Abnormal orotic acid metabolism associated with acute hyperammonaemia in the rat

Diagnostic value of urinary orotic acid levels: applicable separation methods

Urinary orotic acid determination is a useful tool for screening hereditary orotic aciduria and for differentiating the hyperammonemia disorders which cannot be readily diagnosed by amino acid chromatography, thus reducing the need for enzyme determination in tissue biopsies. This review provides an overview of metabolic aberrations that may be related to increased orotic acid levels in urine, and summarises published methods for separation, identification and quantitative determination of orotic acid in urine samples. Applications of high-performance liquid chromatography, gas chromatography, and capillary electrophoresis to the analysis of urinary specimens are described. The advantages and limitations of these separation and identification methodologies as well as other less frequently employed techniques are assessed and discussed.

Hepatic carbamoyl phosphate metabolism. Role of cytosolic and mitochondrial carbamoyl phosphate in de novo pyrimidine synthesis

The interrelationship between the two carbamoyl phosphate pools in intact hepatocytes and intact liver was studied with respect to de novo pyrimidine synthesis by use of selective inhibitors of the mitochondrial and the cytosolic carbamoyl-phosphate synthetase. Inhibition of mitochondrial carbamoyl phosphate synthesis by 4-pentenoate was without effect on galactosamine-stimulated pyrimidine synthesis. Conditions favouring mitochondrial carbamoyl phosphate accumulation, like excess ammonium ions or L-norvaline, led to an increase in pyrimidine synthesis bypassing the feedback inhibition of cytosolic carbamoyl-phosphate synthetase by UTP. A stimulation of pyrimidine synthesis was not observed when the carbamoyl phosphate accumulation was due to aspartate deficiency in the presence of aminooxyacetate. The full response of pyrimidine synthesis to excess ammonium ions was restored, even in the presence of aminooxyacetate, when aspartate was substituted. This is explained by an inhibition of aspartate carbamoyltransferase flux [in view of the Km (aspartate = 0.7 mmol/l) of this enzyme] resulting from a 90% decrease in aspartate tissue levels. Acivicin, the inhibitor of cytosolic carbamoyl-phosphate synthetase, completely abolished the galactosamine-induced stimulation of pyrimidine synthesis, but was without effect on the stimulation of pyrimidine synthesis by ammonium ions and L-norvaline. It is concluded that experimental changes in mitochondrial carbamoyl phosphate content exert effects on de novo pyrimidine synthesis; however, it is considered unlikely that relevant amounts of mitochondrial carbamoyl phosphate are used for pyrimidine synthesis under physiological conditions. In addition the data point to a potential regulatory role of aspartate in hepatic pyrimidine synthesis.

