1. A compound identified as orotidine has been found in the erythrocytes of all subjects on allopurinol.
2. The erythrocyte orotidine concentrations were much higher in patients with renal failure or with the Lesch-Nyhan syndrome.
3. In addition, increased amounts of oxypurinol-7-riboside were excreted in the urine by both of these groups compared with control subjects or with patients with normal renal function on allopurinol.
4. A good correlation was found between urinary oxypurinol-7-riboside excretion and erythrocyte orotidine concentrations.
5. Increased erythrocyte levels of the pyrimidine-sugar UDP-glucose were also found in patients with the highest orotidine levels.
6. The combined results suggest a derangement of pyrimidine nucleotide metabolism during allopurinol therapy. We propose that erythrocyte orotidine formation results primarily from inhibition of orotidine-5′-mono-phosphate decarboxylase by oxypurinol-7-ribotide.