A two-step mechanism for the regulation of tryptophan pyrrolase

Abstract Allopurinol, (4-hydroxypyrazolo[3,4-d]pyrimidine) an inhibitor of xanthine oxidase, prevents the corticoid-stimulated increase in tryptophan pyrrolase activity but does not impair the hormone-mediated synthesis of this enzyme. We conclude that the regulatory mechanism occurs via two steps: 1) the synthesis of inactive tryptophan pyrrolase molecules; and 2) the activation of these molecules by xanthine oxidase.