Orotate urinary excretion

On a low protein diet, when the blood ammonia is considerably decreased, the urinary excretion of these compounds is also reduced. Even so some orotic acid and uracil is always found, although uridine may be completely absent from the urine (L6). 

There are several studies on the effect of allopurinol and its metabolic derivatives on orotate phosphoribosyltransferase and orotidylic acid decarboxylase [127-129]. The administration of allopurinol to rats results in an increased urinary excretion of orotic acid and orotidine [127,130,131], and in elevated activities of orotate phosphoribosyltransferase and orotidylic acid decarboxylase in erythrocytes [128,129]. Also, in man, the administration of allopurinol and oxipurinol leads to an increase in the specific activity of orotate phosphoribosyltransferase and orotidylic acid decarboxylase [129]. The enzymes were found to exist in a complex as three different molecular species with molecular weights of 55000, 80000 and 113 000 daltons. The larger forms of the complex were more stable than the smaller one. In the presence of allopurinol or oxipurinol ribonucleotides (but not the corresponding free bases) the largest, most stable species predominated [129]. 

Classical orotic aciduria is a rare autosomal recessive disorder which is characterized by retarded growth and excretion of large quantities of orotic acid in the urine [221,222]. The disease was described in 1959 as an inborn error of pyrimidine biosynthesis in patients with crystals of orotic acid in the urine [223]. The urinary excretion of orotic acid by these patients was 1.34 g per day in contrast to approximately 0.014 g per day excreted by normal individuals [222,224]. When the diet of patients was supplemented with uridine, clinical remission and a remarkable reduction in orotic acid excretion took place [221,225,226]. 

The reduction in urinary excretion of both compounds following uridine therapy reflects the utilization of uridine for the formation of UMP by the salvage pathway. A similar phenomenon was observed in hereditary orotic aciduria following uridine replacement therapy which bypasses the congenital enzyme defect (Chapter 5). The reversal of 6-azauridine-induced orotic aciduria by hydroxyurea, methotrexate and cyclophosphamide [251] (i.e. by the drugs affecting the synthesis of DNA without any effect on orotic acid synthesis) suggests that the control of pyrimidine synthesis de novo is linked to DNA synthesis. 

An inhibitory effect on orotidylic acid decarboxylase was also observed following 5-azacytidine [253,254], another highly active cytostatic agent [253-256]. The direct action of 5-azacytidine 5 -phosphate on enzyme activity in vitro has not yet been measured and the evidence for its interaction with the transformation of orotic acid came from the observation that 5-azacytidine increases its urinary excretion in mice [257,258]. The activity of orotidylic acid decarboxylase in liver extracts from 5-azacytidine-treated animals was also decreased in comparison to controls [258]. 

Weissmann et al. (4) have calculated that in man approximately 800 to 1000 mg of uracil are synthesized de novo per day. The normal urinary excretion of orotate is approximately 1.4 mg per day, and that of orotidine, 2.5 mg per day (6). In patients with orotic aciduria due to decreased orotidylate decarboxylase, the excretion of orotate may increase 20-fold (5), although the excretion of orotidine does not necessarily increase. 

The accumulation in blood and the excessive urinary excretion of orotic acid are postulated to result from the absence of either decarboxylase or the pyrophos-phorylase. These enzyme activities could not be directly assayed in the patient s tissues, but assays in the parents and two of the siblings indicated that the activities of both decarboxylase and phosphorylase were defective. In the patients described, the orotic aciduria and the megaloblastic anemia that it causes could be relieved by injecting nucleotides (cytidylic and uridylic acid). And in addition to its therapeutic significance, this observation also provides some invaluable information on the functioning of the pyrimidine metabolic pathway in vivo. 

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 hypochromic 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-5NX (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. 

J.W. Stoop and S.K. Wadman, Urinary excretion of orotic acid, orotidine and other pyrimidines in a patient with purine nucleoside phosphorylase deficiency, Clin. Chim. Acta, 93 ... 

Table 3. Reduction of urinary excretion of orotic acid and orotidine by dietary purines.

The resultant clinical state is characterized by failure of growth and development, megaloblastic anemia, and the increased urinary excretion of orotic acid (Types I and II) and orotidine (Type II). 

Fig. 2. The effect of allopurinol therapy, 800 mg per day for 8 days, on the metabolism of orotic acid-T- c in vivo A, urinary excretion of 14c-metabolites of orotic acid B, excretion of C02 in expired air. (From Beardmore and Kelley, 1971).