Influence of ammonium ions on hepatic de novo pyrimidine biosynthesis

Carbamyl phosphate (CP) is synthesized in the liver by two separate enzymes, CPS I and CPS II. CPS I, an intramitochondrial enzyme involved in ureogenesis, has a relative activity of 500- to 1000-fold greater than CPS II, a cytoplasmic enzyme which initiates the sequence of reactions for pyrimidine biosynthesis. The contributions of NH4Cl (substrate for CPS I) ang glutamine (substrate for CPS II) as precursors for pyrimidine biosynthesis in isolated hepatocytes were compared by measuring their effect on uracil nucleotide pool size, the incorporation of NaH14CO3 into these pools, and the accumulation of orotic acid. Physiological concentrations of NH4Cl caused a marked stimulation of incorporation of radioactivity into uracil nucleotides (6-fold increase at 0.5 mM NH4Cl), and radioactive orotate appeared in both the cells and the medium. In contrast, glutamine (at concentrations up to 10 mM) had no effect on the incorporation of radioactivity into uracil nucleotides, and no orotic acid was detected. Uracil nucleotide pools were expanded up to 50% by low levels of NH4Cl, but there was no expansion of this pool in the presence of added glutamine. NH4Cl-driven pyrimidine de novo biosynthesis was insensitive to feedback inhibition by an expanded uracil nucleotide pool, to galactosamine treatment, and to acivicin treatment, indicating that NH+4 stimulated pyrimidine biosynthesis as a result of CP synthesis by mitochondrial CPS I. The consequence of intramitochondrially produced CP being available for pyrimidine biosynthesis is that the controlling step of this pathway (CPS II) is bypassed. The appearance of orotic acid following NH4Cl stimulation indicated that the rate-controlling step of hepatic de novo pyrimidine synthesis under these conditions was orotate phosphoribosyl transferase. These data indicate that, at physiological concentrations of NH+4, the majority of uracil nucleotides synthesized in isolated rat hepatocytes was derived from intramitochondrially generated CP. The effect of NH4Cl on the output of uridine by the isolated perfused rat liver was examined. In the presence of a single addition of 20 mM NH4Cl, the excretion of uridine was increased from 100-200 to 375 nmol h-1 g-1 liver and orotic acid was released into the circulating perfusate reaching a maximum of 2 microM (in 220 ml of perfusate) after 2 h. With 40 mM NH4Cl, uridine export was increased to 450 nmol h-1 g-1 and a maximum of 5 microM orotic acid was released into the perfusate after 2 h.(ABSTRACT TRUNCATED AT 400 WORDS)

Ammonia: its effects on biological systems, metabolic hormones, and reproduction

The physical, chemical properties of ammonia, its sources and detoxification, its effects in biological systems, its influence upon insulin action and glucose metabolism, and its possible effects on reproduction are discussed. Present chemical methods do not distinguish nonionic from ionic forms. At physiological pH, nonionic ammonia concentrations remain low but are primarily responsible for toxic effects. Thus, biologically significant changes of ammonia concentrations may not be revealed by determinations of ammonia in blood plasma. For these and other reasons the subacute toxicity of ammonia often is unrecognized, and its effects on intermediary metabolism and the hormonal milieu in normal and disease states remain poorly understood. Effects of ammonia may be stimulatory at low concentrations and inhibitory as concentrations rise or exposure is extended. Extensive experiments in eight ureotelic species, including man, show that urinary excretion of orotic acid becomes significantly elevated when the quantity of ammonia presented to the liver exceeds the capacity for normal detoxification. Present evidence with arginine and other intermediates of the urea cycle suggest that these substances influence glucose metabolism and insulin action. Recent studies of dairy cattle provide speculative evidence that high protein feeding or forms of protein that lead to elevated ammonia concentrations in tissue decrease conception rates and increase the calving to conception interval of dairy cows. Limited data concerning luteinizing hormone concentrations and steroid hormone metabolism are insufficient to establish whether differences in reproductive performance are due to changes of hormonal physiology, intrauterine environment, or metabolism.

Effect of ammonium ion on pyrimidine synthesis de novo in isolated rat hepatocytes

Addition of ammonium ions to isolated rat hepatocytes stimulated the rate of synthesis of pyrimidines. Isolation and quantification of pyrimidine nucleotides orotic acid and the acid-hydrolyzed product of carbamoyl-aspartic acid by ion-exchange chromatography and high-pressure liquid chromatography show a marked stimulation in the incorporation of [14C]bicarbonate in incubations with added ammonium ions. The incorporation into total uridine nucleotides (sigma UMP) was increased twofold in the presence of 5 mM ammonium ion, and approximately eightyfold into orotic acid. There was a parallel increase in labelling of carbamoylaspartic acid from undetectable to a level similar to that of orotic acid. The specific activity of urea formed during the incubations did not change during incubations or in the presence of ammonium ions confirming that the change in labelling of pyrimidine was not due to a change in the specific activity of precursor. Despite the stimulation in incorporation of label into pyrimidines there was no increase in the hepatocyte content of sigma UMP, which was 11.5 mumol/g dry weight, although the orotic acid content increased from 0.09 mumol/g dry weight in the absence of added ammonium ions (but in the presence of 2 mM L-glutamine) to 8.6 mumol/g dry weight with 5 mM ammonium ion. The stimulated incorporation of [14C]bicarbonate in the presence of 5 mM and 10 mM ammonium ion was shown to be due to a stimulated synthesis of carbamoyl phosphate, since greater than 80% of label in the uracil ring was present at position C-2. Incubation of hepatocytes in basal medium (Eagles) containing 2.5% foetal calf serum and 20 mM bicarbonate showed that there was a significant stimulation of pyrimidine synthesis with 1 mM ammonium ion. The stimulatory effect of ammonium ions on incorporation of bicarbonate into pyrimidines was almost completely reversed by 5 mM L-ornithine and was partially reversed by 1 mM L-ornithine. Evidence for a contribution of the urea cycle carbamoyl phosphate synthetase to pyrimidine synthesis is discussed.

