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Orotic aciduria is a rare disorder which has thus far been described only in the propositus (1). This patient, a 9 month old child, exhibited retarded growth and development associated with megaloblastic anemia unresponsive to vitamin B12, folic acid, ascorbic acid and iron. There was heavy crystalluria, demonstrated on subsequent study to be due to the excretion of large amounts of orotic acid, a pyrimidine nucleotide precursor absent from normal urine. Improvement of the anemia without reversal of the megaloblastic marrow followed treatment with glucocorticoid hormones. The administration of a mixture of pyrimidine nucleotides led to a complete hematological remission, a remarkable improvement in general well-being, and a prompt reduction in orotic acid excretion. The patient died of an overwhelming varicella infection in 1958. His surviving parents and three siblings were normal hematologically and exhibited no orotic acid in the urine on chromatographic examination. These clinical studies indicated the presence of a metabolic block in the pathway of pyrimidine biosynthesis, with impaired conversion of orotic acid to uridine-5'-phosphate (Figure 1). This report describes the development of assays for orotidylic pyrophosphorylase and orotidylic decarboxylase, the sequential enzymes which catalyze this conversion. Reduced activities of both enzymes were found in circulating erythrocytes from the parents and two of the three siblings of the deceased propositus of orotic aciduria.

PYRIMIDINE METABOLISM IN MAN. IV. THE ENZYMATIC DEFECT OF OROTIC ACIDURIA*