Search

Hippurate (Hip), an endogenous conjugate, belongs to the group of uremic toxins. Hip stimulates P-independent glutaminase (PIG) localized at the proximal luminal membrane, desamidating glutamine with the formation of ammonia, a dominant and adaptive elimination product of H+. This appears to be important because metabolic acidosis (MAC) does not stimulate PIG. Moreover, Hip inhibits ammonia production by P-dependent mitochondrial glutaminase (PDG) that is primarily stimulated by MAC. By this mechanism, it shifts the ammonia production from mitochondria to proximal tubular lumen. MAC stimulates Hip synthesis in the liver and kidney and increases Hip plasma concentration and even fractional excretion by the kidney, which creates an effective regulatory loop of ammoniagenesis. Thus, it appears that Hip by its participation in the correction of MAC possesses the modulatory function.

The key regulatory enzymes of kidney ammoniagenesis appear to be P-dependent (PDG) and P-independent (PIG) glutaminases. While the participation of PDG has been satisfactorily elucidated, the significance of PIG remains doubtful. Rat kidney cortex slices synthesized ammonia even under basal conditions. Metabolic acidosis, hippurate and insulin stimulated ammonia production. Under basal conditions, PDG activity in kidney homogenate, was twice as high as PIG activity. Metabolic acidosis stimulated ammonia production by the stimulation of both PDG (100%) and PIG (57%) activities. Hippurate stimulated only PIG activity both under basal conditions (90%) and in metabolic acidosis (52%), while it inhibited PDG activity only insignificantly under basal conditions and markedly (53%) in metabolic acidosis. Insulin stimulated both PIG and PDG activities under basal conditions as well as in metabolic acidosis and potentiated the PIG stimulation by hippurate while it potentiated the hippurate inhibition of PDG both under basal conditions and in acidotic rats. In conclusion, both PDG and PIG participate in ammoniagenesis and are stimulated by metabolic acidosis and insulin. Hippurate stimulates PIG, while it inhibits PDG in metabolic acidosis and even after insulin administration. The effect of hippurate appears to be of physiological interest.

A number of methods have been developed for the determination of acetylsalicylic acid (ASA, aspirin). However, they are not sensitive enough for the simultaneous determination of ASA and its major metabolites salicylic (SA) and salicyluric (SUA) acids at the low dosage schedules (30-100 mg ASA/d). The HPLC method with UV detection described here fulfills these requirements. The calibration curves were linear at the range 0.18-10 mumol/l. Coefficients of variation were 3.9% for SUA, 7.89% for ASA and 5.88% for SA. The recovery of ASA, SA and SUA was between 90-105%. ASA administered in doses of 30-400 mg was rapidly absorbed from gastrointestinal tract and deacetylated, forming the dominant plasma metabolite SA. SA was eliminated to about 60% by conjugation with glycine and therefore SUA was a dominant metabolite in urine. ASA was never found in urine at the low-dose ASA treatment. For this reason, SUA, but not ASA, can be determined in urine and may be used for monitoring patient compliance.

The pharmacokinetics of acetylsalicylic acid (ASA) and its metabolites salicylic acid (SA) and salicyluric acid (SUA) were studied in 12 healthy young volunteers after oral administration of low (30 and 100 mg) and moderate (400 mg) doses. Plasma and urine were assayed for the above drugs by high-performance liquid chromatographic method. Individual pharmacokinetic parameters were estimated by compartmental modeling (ASA and SA) and by model-independent methods (SUA). ASA parameter values estimated in this study were in agreement with those reported by other authors after administration of higher doses, which confirms the linearity of ASA pharmacokinetics in a broad dose range. On the contrary, both metabolic and renal elimination routes for SA were found to be saturable. The relative changes in SA renal clearance with the dose were more pronounced than those in metabolic clearance. Particularly, there was no statistically significant difference in SA metabolic clearance between 30 and 100 mg doses, indicating the linear kinetics in this dose range. Further increase in the dose resulted in significant decrease in SA metabolic clearance. At the same time, both SA excretion rate constant and fraction excreted significantly diminished across the entire dose range studied. The dependence of SUA renal clearance upon the dose was shown to be complex, reflecting possible saturability of its excretion.

In 8 subjects with chronic renal failure treated conservatively the renal hippurate clearance (CHip), polyfructosan (CPF) and creatinine (Ccr) clearance was assessed. The mean values of the investigated variables were as follows: CHip = 1.06 +/- 0.79 ml/s, CPF = 0.15 +/- 0.08 ml/s, Ccr = 0.27 +/- 0.17 ml/s. The ratios between the investigated clearance values were: CHip/CPF = 6.9 +/- 4.2 and CHip/Ccr = 4.2 +/- 1.4. The ratio of CHip/CPF was in some instances as high as 10 or more. Between values of CHip/CPF and the plasma hippurate concentration (PHip) a significant negative (non-linear correlation was found (p < 0.01). When the PHip values were very high, the value of the CHip/CPF ratio was close to 1. The recorded findings confirm that in subjects with chronic renal failure in the residual nephrons hippurate is synthetized and its urinary excretion in relation to glomerular filtration is increased. The findings suggest also that in these patients the tubular transport for hippurate is saturated. It will be necessary to take into account the possible saturation of tubular transport for hippurate when interpreting changes of drug excretion (i.e. those excreted by the same mechanism as hippurate) in subjects with chronic renal failure.

