3-carboxymuconate-6-semiardehyde decarboxylase (ACMSD; formerly termed picolinic carboxylase) is an important enzyme regulating the generation of quinolinate. In a series of experiments, we investigated alterations of ACMSD expression in rats by feeding a high protein diet and by inducing diabetes with streptozotocin (STZ). Male Sprague-Dawley rats (5-wk-old) were fed a diet containing 40% casein for 11 d, and hepatic ACMSD activity and mRNA expression were determined at intervals. The enzyme activity had increased at d 2, and it continued to increase through d 11. ACMSD mRNA expression had increased at d 1 and the elevated levels were maintained through d 11. Shifting from the 40% casein diet to a 20% casein diet restored hepatic ACMSD activity and mRNA expression to normal levels within 5 d and 2 d, respectively. In another series of experiments, male Wistar rats were injected with STZ (50 mg/kg) and the time-course (d 0, 1, 2, 4, 8 and 14) of the change in hepatic ACMSD activity and mRNA expression were examined. The activity increased dramatically after d 4, while mRNA expression was significantly elevated at d 2, followed by slight increases through d 14. Insulin administration (2 U/12 h) reduced the elevated ACMSD activity and fully suppressed the elevated ACMSD mRNA expression due to STZ injection. These results indicated that the fluctuation of hepatic ACMSD mRNA expression was followed by that of ACMSD activity.

New evidence has been obtained suggesting that the "monitor" peptide is an essential intraluminal mediator in the stimulation of pancreatic enzyme secretion in response to protein intake in rats. Experiments were performed in vivo using a mixture of 50 mg of ovalbumin, alpha-lactalbumin or casein, 2 micrograms of purified protease-sensitive, cholecystokinin-releasing monitor peptide and 1 mg of porcine trypsin which was infused by cannula into the duodenum of atropine-treated rats. The small intestine had previously been washed with bicarbonate to eliminate proteases and the pancreatic juice was diverted. The amount of trypsin secreted in 2 h was comparable to that of rats in which the pancreatic juice was returned into the duodenum. However, in the presence of a monitor peptide--specific antibody which recognizes the N-terminal region of the peptide, the monitor peptide did not induce any pancreatic response. Therefore, the characteristic pattern of pancreatic enzyme secretion in response to protein intake can be reproduced by infusing only three components--dietary proteins, porcine trypsin and the purified monitor peptide.

We purified a peptide from rat pancreatic juice that enhances pancreatic enzyme secretion in a manner different from that of exogenous trypsin inhibitors, such as soybean trypsin inhibitor, when it is infused into the rat intestine. In this paper, we present evidence for the release of gut hormone(s) into the blood of rats after intestinal administration of the peptide. In the presence of atropine, an anesthetized rat small intestine was washed out with saline containing soybean trypsin inhibitor (Kunitz type) to eliminate proteases. Under these conditions, the rat small intestine was divided into four equal parts by ligation. Administration of the peptide into the first quarter of the small intestine stimulated pancreatic enzyme secretion. However, administration into the more distal parts did not stimulate the enzyme secretion. Moreover, intravenous injection of 1 mL of plasma from rats in which the peptide had been infused into the duodenum caused stimulation of pancreatic enzyme secretion in the recipient rats. It was suggested that the purified peptide acts in the proximal small intestine and that it stimulates the release of gut hormone(s) into the blood to enhance pancreatic enzyme secretion. These findings support the hypothesis that the peptide we purified is responsible in part for the humoral control of pancreatic enzyme secretion in the response to food intake.

The effect of atropine on pancreatic enzyme secretion in response to removal of intraluminal protease was investigated using rats with an indwelling bile-pancreatic cannula. Stimulation of pancreatic enzyme secretion by diversion of bile-pancreatic juice was blocked by intravenous atropine [100 micrograms/(kg X h)] injection. On the other hand, stimulation of pancreatic enzyme secretion by intraluminal infusion of soybean trypsin inhibitor (SBTI) was not blocked by atropine administration. These results suggest that the feedback response of pancreatic enzyme secretion after removal of the intraluminal protease is controlled by at least two different systems, an atropine-sensitive mechanism and an atropine-nonsensitive mechanism. We detected and purified a peptide from rat bile-pancreatic juice that enhanced pancreatic enzyme secretion when the peptide was infused into the proximal intestine. In the atropine-administered rat whose small intestine was deprived of protease after intraluminal washing with saline containing SBTI, intraluminal infusion of the purified peptide stimulated pancreatic enzyme secretion markedly. These findings suggest that the atropine-nonsensitive feedback response of pancreatic enzyme secretion is at least in part mediated by the peptide.