Urinary pyrimidine excretion in arginase deficiency

A high-performance liquid-chromatographic method was used to separate and identify uracil, uridine, pseudouridine and orotic acid after preliminary extraction in two patients (McKusick 20780). Urinary uracil excretion was 10-35 times normal in both patients with arginase deficiency. Uridine and orotic acid, not normally detected, were excreted in large amounts and were directly influenced by protein intake. Their excretions were correlated with urinary arginine excretion. Urinary uracil levels remained consistently high and showed minimal variations with increased protein intake or urinary arginine levels. The measurement of urinary pyrimidines appears to be useful for the detection, differential diagnosis and dietary monitoring of patients with urea cycle disorders. The data presented extends this observation to include patients with arginase deficiency.

Measurement of urinary pyrimidine bases and nucleosides by high-performance liquid chromatography

A rapid procedure for the isolation, separation, identification and measurement of urinary pyrimidine bases and nucleosides by high-performance liquid chromatography (HPLC) is presented. The initial isolation of these compounds from urine was accomplished with small disposable ion-exchange columns. HPLC was performed on a silica gel column with a mobile phase composed of methylene chloride, methanol and 1 M aqueous ammonium formate buffer. Peaks were recorded at both 254 nm and 280 nm and the response ratio was used in combination with the elution volume for compound identification. The minimum detectable amount (signal-to-noise ratio = 2) ranged from 0.2 ng for uracil to 2.2 ng for cytidine. Linearity and recovery for thymine, uracil, uridine, pseudouridine, orotic acid and orotidine added to urine was demonstrated over almost a 10(3) concentration range. The potential application of this method for the study of inborn errors in the urea cycle is discussed.

The urinary excretion of orotic acid and orotidine, measured by an isotope dilution assay

Unknown concentrations of orotic acid can be measured by competition with a known amount of [carboxyl-14C]orotic acid for reaction with a limiting amount of phosphoribosylpyrophosphate in the presence of orotate phosphoribosyltransferase and orotidine monophosphate decarboxylase. The dilution of the specific radioactivity in the product 14CO2 is a sensitive and accurate measure of the amount of orotic acid present in the sample. Orotidine can also be determined after hydrolytic cleavage to orotic acid. The method was used to measure orotic acid and orotidine in urine samples from newborns, healthy controls and patients with gout or deficiency of hypoxanthine-guanine phosphoribosyltransferase receiving allopurinol. Urinary excretion of orotic acid and orotidine in newborns was similar whether the infants were breast-fed or received milk powder. The excretion of orotidine was increased in all patients receiving allopurinol. After allopurinol administration orotic acid excretion was increased in gouty patients but close to normal values in patients with deficiency of hypoxanthine-guanine phosphoribosyltransferase. The results are discussed in relation to the mechanism by which allopurinol inhibits pyrimidine metabolism.

