Mechanism of inhibition of carbonyl reductase from rabbit kidney by phenylbutazone.

Phenylbutazone showed significant inhibition against the metabolic reduction of acetohexamide catalyzed by carbonyl reductase purified from rabbit kidney. Thus, the inhibitory effect of phenylbutazone was kinetically examined. Phenylbutazone was a competitive inhibitor for the enzyme with respect to NADPH, whereas it noncompetitively inhibited the enzyme activity with respect to acetohexamide. A fluorescence study revealed that phenylbutazone decreases the binding of NADPH to the free enzyme (apoenzyme). These results suggest that phenylbutazone causes the inhibition of carbonyl reductase by competing with NADPH in its coenzyme-binding domain.