Erythrocytes orotate metabolism

Orotic acid added to rat diet also provokes an. excessive biosynthesis of porphyrins in liver, erythrocytes and bone marrow. Administration of adenine monophosphate (AMP) counteracted this effect of orotic acid intoxication [165]. Haemorrhagic renal necrosis in rats, caused by choline deficiency, can be relieved by orotic acid [166], Simultaneous supplementation of the diet with adenine does not influence the protective effect of orotic acid. It has been suggested that orotic acid may lower the body requirement for choline through a metabolic interaction—orotic acid may stimulate the cytidine phosphate choline pathway of lecthin biosynthesis [166]. 

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]. 

Orotic Aciduria. Although orotic aciduria is a rare disease, the study of the few patients afflicted with it has provided a great deal of information on the role of pyrimidines in disease and the in vivo metabolism of pyrimidine nucleotides. The first patient was studied when he was 9 months old. At that time, he had two main groups of symptoms a rather unique hematological picture and massive excretion of orotic acid in the urine. The blood smear contained hypochromic erythrocytes and revealed anisocytosis and poikilocytosis. A leukocyte count demonstrated moderate leukopenia and monocytosis. Bone marrow examination established hyperplasia of the marrow with a marked increase in the number of megaloblastic cells. 

In summary, it is proposed that the most hopeful prospect for therapy of this condition would be to search for a specific inhibitor of orotate transport across cell membranes. This is likely to affect the metabolism of pyrimidines in only two tissues, liver and erythrocytes, and, unless it gives rise to toxic accumulations of orotate in the liver, could help to reduce pyrimidine nucleotide levels in pyrimidine 5 nucleotidase deficient erythrocytes. A trial of a specific inhibitor of uridine kinase might also be appropriate, and the final choice of a therapeutic approach will depend on the demonstration of which of these two alternative pathways amenable to therapeutic intervention contributes most to erythrocyte pyrimidine nucleotide accumulation. 

Berman, P., and Harley, E. H., 1982, Orotic acid uptake and metabolism by human erythrocytes, Adv. Exp. Med. Biol. this volume. 

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. 

The findings described above are sumarized in Figure 5. Erythrocytes are able to take up orotate from the medium, metabolize it to uridine and secrete this uridine back into the medium when it becomes available for nucleic acid synthesis in nucleated cells. 

A possible physiological role for this pathway would be the conver sion of orotate derived from the diet or endogenous sources to a form capable of utilisation by peripheral tissues. Experiments are under way to determine whether the transport system for orotate across liver cells function primarily to export orotate for metabolism by erythrocytes and distribution as uridine to peripheral tissues. 

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