Interrelationships between gluconeogenesis and uricogenesis in chicken liver

1. Experiments performed on isolated hepatocytes and perfused liver of starved chickens showed that gluconeogenesis from lactate, glycerol and fructose was inhibited by 22-100% on addition of urate precursors. 2. The inhibition was associated with an increased rate of urate formation. 3. 2,4-Dinitrophenol (40 microM), 2-bromooctanoate (2 mM) and 3-mercaptopicolinate (3MPA) (0.5 mM) were inhibitory with respect to gluconeogenesis but did not significantly affect the rate of urate formation. 4. The possible interrelationships between gluconeogenesis and uricogenesis are considered in terms of a competition for ATP and for other metabolites between the two pathways. 5. An interplay of both pathways at the level of anion transfer across the inner mitochondrial membrane is also discussed.

Adaptation of urate synthesis in chicken liver

1. Urate synthesis was measured in hepatocytes from chickens after starvation or high-protein feeding. Adaptation occurred only on the high-protein diet. 2. The theoretical balances of reactions from alanine (5 alanine + 3 O2 = urate + 1.5 glucose + glycine) and asparagine (3 asparagine + 2 O2 = urate + ammonia + 0.5 glucose + glycine) agree reasonably well with the experimental results. 3. Enzymes directly involved in urate synthesis from these amino acids increase up to 12-fold on the high-protein diet; only amidophosphoribosyltransferase activity appears to be rate-limiting for urate synthesis. 4. The processes of nitrogen disposal in chicken and rat are compared and discussed.

[Different effect of taurolithocholate and chenodeoxycholate on structure and function of isolated hepatocytes (author's transl)]

Alterations of cellular membranes under the influence of bile acids seem to be of pathophysiological importance in cholestasis. The effect of taurolithocholic acid (TLCA) and chenodeoxycholic acid (CDCA) on membrane structure and release of cellular enzymes was studied on isolated rat hepatocytes. The response of urea synthesis to glucagon was used as a parameter of membrane function. The threshold dose of TLCA, marked by rapidly increasing enzyme release, was about 100 micrometers, whereas that of CDCA was between 500 and 1,000 micrometers. Addition of albumin (1 g-%) increased the threshold dose of CDCA; this occurred for TLCA only 8 g-%. Electron-microscopical alterations of the endoplasmic reticulum and submembranous areas were found with concentrations below these threshold doses even in the presence of albumin. These alterations are interpreted as disturbance of cellular transport and energy metabolism. TLCA inhibited glucagon response of cells in concentrations below 100 micrometers. These results demonstrate an influence of the bile acids studied on structure and function of liver cell membranes, which may be of importance in the pathogenesis of cholestasis. The rough endoplasmic reticulum could be another cellular structure which is affected by these bile acids.

Catecholamine-induced hyperuricemia in eviscerated rats with functional hepatectomy

The presence of extra-hepatic systems for uric acid production and their role in catecholamine-induced hyperuricemia were studied in eviscerated rats with functional hepatectomy. In these animals, the plasma uric acid level progressively increased with a decrease of allantoin, and isoproterenol subcutaneously administered in low doses produced an evident hyperuricemia. The effect of isoproterenol was seen even in nephrectomized animals, but all effects were abolished by pretreatment with allopurinol. Epinephrine and norepinephrine also produced hyperuricemia, though to a lesser extent than isoproterenol. Propranolol inhibited the hyperuricemic effects of isoproterenol and epinephrine, while phentolamine potentiated the effects of epinephrine. Electrical stimulation of the splanchnic nerve also produced hyperuricemia in eviscerated rats with functional hepatectomy, and this state was abolished by pretreatment with propranolol and by adrenalectomy but was not affected by pretreatment with phentolamine. Thus, beta adrenoceptor agonists stimulate production of uric acid in tissues other than the liver and the viscera, and hyperuricemia results. An effect similar to that on the uric acid level was also observed for the levels of plasma lactate, creatinine and glutamic oxalacetic transaminase in rats given isoproterenol.

Purine metabolism studies in the high and low uric acid containing lines of chickens: de novo uric acid synthesis and xanthine dehydrogenase activities

