Orotic Aciduria Research Articles

CYCLIC VOMITING SECONDARY TO OCT DEFICIENCY

When other signs or symptoms are absent, cyclic vomiting in young children frequently results in psychiatric referral that proves ineffectual. Clinical experience with two female grade school children with cyclic vomiting and hyperammonemia suggests an organic etiology. Protein loading studies in these children provoked hyperammonemia, orotic aciduria, and hyperaminoacidemia consistent with the diagnosis of OCT deficiency. Protein restriction resulted in clinical improvement and was well tolerated. We suggest that, when severe cyclic vomiting occurs in female children, the possibility of OCT deficiency should be considered before psychiatric referral.

Urea Cycle Function in the Dog with Emphasis on the Role of Arginine

English Pointer puppies were used in experiments designed to assess urea cycle function. A preliminary assay revealed a dietary arginine (Arg) requirement of 0.40% for maximal weight gain of puppies fed a chemically defined L-amino acid diet. Subsequent studies showed that consumption of an Arg-free diet resulted in growth failure, emetic episodes, mild hyperammonemia, decreased plasma Arg and urea, and orotic aciduria. Oral administration of ornithine (Orn), equimolar to 0.40% Arg, failed to correct these Arg deficiency symptoms. Consumption of an equimolar concentration of citrulline (Cit), on the other hand, resulted in near normal weight gains, but blood and urine metabolite levels still did not parallel those of dogs fed Arg. Plasma glucose concentration was unaffected by dietary treatment. Older dogs (20 weeks of age) performed no better than younger dogs (7 weeks of age) when fed an Arg-free diet. In a serial bleeding study, plasma ammonia increased after ingestion of an Arg-free diet, reached a peak during emesis and declined immediately thereafter. In contrast, plasma ammonia of dogs fed an Arg-adequate diet remained relatively constant. It was apparent that in terms of meaningful Arg replacement value, Orn had minimal activity. Cit could replace Arg for growth, but was not as efficient as Arg for urea cycle function.

Species Comparison of the Influence of Ammonia on Orotic Acid and Urea Biosynthesis in Liver

Although orotic aciduria occurs in both male and female rats fed an arginine-deficient diet, only hepatocytes from male rats exhibited an enhanced rate of orotate biosynthesis to increasing ammonium compared to controls. Inhibition of incorporation [14C]NaHCO3 into urea by norvaline in the rat and mouse was accompanied by a 20 and 25% increase in orotate biosynthesis, respectively. Injection of ammonium chloride (2 mmol/kg) or consumption of a diet devoid of arginine resulted in an increased urinary orotate excretion in the rat. Injection of a similar quantity of ammonium chloride in mice did not result in an orotic aciduria. The influence of ammonia and arginine on the biosynthesis of orotic acid and urea in isolated liver slices from various mammals was also examined. In rat, mouse and pig liver, increasing quantities of ammonia stimulated the incorporation of [14C]NaHCO3 into orotic acid. In porcine, bovine and ovine liver the incorporation of [14C]NaHCO3 into orotic acid was low and was minimally effected by supplemental ammonia. Addition of arginine to the incubation medium diminished the incorporation of [14C]NaHCO3 into orotic acid in the liver of all species examined except the cow and sheep. This decrease was accompanied by a stimulation of urea biosynthesis in the rat, but a depression in the pig. Addition of ammonium (5.0 mM) markedly increased the incorporation of [14C]NaHCO3 into urea in all mammalian liver slices examined. These studies show that the liver of various mammals can respond to increasing ammonia by increasing orotic acid biosynthesis. However, species differences in the response to ammonia are evident.

New defects of pyrimidine metabolism.

The number of inherited defects of the pyrimidine metabolism described so far is small, compared to that of the purine metabolism. Combined deficiency of orotate phosphoribosyltransferase (OPRT) (EC 2.4.2.10) and orotidine 5′-monophosphate decarboxylase (ODC) (EC 4.1.1.23), designated as type I hereditary orotic aciduria, presents with characteristic clinical features such as hypo-chromic anemia with a megaloblastic bone marrow and crystalluria. Only six patients have been described and, as far as we know, new cases have not been discovered recently. ODC deficiency with similar clinical phenomena and leading to increased urinary excretion of orotate and orotidine has been detected in only one patient (1). A third defect, a deficiency of pyrimidine 5′-nucleotidase (Py-5N). (EC 3.1.3.5.) in erythrocytes, is associated with chronic hemolytic anemia and prominent basophylic stippling of the erythrocytes due to accumulated pyrimidine nucleotides. An increasing number of patients have been reported, their detection being facilitated by the typical phenomena. We do not know whether the urinary pyrimidine profile in this condition is abnormal.

