Propylthiouracil. A substrate for the glutathione S-transferases that competes with glutathione.

In previous studies, we have observed that propylthiouracil (PTU) binds to Sephadex 6-100 eluates of rat liver cytosol associated with glutathione (GSH) Stransferase activity (EC 2.5.1.18). In order to study further this association between propylthiouracil and the GSH S-transferases, we examined the effect of propylthiouracil on the kinetics of the enzymatically catalyzed reaction between GSH and l-chloro-2,4-dinitrobenzene and found that propylthiouracil inhibited the reaction uncompetitively with respect to the latter reactant, but competitively with respect to the former (Ki = 0.62 mM). In addition to its activity as an inhibitor of the GSH S-transferases, propylthiouracil appeared to be a substrate. Reaction products between propylthiouracil and several of the lipophilic substrates for the transferases were identified and the enzymatically catalyzed formation of these products obeyed Michaelis-Menten kinetics. The specific activities of the partially purified GSH S-transferases AA, A, B, and C in catalyzing the reaction between propylthiouracil and l-chlor0-2,4-dinitrobenzene were 40, 52, 141, and 58 nmol/mg/min, respectively, approximately 1 to 20% of the specific activities of these enzymes in catalyzing the reaction between GSH and l-chloro-2,4-dinitrobenzene. Similar results were obtained with GSH S-transferases AA, B, and C purified to homogeneity. These studies suggest that propylthiouracil inhibits the GSH S-transferases in a unique fashion by competing with GSH as a substrate in reactions catalyzed by the enzymes.