De novo purine synthesis has been examined in two genetic lines of chickens selected for their plasma uric acid levels. Synthesis rates were determined using aqueous extracts of tissue acetone powders. The high uric acid line (HUA) had significantly greater kidney synthesis rates compared to the low uric acid (LUA) birds (P less than .05). The mean value of kidney de novo uric acid synthesis rates was 5.65 +/- .65 micrograms and 3.6 +/- .24 micrograms uric acid synthesized/mg acetone powder/hour, respectively, for the HUA and LUA birds. Kidney uric acid synthesis rates were significantly correlated to plasma uric acid levels (P = .07). No correlation was seen between liver de novo uric acid synthesis rates and plasma uric acid levels. Plasma uric acid levels were monitored in both lines from hatching through 42 weeks of age. In the HUA line peaks of uric acid level were noted during the 2nd, 4th, and 8th week followed by a fairly abrupt rise in levels after the 20th week, whereas in the LUA males a broad peak in the early weeks was followed by a gradual rise in levels after the 20th week. The LUA females had slightly elevated levels during the first weeks; however, no significant changes occurred after the 20th week. Xanthine dehydrogenase (XDH) activities in kidney acetone powders from the HUA line were significantly higher compared to those from the LUA line (P less than .05). The mean value of kidney XDH activity was 85.41 +/- 13.81 and 46.35 +/- 6.71 mmoles NAD reduced per milligram acetone powder per hour, respectively, for the HUA and LUA lines. There was no significant correlation between liver XDH activities and plasma uric acid levels; however, the activity was significantly higher in the HUA than in the LUA line.

Formation of hydrogen peroxide in normal and copper-treated rat endometrium

An intrauterine copper device stimulated endogenous hydrogen peroxide formation in whole homogenates and in the mitochondrial and microsomal (but not the nuclear) fractions of rat endometrial tissues. Uric acid also accumulated in the endometrium of copper-treated rats, but not in those fitted with a nylon device or sham operated. It is suggested that the contraceptive effect of copper may be related to these events.

Primary overproduction of urate caused by a partial deficiency of hypoxanthine-guanine phosphoribosyl transferase

Inherited enzyme deficiencies are found in a small proportion of patients with gout who produce an excess of uric acid. The clinical, biochemical and therapeutic aspects of a case of hyperuricaemia caused by an atypical mutant hypoxanthine-guanine phosphoribosyl transferase are presented. Urate overproduction was moderate and controlled by allopurinol therapy.

Quantification of renal uric acid synthesis in the rat

The excretion of nephrogenic uric acid in the urine of Sprague-Dawley rats was estimated by use of the isotope-dilution technique. In nonfasted rats the urine-to-plasma specific activity ratio (SAR) of [14C]uric acid was 0.93, suggesting that a minimum of 7% of the uric acid excreted in the urine is synthesized in the kidney. During hypoxanthine infusion the SAR decreased in a dose-related fashion, indicating that the rat kidney is capable of synthesizing relatively large amounts of uric acid and that circulating precursor levels may in part regulate the renal synthesis of uric acid. During allopurinol infusion the SAR increased to 1.0, demonstrating that the SAR is a valid indicator of the contribution of nephrogenic uric acid excreted into the urine. Results of perfusion studies in isolated rat kidneys suggest that uric acid is the major end product of purine catabolism in the rat kidney and that some uric acid formed in the kidney may be absorbed directly into the circulation.

Variant human phosphoribosylpyrophosphate synthetase altered in regulatory and catalytic functions

An inherited, structurally abnormal and superactive form of the enzyme 5-phosphoribosyl 1-pyrophosphate (PP-ribose-P) synthetase (EC 2.7.6.1) has been characterized in fibroblasts cultured from a 14-yr-old male (S.M.) with clinical manifestations of uric acid overproduction present since infancy. PP-ribose-P synthetase from the cells of this child showed four- to fivefold greater than normal resistance to purine nucleotide (ADP and GDP) feedback inhibition of enzyme activity and hyperbolic rather than sigmoidal inorganic phosphate (Pi) activation in incompletely dialyzed extracts. Excessive maximal velocity of the enzyme reaction catalyzed by the mutant enzyme was indicated by: enzyme activities twice those of normal at all concentrations of Pi in chromatographed fibroblast extracts; normal affinity constants for substrates and for the activator, Mg2+; and twofold greater than normal activity per immunoreactive enzyme molecule. The mutant enzyme thus possessed deficient regulatory and superactive catalytic properties, two mechanisms previously demonstrated individually to underlie the excessive PPRribose-P and uric acid synthesis of affected members of families with superactive PP-ribose-P synthetases. Increased PP-ribose-P concentration (4-fold) and generation (2.7-fold) and enhanced rates of PP-ribose-P dependent purine synthetic reactions, including purine synthesis de novo, in S.M. fibroblasts confirmed the functional significance of this patient's mutant enzyme. Diminished stability of the variant PP-ribose-P synthetase was manifested in vitro by increased thermal lability and in vivo by deficiency of enzyme activity at Pi concentrations greater than 0.3 mM in hemolysates and by an accelerated, age-related decrement in enzyme activity in lysates of erythrocytes separated by specific density. Despite the diminished amount of PP-ribose-P synthetase in the S.M. erythrocyte population, S.M. erythrocytes had increased PP-ribose-P concentration and increased rates of incorporation of [14C]adenine and hypoxanthine into acid-soluble nucleotides during incubation at 1 mM Pi. These findings provided further confirmation of the extent to which PP-ribose-P synthesis is modulated in the normal cell at physiological Pi concentration by purine nucleotide inhibition of PP-ribose-P synthetase. The activity and kinetic characteristics of PP-ribose-P synthetase from fibroblasts of the mother of patient S.M. indicated that this woman was a heterozygous carrier of the enzyme defect expressed in hemizygous manner by her son.