Hyperammonemia and Orotic Aciduria in Portacaval-Shunted Rats

The effect of a portacaval shunt-induced alteration in liver function on nitrogen metabolism was studied in rats. Within a few days after surgery, portacaval-shunted rats grew with an average daily gain in body weight equal to shamoperated control rats. Within 1 week after surgery, portacaval-shunted rats excreted 20% more orotic acid in their urine compared to control rats. The difference increased to 37% after 3 weeks. Plasma ammonia levels were elevated by 78% in portacaval-shunted rats compared to control rats after 2 weeks. Portacaval-shunted rats injected with a challenging load of ammonium chloride (5 mmol/kg) excreted half as much orotic acid in their urine over a 24-hour period as similarly injected controls. The simultaneous injection of 1.5 mmol/kg of arginine prevented the ammonia-induced increase in orotic acid excretion in both shunted and control rats. However, feeding rats diets supplemented with 1% arginine did not prevent the chronic hyperammonemia and orotic aciduria produced by the construction of portacaval shunts. Similar experiments with diets supplemented with 1% sodium benzoate to induce alternative pathways for nitrogen excretion were also without effect. These results are in contrast to recent clinical studies reporting the effectiveness of sodium benzoate in treating hyperammonemia in patients with urea cycle enzyme defects.

Influence of purines on pyrimidine metabolism in rat hepatocytes.

Nutrition experiments showed that dietary nucleic acids or purines diminish allopurinol induced orotic aciduria (Zöllner and Gröbner 1971, Zöllner and Gröbner 1977). Using lymphoblasts cultures we showed that this might be due to an inhibition of pyrimidine synthesis by purines (Banholzer et al. 1980, Banholzer et al. 1981). In this study the influence of purine bases and nucleosides as well as of oxipurinol on pyrimidine biosynthesis was investigated using rat hepatocytes in suspension. By measuring bicarbonate-14C incorporation into orotic acid as well as the incorporation of orotic acid-14-C or uridine-14C into nucleic acids it was possible to study the first and second part of pyrimidine biosynthesis and the salvage pathway separately.

Arginine requirement of the young pig.

Four experiments were conducted with an arginine (Arg)-deficient, semipurified diet to determine the Arg requirement of the weanling pig. The semipurified diet contained .18% bioavailable Arg as determined by a chick bioassay procedure and, when properly supplemented with Arg, allowed rat growth equal to that obtained with a 19.2% protein corn-soybean meal diet. A preliminary experiment with pigs indicated that the semi-purified diet was markedly deficient in Arg. The Arg requirement of the pig was estimated by adding L-Arg to the semipurified diet such that final dietary Arg levels were .18, .23, .28, .38 and .48% (Exp. 2 and 3) or .48, .58 and .68% (Exp. 4). Weight gain and efficiency of feed utilization were maximized at .48% dietary Arg. Plasma urea-N was increased and plasma NH3-N decreased by .48% Arg compared with lower levels. A slight orotic aciduria was observed in pigs fed Arg levels below .48%. From these results, it is concluded that the Arg requirement of the weanling pig is .48% of the diet, a level twice the current NRC estimate.

Structural gene coding for multifunctional protein carrying orotate phosphoribosyltransferase and OMP decarboxylase activity is located on long arm of human chromosome 3.

In humans, deficiency in the last two enzymes of UMP biosynthesis, orotate phosphoribosyltransferase (OPRT) and OMP decarboxylase results in the inborn error of metabolism hereditary orotic aciduria, type 1. In this manuscript, we present immunologic, molecular, biochemical, and genetic evidence that the gene coding for this set of enzymatic activities is located on the long arm of human chromosome 3. The evidence presented here is consistent with both these activities being carried on the same multifunctional protein in mammalian cells. These studies allow further genetic analysis of human chromosome 3, confirming that human markers ACY-1, previously assigned to 3p21, and beta-gal, previously assigned by others to the region 3(p21-q21), must be in the region 3 (cen-p21) and confirming the regional assignment of a human DNA segment, D3S1, to 3q12. The significance of these studies to genetic analysis of genes on human chromosome 3, some of which appear to play a role in some forms of malignancy, is discussed.

