Studies on the coordinate activity and liability of orotidylate phosphoribosyltransferase and decarboxylase in human erythrocytes, and the effects of allopurinol administration

Thiopurinol and purine metabolism. Metabolic and radioisotope studies

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Influence of allopurinol on drug metabolism in man

1. Elimination rates of phenylbutazone and warfarin after single oral doses in human volunteers, and steady-state plasma concentrations of these drugs in patients have been measured before, during and after administration of allopurinol.2. Allopurinol, in usual clinical doses, had no significant influence on the elimination rate of either drug in the group of volunteers as a whole although in a few individuals an apparent inhibitory effect was observed.3. No significant changes in the steady-state plasma concentrations of either phenylbutazone or warfarin were observed in patients during the administration of allopurinol.4. It is concluded that although allopurinol may have an apparent inhibitory effect on drug elimination in a few individuals, its administration to most patients on anticoagulant therapy is unlikely to alter dose requirements.

Mechanism of allopurinol-mediated increase in enzyme activity in man

Allopurinol therapy in man interferes with pyrimidine biosynthesis de novo by inhibition of one or both of the two enzymes, orotate phosphoribosyltransferase (OPRT) and orotidylic decarboxylase (ODC), responsible for the conversion of orotic acid to uridine-5'-monophosphate. Inhibition of this pathway in vivo is followed in 1-3 wk by an increase in the activity of both of these enzymes in erythrocytes and of ODC in circulating leukocytes. This drug-mediated increase in enzyme activity in erythrocytes could not be attributed to enzyme stabilization or induction in vivo but appeared to be due to enzyme "activation." "Activation" of the OPRT enzyme was directly demonstrated in erythrocytes studied in vitro after incubation with oxipurinol, and to a lesser extent, with allopurinol. No evidence for "activation" of the ODC enzyme was demonstrated in vitro. This response to allopurinol therapy provides an excellent model for examining the mechanism of increased enzyme activity in response to drug administration.

Hypoxanthine-guanine phosphoribosyltransferase: characteristics of the mutant enzyme in erythrocytes from patients with the Lesch-Nyhan syndrome

The Lesch-Nyhan syndrome is characterized clinically by choreoathetosis, spasticity, selfmutilation, and mental and growth retardation. Biochemically, there is a striking reduction of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity in affected individuals. We have examined erythrocytes from 14 patients with the Lesch-Nyhan syndrome for the presence of hypoxanthine-guanine phosphoribosyltransferase activity and enzyme protein. In contrast to the usual finding of no detectable hypoxanthine-guanine phosphoribosyltransferase activity, we have found low levels (0.002-0.79 nmoles/mg protein per hr) of hypoxanthine-guanine phosphoribosyltransferase activity in erythrocyte lysates from five of these patients. In three of the five patients, hypoxanthine-guanine phosphoribosyltransferase activity appeared to be substantially more labile in vivo than normal using erythrocytes which had been separated according to their density (age). Immunochemical studies using a monospecific antiserum prepared from a homogeneous preparation of normal human erythrocyte hypoxanthine-guanine phosphoribosyltransferase revealed immunoreactive protein (CRM) in hemolysate from all 14 patients with the Lesch-Nyhan syndrome. The immunoreactive protein from each patient gave a reaction of complete identity with normal erythrocyte hypoxanthine-guanine phosphoribosyltransferase and was present in quantities equal to those observed in normal erythrocytes. In addition, a constant amount of CRM was found in erythrocytes of increasing density (age) from patients with the Lesch-Nyhan syndrome despite the decreasing hypoxanthine-guanine phosphoribosyltransferase activity. These studies confirm previous data which indicate that the mutations leading to the Lesch-Nyhan syndrome are usually, if not always on the structural gene coding for hypoxanthine-guanine phosphoribosyltransferase. In addition, although the mutant proteins appear to be present in normal amounts, they are often very labile in vivo with respect to enzymatic activity. These observations suggest that therapy directed at stabilization or activation of enzyme activity in vivo may be of potential benefit.