The influence of ammonia on purine and pyrimidine nucleotide biosynthesis in rat liver and brain in vitro

1. The effect of ammonia on purine and pyrimidine nucleotide biosynthesis was studied in rat liver and brain in vitro. The incorporation of NaH(14)CO(3) into acid-soluble uridine nucleotide (UMP) in liver homogenates and minces was increased 2.5-4-fold on incubation with 10mm-NH(4)Cl plus N-acetyl-l-glutamate, but not with either compound alone. 2. The incorporation of NaH(14)CO(3) into orotic acid was increased 3-4-fold in liver homogenate with NH(4)Cl plus acetylglutamate. 3. The 5-phosphoribosyl 1-pyrophosphate content of liver homogenate was decreased by 50% after incubation for 10min with 10mm-NH(4)Cl plus acetylglutamate. 4. Concomitant with this decrease in free phosphoribosyl pyrophosphate was a 40-50% decrease in the rates of purine nucleotide synthesis, both de novo and from the preformed base. 5. Subcellular fractionation of liver indicated that the effects of NH(4)Cl plus acetylglutamate on pyrimidine and purine biosynthesis required a mitochondrial fraction. This effect of NH(4)Cl plus acetylglutamate could be duplicated in a mitochondria-free liver fraction with carbamoyl phosphate. 6. A similar series of experiments carried out with rat brain demonstrated a significant, though considerably smaller, effect on UMP synthesis de novo and purine base reutilization. 7. These data indicate that excessive amounts of ammonia may interfere with purine nucleotide biosynthesis by stimulating production of carbamoyl phosphate through the mitochondrial synthetase, with the excess carbamoyl phosphate in turn increasing pyrimidine nucleotide synthesis de novo and diminishing the phosphoribosyl pyrophosphate available for purine biosynthesis.

[Biochemical and pathophysiological aspects of hyperammonaemia (author's transl)]

1. Ammonia liberated continuously in large amounts in muscle, kidney and brain is used immediately for the synthesis of mainly glutamine because of the toxic effects of elevated ammonia concentrations. After glutamine hydrolysis in the liver ammonia serves as substrate for the urea biosynthesis. In ureotelic animals urea is the quantitatively most important product for the elimination of surplus nitrogen. 2. The rate of urea biosynthesis depends on the amount of surplus nitrogen and acts as regulatory factor for the nitrogen balance of the adult organism. 3. Urea cycle abnormalities in liver diseases or inborn enzymatic defects are important factors leading to hyperammonaemia in patients. 4. The hyperammonaemia induces an increase of the rate of hepatic pyrimidine nucleotide biosynthesis as a consequence of an ineffective feedback inhibition of the glutamine-dependent carbamoyl phosphate synthetase. 5. The distribution of ammonia between intra- and extracellular space and the amount of ammonium ions excreted in the urine depend on the pH value. An alkalosis induces an intracellular ammonia load and inhibits the urinary ammonium ion excretion, which is increased in acidosis as one mechanism of protein elimination. 6. The ammonia-induced inhibition of the citric acid cycle by an alpha-ketoglutarate deficiency is one important reason for the neurotoxicity of ammonia, which is the main point in the pathogenesis of hepatic coma.

Urea-cycle enzyme deficiencies and an increased nitrogen load producing hyperammonemia in Reye's syndrome

Assay of urea-cycle enzymes in liver tissue showed ornithine transcarbamylase activities of 18 to 72 per cent of the normal mean in eight patients with Reye's syndrome, below the range of normal in seven of eight, and, in six cases, as low as those in females with X-linked deficiency of this enzyme. Carbamyl phosphate synthetase activities were less than 32 per cent of controls in two patients. Argininosuccinate synthetase and lyase activities were normal in seven patients. Arginase was normal in two biopsy specimens, but below normal in four of five autopsy specimens. The Km's for ornithine and carbamyl phosphate, pH optimum, and heat lability of ornithine transcarbamylase were normal. Two patients excreted 0.64 and 0.58 g per kilogram per day of urinary nitrogen at the peak of hyperammonemia, in spite of peritoneal dialysis. The hyperammonemia of Reye's syndrome apparently results from excess waste nitrogen that overwhelms the ability of reduced ornithine transcarbamylase (and occasionally carbamyl phosphate synthetase) to detoxify the ammonia load.