Ornithine Carbamoyltransferase Deficiency in an Adult Male Patient: Significance of Hepatic Ultrastructure in Clinical Diagnosis

Ornithine carbamoyltransferase (OCT) activity was deficient (8% of control) in the liver of a 21-year-old man who died after suddenly becoming comatose. Activities of other enzymes of the urea cycle in the liver were normal. There was no known prior illness or injury; the patient, however, had been taking liquid protein supplements to his diet. Hyperammonemia and orotic aciduria were present, and the concentration of lysine in the plasma was elevated. Survey of earlier reports indicates that neither the specific deficiency of hepatic OCT nor the urine and plasma findings provide a basis for definitive diagnosis of the patient's illness as primary OCT deficiency or as Reye's syndrome. Indeed, the age of the patient at onset of symptoms and the absence of any prodromal infection argue against the OCT deficiency being either primary or a sequel to Reye's syndrome. We suggest that it was secondary to mitochondrial injury caused by an unknown agent. Electron microscopic study of hepatocyte ultrastructure lends support to this view; abnormalities of the patient's mitochondria (bizarre, elongated shapes) do not resemble those seen in Reye's syndrome, nor have abnormalities been found in primary OCT deficiency.

Hereditary orotic aciduria, Lesch-Nyhan syndrome, and xeroderma pigmentosum probed by herpes simplex virus: 125I-iododeoxycytidine incorporation as an assay for viral growth.

Abstract125I‐Iododeoxycytidine (125IdC) incorporation into acid‐insoluble material was a sensitive, rapid, and quantitative assay for the growth of herpes simplex virus type 1 (HSV‐1) in human fibroblasts. Cellular utilization of the isotope was 10 to 25% of the incorporation by infected cells and could be 80% inhibited by tetrahydrouridine (THU). Viral utilization was inhibited by acycloguanosine, thioguanine (TG), and cytosine arabinoside. Isotope was incorporated equally well by growing or quiescent infected cells. HSV‐1 was used to probe the metabolic capabilities of three mutant human fibroblast strains. 125IdC incorporation quantitatively measured the ability of the virus to grow in these cells. Viral 125IdC incorporation was sensitive to TG in normal fibroblasts but showed a 8‐ to 10‐fold greater resistance to TG in fibroblasts derived from patients with Lesch‐Nyhan syndrome (LN). Similarly, the growth of ultraviolet irradiated HSV‐1 in normal fibroblasts, as judged by 125IdC incorporation, was 5‐fold greater than in fibroblasts derived from patients with xeroderma pigmentosum. In fibroblasts derived from patients with hereditary orotic aciduria, viral 125IdC incorporation was sensitive to adenosine (AD) at concentrations which were slightly stimulatory in normal fibroblasts. This was a 2‐fold difference in AD sensitivity, which the radioassay reliably and quantitatively documented. Previous attempts to quantify and characterize this difference by immunofluorescence were unsuccessful. HSV‐1 infected cells could be individually identified by their incorporated 125IdC; such cells had blackened nuclei in autoradiograms prepared 12 hr after infection. Normal cells infected in the presence of TG had many fewer labeled nuclei than LN cells similarly infected in the presence of the drug. Thus, this viral assay can be a rapid and accurate probe of cellular function and has potential for the identification of mutant cells in amniocentesis samples or tumor speciments. Towards this end, we determined that the AD and TC dose responses of infected amniotic fluid cells closely paralleled that of the normal human fibroblasts.

Arginine Utilization of Young Rats Fed Diets with Simple versus Complex Carbohydrates

