SLOW-BINDING INHIBITION OF 6-PHOSPHOGLUCONATE DEHYDROGENASE BY ZINC ION

6-Phosphogluconate dehydrogenase (EC 1.1.1.44) has been purified from Cryptococcus neoformans, an encapsulated yeast that is an opportunisti c pathogen of AIDS patients, The dimeric enzyme had a subunit molecula r weight of 50,000, a specific activity of 50 units mg(-1), and K-m va lues of 13 mu M for 6-phosphogluconate and 0.89 mu M for NADP, This en zyme, like many fungal dehydrogenases, was inhibited by Zn2+, with the inhibition pattern being competitive versus the nonnucleotide substra te, In the presence of micromolar Zn2+, the reaction was biphasic, wit h the reduction of NADP proceeding initially at the control rate, but, over the time course of 20-300 s, this initial nonlinear phase reache d a final, linear steady state with a slower velocity, This pattern is indicative of a slow binding inhibition process, for which we have ca lculated the following kinetic constants: k(6), the limiting rate cons tant for the transition from initial to final steady state was 0.0024 s(-1), corresponding to a half-time of 300 s; K-i, the overall equili brium constant for the dissociation of EZn2+ to E + Zn2+ was 0.021 mu M. (C) 1996 Academic Press, Inc.