Organic anions retained in patients with renal failure participate in a substantial way in the metabolic acidosis and thus also the catabolism in renal failure. The basic disorder is the reduced capacity of the transport system of organic ions. Therefore drugs are sought to enhance its activity. Acute investigation of healthy volunteers revealed that 150 mg dipyridamol increased the urinary hippurate excretion, while the serum hippurate concentration declined. In patients with renal failure there is the same tendency, though less marked, and due to the variability of functions of individual patients this trend is insignificant. It is assumed that this hitherto not described action of dipyridamol may be of therapeutic importance when administered to patients with impaired renal function.

54 healthy volunteers or patients with normal kidney and liver function, 17 patients with decreased kidney function and 12 dialysed patients were evaluated for their serum hippurate accumulation and kidney excretion. It was found that there was an inverse relationship between serum hippurate and the clearance of endogenous creatinine (CCr) and a free relationship between fractional excretion of hippurate and CCr. The excretory capacity in residual nephrons was increased. This was caused by the greater glomerular filtration load which increased up to 25 times and tubular secretion which increased 7 times in dialysed patients. The relative contribution of glomerular filtration to hippurate excretion rose from about 20% in controls to almost 50% in dialysed patients. True kidney adaptation was localized in the organic anion transport system of proximal tubules.

Hippurate action on glucose utilization was evaluated in rat kidney cortex slices. Studies have shown the following. (1) Hippurate inhibits markedly basal as well as insulin-stimulated glucose utilization and basal gluconeogenesis. (2) Ca deficiency and specific Ca channel blockers diltiazem and isradipine abolish the hippurate inhibition of glucose utilization. (3) K+ channel blockers, i.e. the increased K+ concentration in incubation medium, procaine and sulfonylurea drugs also abolish the hippurate inhibition of glucose utilization. It is concluded that hippurate and benzoate operate through the ATP-dependent K+ channel.

The authors studied 1-14C-palmitate metabolism in rat muscle, renal cortex and liver incubated with synthetic hippurate in vitro (1 mmol/l). a) Hippurate did not affect 1-14C-palmitate uptake and utilization in the muscle (hemidiaphragm). b) In the renal cortex it stimulated only the incorporation into total lipids and from the individual lipid fractions into mono- and diglycerides and free fatty acids (FFA). c) In the liver it stimulated the uptake, oxidation to 14CO2 and incorporation into total lipids and, out of the individual lipid fractions, into phospholipids, triacylglycerols and free fatty acids. d) Hippurate already had a significant effect in the concentration of 0.5 mmol/l, i.e. during the development of the disturbance and not just as a supplementary factor in advanced renal insufficiency. It is concluded that, by interfering with fatty acid metabolism, the hippurate present in the serum of patients with renal insufficiency plays an active role in the development of dyslipoproteinaemia in such patients.

In a group of 12 healthy volunteers and 10 patients with livers cirrhosis the concentration of glucose, insulin, and hippurate was studied during glucose tolerance test after peroral administration of 75 g glucose. Basal values of glycemia and insulin were increased. The course of the glycemic curve was pathological in all intervals studied (p less than 0.001). After a slight latency, the insulin level exhibited a sharp rise in the 30th min (p 0.001). The basal hippurate level was slightly diminished and its decrease in the course of the oral glucose tolerance test was similar to that seen in the volunteers. Reduced hippurate synthesis in the liver and its alternative synthesis in the kidneys is being documented.

Patients with renal insufficiency suffer from inhibited glucose utilization which can be tested in vitro by the incubation of tissues or isolated cells in diluted uraemic serum. 1. Systematic isolation from the serum of patients with renal insufficiency has revealed that hippurate participates in the inhibition of glucose utilization in the muscle and, to a lesser extent, in the kidney. It does not inhibit glucose utilization in the brain, red blood cells and platelets. 2. Maximal inhibition is 86.6% and AD50 is 0.432 mmol hippurate/l. These hippurate concentrations are achieved in the early stages of renal insufficiency. 3. While the inhibitory effect is evident at normal and elevated glucose concentrations, it does not manifest itself at hypoglycaemic concentrations. 4. A less pronounced inhibitory effect, compared with hippurate, is exerted also by O-OH hippurate, but it is absent altogether in P-amino-hippurate and O-OH hippuryl-beta-glucuronide. It is concluded that, through its inhibition of glucose utilization, hippurate may participate in the development of muscular weakness in patients with renal insufficiency.

Hippurate had antitumor effects both in vitro and in vivo by killing dose-dependently the cells derived from Skalsky lymphoma, Németh-Kellner lymphoma, L-asparaginase sensitive 6C3HED Gardner lymphoma, and LP-2 plasmacytoma. There was no significant difference between the effect of hippurate and its maternal compound benzoate. Hippurate administered intraperitoneally twice a day at a dose of 1 g/kg body weight significantly inhibited the growth of solid 6C3HED Gardner lymphoma even in the case that the treatment was started when the tumors reached a weight of over 400 mg. It was concluded that benzoates could be an important endogenous group of substances with antitumor effects.