We proposed that increasing dietary complex carbohydrates would, by increasing fecal nitrogen loss and thus decreasing urinary nitrogen and the need for urea synthesis, ameliorate the effects of arginine deficiency. Two experiments using male, weanling. Sprague-Dawley rats with ad libitum access to water and isocaloric, isonitrogenous, 19% L-amino acid diets were carried out to test this hypothesis. The first was performed in two identical blocks of a 4 × 4 factorial experiment each with six rats per group. Four levels of arginine were fed: 0.0, 0.4, 0.8 and 1.2% of diet with four variations in dietary carbohydrates: sucrose alone, replacement of ⅔ of sucrose with dextrin, and 5 g of wheat bran or guar gum added to 100 g of the sucrose-based diets. Varying dietary carbohydrate quality did not influence growth of rats fed the 1.2% arginine diets, however, when added to the diet with 0.0 arginine, guar gum but not dextrin or wheat bran increased growth. This guar gum-induced weight increase was associated with partial reversal of arginine-deficiency-induced liver lipidosis and orotic aciduria and with higher fecal nitrogen and lower urinary nitrogen. The specificity of this guar gum effect was tested in a second experiment in which guar gum was added to diets containing 33% of the control level of arginine, histidine, leucine, methionine or phenylalanine. Only rats fed the diet deficient in arginine showed significant improvement in growth with added guar gum. We conclude that dietary guar gum but not dextrin or wheat bran, by increasing fecal nitrogen loss and decreasing the need for urea synthesis, partially ameliorates the effects of a low arginine intake. The practical application of feeding guar gum to alleviate certain ammonia-toxicity-related medical conditions is discussed.

Neonatal diagnosis of orotic aciduria: An experience with one family

2. Ulick S: Diagnosis and nomenclature of disorders of the terminal portion of the aldosterone biosynthetic pathway. J Clin Endocrinol Metab 43:92, 1976. 3. Royer P, Lestradet H, deMenibus CH, Vermeil G: Hypoaldosteronisme familial chronique ~. debut neo-natal. Ann Pediatr (Paris) 8:133, 1961. 4. Visser HKA, Cost WS: A new hereditary defect in the biosynthesis of aldosterone: Urinary C2~-corticosteroid pattern in three related patients with a salt-losing syndrome, suggesting an 18-oxidation defect. Acta Endocrinol (Copenh) 47:589, 1964. 5. David R, Golan S, Drucker W: Familial aldosterone deficiency: Enzyme defect, diagnosis, and clinical course. Pediatrics 41:403, 1968. 6. R6sler A, Rabinowitz D, Theodor R, Ramirez LD, Ulick S: The nature of the defect in a salt-wasting disorder of Jews of Iran: J Clin Endocrinol Metab 44:279, 1977. 7. Veldhuis JD, Kulin HE, Santen R J, Wilson TE, Melby JC: Inborn error in the terminal step of aldosterone biosynthesis: Corticosterone methyloxidase type II deficiency in a North American pedigree. N Engl J Med 303:117, 1980. 8. Kowarski A, Katz H, Migeon C J: Plasma aldosterone concentration in normal subjects from infancy to adulthood. J Clin Endocrinol Metab 38:489, 1974. 9. Siegler RL, Crouch RH, Hackett TN Jr, Walker M, Jubiz W: Potassium-renin-aldosterone relationships during the first year of life. J PEDIATR 91:52, 1977. 10. Pratt JH, Holbrook MM, Dale SL, Melby JC: The measurement of urinary tetrahydroaldosterone by radioimmunoassay. J Steroid Biochem 8:677, 1977. 11. Mattox VR, Nelson AN: Determination of urinary tetrahydroaldosterone glucosiduronic acid by radioimmunoassay. J Steroid Biochem 14:243, 1980. 12. Zurbriigg RP: Congenital adrenal hyperplasia. In Gardner LI, editor: Endocrine and genetic diseases of childhood and adolescence, ed 2. Philadelphia, 1975, WB Saunders, p 476.

Consequences of UMP synthase deficiency in cattle.

Several dairy cows have been identified as partially deficient in UMP synthase. Although erythrocytes of normal cows contained 2.54 units of enzyme per ml, four cows were discovered with only 1.08 units per ml. Cows deficient in UMP synthase secreted milk with abnormally high levels of orotate, 300-1,000 micrograms of orotate per ml compared to 80 micrograms/ml for normal cows. The deficiency also was accompanied by a lactation-induced orotic aciduria. Although bovine urinary orotate was generally less than 10 micrograms/ml, the urine of the deficient cows, when lactating, contained 20-200 micrograms/ml. Their plasma orotate also was elevated. Genetic transmission of the condition was suggested by a common bull in the pedigrees of all deficient animals. Indeed, these cows, with half the normal level of UMP synthase, are probably heterozygotes with a 50% chance of passing the deficient allele to their progeny. For these putative heterozygotes, the condition is apparently benign because longevity and production were unaffected. However, the existence of a gene for UMP synthase deficiency in the dairy cow population poses a hazard with respect to the conception of homozygotic, deficient animals. These, in analogy with a comparable human condition, would be expected to exhibit high perinatal morbidity and mortality.