When guanine was given together with the allopurinol, the increased urinary excretion of orotic acid and orotidine remained unchanged at 50 mg/2 hours. This is in contrast to the findings in man, where exogenous RNA fed together with allopurinol eliminated the increase in excretion of orotic acid and orotidine (3) ... 

Elevated concentrations of orotic acid and uracil may be found in the urine of heterozygote carriers for urea cycle disorders, most commonly ornithine carbamoyltransferase (OCT) deficiency. This is due to the increased flux through the pyrimidine pathway which occurs, especially where the relevant enzymes are expressed only in liver tissue, as is the case when urea cycle enzymes are defective. A protein load was used previously to stress this route and the elevation in orotic acid excretion used as a diagnostic marker. However, the test frequently failed to detect known carriers. The al-lopurinol loading test (measurement of the increment in urinary orotic acid and orotidine in three separate 8 hour urine collections over the 24 hours following a 300 mg allopurinol tablet) is the most reliable test so far for carrier detection for such disorders [17]. 

The underlying biochemical defect is a failure of mitochondrial uptake of ornithine. This results in a failure of citrulline synthesis and a consequent hyperammonemia. Urinary orotic acid is high, presumably because of underutilization of carbamyl phosphate. In contrast, excretion of creatine is low, reflecting the inhibition of glycine trans-amidinase by excessive levels of ornithine. 

Since allopurinol blocks xanthine conversion to uric acid, urinary xanthine excretion is increased, creating a risk of xanthine crystal formation in the urinary system or even in muscles this can result in nephrolithiasis (12). It is still an open question whether a predisposition to renal disease or renal disease itself is required to precipitate these adverse effects. It is also not known whether increased excretion of orotic acid, due to an interaction of allopurinol with pyrimidine formation, has any consequences for these adverse effects or for its role in reducing glucose tolerance. 

Certain metabolites of the pyrimidine pathway are excreted in excess of the normal in inherited metabolic disorders of the urea cycle. They include orotic acid, uridine, and uracil. Of these substances, only uracil is a normal constituent of urine, the other two either being absent or present in very small amount. They are readily detected qualitatively as dark bands at the appropriate Rf values when a paper chromatogram of the urine is examined under ultraviolet light. They may be estimated by an ion exchange method similar to that for urinary pseudouridine (R13). 

Ornithine carbamoyltransferase (ornithine transcar-bamylase) (EC 2.1.3.3). Gross elevation of blood ammonia. Elevated glutamine in plasma and cerebrospinal fluid. Urea excretion low. Urinary orotic add increased. Uracil and uridine in urine. Gene for enzyme X-linked. Condition severe in boys (0-0.2% of normal enzyme activity in liver), who die in postnatal period (some cases of late onset have been reported). Girls have 5-10% normal liver enzyme activity. Some girls have died in late infancy or childhood, and others have survived with restricted protein intake. Abnormal EEC, mental retardation, brain atrophy and hepatomegaly. 

Pigs given allopurinol showed an increase in urinary orotic acid and orotidine excretion from a mean of 5 to a mean of 50 mg/2 hours. Although the dose of allopurinol was constant the levels of orotic acid and orotidine fell gradually from their initial peak. When the drug was stopped levels returned rapidly to normal. These results are similar to those obtained in rat and man (I), where this effect has been attributed to inhibition of the enzyme orotidylic decarboxylase (2). 

As renal function deteriorated there was a proportional increase in urinary levels of orotic acid and orotidine. This increased excretion would appear to be related to the increase in plasma oxipurinol levels which have been reported in renal failure (5) ... 

Oxonic acid is a uricase inhibitor (6) and has been given to rodents to produce an animal model for gout (7). When given to pigs urinary allantoin excretion decreased, as uric acid excretion increased. These findings indicate inhibition of the enzyme uricase. However, oxonic acid also produced a great increase in orotic acid and orotidine excretion. Urinary levels increased from 5 nig to 900 mg/2A hours, and this would imply a complete or... 

Urinary pyrimidines and uric acid. Administration of PF was followed by the excretion of a large amount of orotic acid and orotidine. Fig. 2 shows the total ion chromatogram of the urine of a patient 2 days after PF infusion started (250 mg/m ). The peaks corresponding to mass spectrayA 230 and 258 on the abscissa are hippuric and citric acid, respectively. In the control urine... 

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