The role of biochemistry in drug research

The present and the future role of biochemistry in the search for a new therapeutic agent is reviewed. It is stated that the great importance of the various disciplines of biochemistry, including pathobiochemistry and pharmacological biochemistry, is presently recognized, and the involvement of biochemistry in drug research is increasing. Biochemistry at the present time and in the future will utilize the already known basic biological principles for the new development of new and more useful medicines. It is emphasized that the limiting factor in new drug discovery today, however, is the lack of new basic discoveries in biology.

On the pathogenesis of stone formation in stone-eliminating patients

The clinical peculiarities, and the etiological and pathogenetic factors of urolithiasis in 296 patients suffering from spontaneous stone elimination were studied. It was established that 209 patients eliminated stones consisting of uric acid, sodium salts and ammonium salts. Moderate hypocalcemia and hyperphosphatemia and also hyperuricemia and hyperuricuria were present. There were 39 'eliminators' of calcium stones. Their blood calcium content was higher, hypercalciuria, inorganic phosphorus and normal uric acid, were noted. Compound stones were present in 48 observations. When carrying out additional biochemical tests in 57 patients with calcium and compound stones, primary hyperparathyroidism was diagnosed in 34 observations; and parathyroidectomy was successfully performed.

Rate-limiting factors in urate synthesis and gluconeogenesis in avian liver

1. Urate synthesis and other metabolic characteristics of isolated chicken hepatocytes were studied. 2. The distinction is made between immediate precursors of the purine ring (glycine, glutamine, aspartate, formyltetrahydrofolate, bicarbonate) and ultimate precursors from which the immediate precursors are formed in the liver. 3. In hepatocytes from well-fed chickens the rate of urate synthesis was not greatly increased by the addition of amino acids or NH(4)Cl, but in hepatocytes from 72h-starved chickens the rate was much increased when alanine or asparagine was added as the only substrate. Other amino acids, when added alone, did not affect the rate. The exceptional effect of alanine and asparagine is due to the ready formation of the immediate precursors. 4. Conditions are described under which glutamine, serine, glycine plus formate, ribose and glucose increased the rate of urate synthesis. 5. At 1mm-NH(4)Cl (a concentration not much higher than that of blood plasma) the rate of urate synthesis in the presence of lactate was increased, but higher concentrations inhibited urate synthesis in the presence of lactate or alanine; with alanine even 1mm-NH(4)Cl was inhibitory. 6. Glucose synthesis from lactate, alanine or dihydroxyacetone was also inhibited by 1mm-NH(4)Cl. 7. NH(4)Cl inhibition of urate and glucose synthesis was paralleled by an increased rate of glutamine synthesis. Thus in the presence of NH(4)Cl the gluconeogenic precursors are diverted from the pathway of gluconeogenesis to that of glutamate and glutamine synthesis. This implies that the synthesis of these amino acids is the primary process in the detoxication of ammonia in the avian liver. 8. Urate synthesis, like urea synthesis, can be looked on as a cyclic process with either phosphoribosyl pyrophosphate or ribose acting as the carrier on which the purine ring is assembled. 9. The energy requirements of urate synthesis depend on whether phosphoribosyl pyrophosphate is regenerated from IMP by pyrophosphorylase or by phosphorylation and pyrophosphorylation of ribose. It is 6 or 9 pyrophosphate bonds of ATP respectively.

Protein intake, hepatic purine enzyme levels and uric acid production in growing chicks

Changes in hepatic purine enzyme activities of chicks fed diets containing 11%, 20%, 43% and 80% protein were correlated with protein intake and uric acid production in order to identify those enzymes with activities that parallel closely and may regulate uric acid production. Nucleoside phosphorylase, xanthine dehydrogenase, adenylosuccinate synthetase and adenosine kinase correlated positively with protein intake and uric acid production. Adenosine deaminase, 5'-nucleotidase (AMP), adenylate deaminase and adenine phosphoribosyltransferase correlated negatively with protein intake and uric acid production. Hypoxanthine phosphoribosyltransferase and 5'-nucleotidase (IMP) were unaffected by protein intake and did not correlate with uric acid production. The ratio of adenosine kinase to adenosine deaminase correlated positively with protein intake and uric acid production. The increased activities of adenylosuccinate synthetase and adenosine kinase, along with the reduced activities of 5'-nucleotidase and adenylate deaminase, in liver from chickens fed the 80% compared with the 11% protein diet demonstrate enhanced synthesis of adenine nucleotides. Since adenine nucleotides are essential cofactors for de novo purine synthesis, it is proposed that adenylosuccinate synthetase, adenosine kinase, 5'-nucleotidase and adenylate deaminase are key enzymes involved in the regulation of purine biosynthesis.