The Influence of Excess Lysine on Urea Cycle Operation and Pyrimidine Biosynthesis

Rats fed a purified l-amino acid diet with 0.72, 1.50, 2.9 or 4.3% lysine excreted 117, 124, 237 and 628 µg/day orotic acid, respectively. Dietary arginine supplementation prevented the orotic aciduria induced by excess dietary lysine. Increased orotic acid excretion was accompanied by a significant depression in urinary urea in rats fed a diet containing 4.3% lysine compared to those fed a diet containing 0.72% lysine. As measured by incorporation of [14C]HCO3, lysine addition to liver slices or isolated hepatocytes resulted in a progressive increase in the rate of orotic acid biosynthesis. The minimum quantity of lysine tested that significantly increased the rate of orotic acid biosynthesis was 0.5 mm or 1 mm for studies with slices and hepatocytes, respectively. Ammonia at concentrations of 0, 0.75, or 5.0 mm NH4Cl linearly increased orotate and urea synthesis. Inhibition of urea biosynthesis resulting from lysine supplementation coincided with an increase in pyrimidine biosynthesis. Addition of 1 mm arginine to the liver incubation media prevented the increased rate of orotic acid biosynthesis caused by lysine. Arginine addition may overcome an approximate 90% competitive inhibition of arginase by excess lysine.lysine orotic aciduria urea arginine

Orotic Aciduria Caused by Feeding Excess Lysine to Growing Rats

Two 14-day factorial experiments were performed to assess changes in growth, feed intake and urinary orotic acid excretion of weanling male Sprague-Dawley rats. The dietary variables investigated included the percentage of dietary casein in the basal diet, its supplementation with 5% lysine-HCl or 1% arginine-HCl and sucrose or two parts dextrin and one part sucrose as the only form of carbohydrate. The response to 5% supplemental lysine was strikingly similar to that seen with arginine-free diets. With 5% lysine added to 15% casein diets, growth decreased 25% and feed efficiency 20% while orotic acid excretion increased significantly. When 5% lysine was added to diets with 7.5 or 30% casein, these responses were smaller and were prevented by the simultaneous feeding of 1% arginine. Growth depression by lysine was partially reversed by replacing two-thirds of the dietary sucrose with dextrin. When unsupplemented, the casein diets containing only sucrose as carbohydrate supported less growth than the same diets containing the dextrin sucrose mixture. This difference was abolished by supplementation with 1% arginine, suggesting that sucrose increases arginine requirements for optimal growth. The data are consistent with the conclusion that orotic acid excretion is a useful index for determining when lysine excess is producing a functional deficiency of arginine.orotic acid lysine arginine

Alterations in liver nucleic acids and nucleotides in arginine deficient rats

Dietary arginine deficiency is associated with retarded growth and depressed feed intake. Nucleic acid biosynthesis as indicated by the incorporation of [6- 14C]-orotic acid and [ 32p]-orthophosphate was significantly depressed in rats fed an arginine deficient diet. The activities of the pyrimidine enzymes, aspartate transcarbamylase (ATC) and dihydroorotate dehydrogenase (DHODH) were significantly increased in rats fed an arginine deficient diet. ATC and DHODH activities in rats fed the arginine deficient diet returned to control activities after 3 wk of feeding. Orotidine 5′ phosphate decarboxylase and orotate phosphoribosyl transferase activities were not affected by dietary arginine availability. In the rat fed an arginine deficient diet there was an increase in total liver pyrimidine nucleotides and a decrease in the total purine nucleotides. Significant alterations in the individual liver nucleotides were also observed. Incubation of various tissues obtained from rats fed an arginine deficient diet or a complete diet with glutamine (5mM) revealed that the liver is the major site of orotic acid synthesis. Orotic acid production in liver slices using glutamine as the nitrogen source was significantly greater in rats fed an arginine deficient diet compared to controls. The orotic aciduria occurring in rats fed an arginine deficient diet is associated with increased synthesis and decreased utilization of pyrimidines.

