Purine and pyrimidine metabolism in hereditary orotic aciduria: some unexpected effects of allopurinol.

Purine and pyrimidine metabolism have been investigated in the longest surviving case of hereditary orotic aciduria after 15 years of chronic uridine therapy. Several unusual features were recorded: 1. Although the uridine dosage (0.5 mmol/kg) was adequate to control an otherwise normal clinical status, orotic acid excretion was still excessive (in congruent 7 mmol/24 h). Urinary drug metabolites (uracil and uridine), however, accounted for less than 7% of the daily uridine dose, and no orotidine, or any abnormal pyrimidines or purines, were identified at any time. 2. Urinary uric acid excretion was high and plasma uric acid low, resulting in a clearance up to 4 times normal. This was attributed to the uricosuric effect of orotic acid. 3. In direct contrast to previous findings in gouty subjects and healthy male controls we noted: (i) no increase in plasma or urinary uric acid levels, or uric acid clearance, following the change from a low to a high nucleoprotein regime (normally up to two-fold); (ii) allopurinol reduced both urinary uric acid and total oxypurine levels by more than 50% on the low (normally unaffected) as well as the high (normally reduced 20-50%) nucleoprotein regime; (iii) a substantial (up to 70%) reduction in orotic acid excretion during allopurinol therapy (normally mild orotic aciduria), of similar magnitude and in parallel with the reduction in uric acid levels. Uric acid and orotic acid excretion were closely related throughout. These findings differ from those of a similar study of hereditary orotic aciduria and suggest there is competitive transport between exogenous (dietary) purines and pyrimidines, as well as an important interdependence between endogenous purine and pyrimidine metabolism, by mechanisms as yet undefined.