Fatty liver of growing rats fed excess lysine and its prevention by adenine or allopurinol

Weanling male Sprague-Dawley rats were fed ad libitum 15% casein diets with and without 5.0% lysine-HCI, 0.25% adenine sulfate or 0.1% allopurinol for 2 weeks. Addition of lysine alone depressed 2-week growth from 94 to 65 g increased average daily urinary orotic acid excretion from 0.39 to 1.77 mg and increased the percentage of total liver lipids from 3.6 to 11.2. Adenine or allopurinol did not change growth but markedly enhanced lysine-induced orotic aciduria and completely prevented lysine-induced fatty livers. Reports by other show that adenine and allopurinol also prevent fatty livers or rats fed arginine-free diets or excess orotic acid. The authors conclude that lysine-induced orotic aciduria results from arginine deficiency caused by antagonism of arginine function by lysine, and that lysine-induced fatty liver probably results from a lesion identical to that produced by feeding excess orotic acid.

1182 EXPERIMENTAL MODELS OF MICROVESICULAR FATTY CHANGE IN RAT LIVER

An association of microvesicular hepatic fatty change with hyperammonemia and orotic aciduria (OAuria) has been noted in several pediatric conditions, including Reye's syndrome. Experimental OA supplementation produces fatty changes in rat liver. In the present study severe hyperammonemia (2-3 mg%), without OAuria was produced by repeated administration of urease (25 U/kg) for periods up to 24 hrs. The hyperammonemic rats were found to have hyperglycemia (374±21 mg%: controls: 172±6 mg%), elevated plasma FFA (815±62 μEq/1: controls 491±25 μEq/1) and hepatic triglycerides (15.6±1.4 mg/gm: controls: 2.7±0.2 mg/gm) (SEM). Liver sections stained with oil red O showed microvesicular fatty changes. To evaluate the role of OA in the development of fatty changes, conversion of OA to uridine monophosphate (UMP) was blocked with azauridine. Under these conditions fatty changes resulting from urease-induced hyperammonemia were markedly less than those observed without azauridine. It is concluded that increased availability of carbamyl phosphate in hyperammonemia leads to overproduction of OA and that the conversion of increased amounts of OA to UMP rather than OA elevation was an essential prerequisite to the induction of hepatic microvesicular fatty changes.

Orotic aciduria in lysinuric protein intolerance: dependence on the urea cycle intermediates.

Urinary excretion of orotic acid was measured in controls and in subjects homozygous and heterozygous for lysinuric protein intolerance under various conditions of amino nitrogen intake. In all situations, the controls excreted less than 28 microgram/kg/hr. Only one of the two heterozygotes studied differed from the controls, The orotic aciduria of homozygotes was normal in fasting but increased on a self-chosen low-protein diet (log mean, 80; range, 8 to 588 microgram/kg/hr in 24-hr urine), after cow's milk protein 0.5 g/kg (769; 251 to 1747 microgram/kg/hr in 4 to 6-hr urine), oral ammonium lactate, 2.5 mmoles/kg (95; 15 to 1127 microgram/kg/hr in 1.5-hr urine), and IV alanine, 6.6 mmoles/kg (519; 47 to 1831 microgram/kg/hr in 6-hr urine). Giving ornithine or citrulline IV with the infusion of alanine prevented the increase in orotic acid excretion. Given orally, citrulline was more efficient than ornithine or arginine. To prevent the hyperammonemic and orotic aciduric responses with ornithine its plasma concentrations needed to be higher than normal. Orotic aciduria is a reliable indicator of the function of the urea cycle in lysinuric protein intolerance and facilitates monitoring of the treatment of this disease.

Adenine-induced orotic aciduria in rats fed diets with and without orotic acid

Diets with 0, 0.05, 0.25, and 1% adenine sulfate, each with and without 1% orotic acid, were fed for 10 days to female weanling rats, 5 animals per treatment. In the absence of dietary orotic acid, urinary excretion of orotic acid was increased 8-fold (0.09 to 0.73 mg/day) by 0.25% adenine sulfate. In the presence of dietary orotic acid, excretion was 50 times greater and was enhanced 3-fold (4.4 to 12.7 mg/day) by 0.25% adenine sulfate. Adenine sulfate at 0.05% was ineffective at stimulating orotic acid excretion; adenine sulfate at 1.0% was as effective as 0.25% but led to symptoms of adenine toxicity. The two high levels of adenine also prevented development of fatty livers induced by orotic acid. Thus, the prevention of such fatty livers is associated with increased excretion of orotic acid. The orotic aciduria could be due to competition for renal reabsorption by adenine metabolites but more likely results from competition between pyrimidine and purine metabolism in the liver.