Wade M L, Voelkel N F, Fitzpatrick F A
Department of Pharmacology C-236, University of Colorado Health Sciences Center, Denver 80262, USA.
Arch Biochem Biophys. 1995 Aug 20;321(2):453-8. doi: 10.1006/abbi.1995.1417.
"Suicide" inactivation accompanied catalysis by isolated prostaglandin I synthase. Inactivation occurred via a saturable, pseudo-first-order process with an apparent binding constant Ki = 8 microM prostaglandin H2 and an inactivation rate constant ki = 0.06 s-1. Enzymatic activity declined as an exponential function of substrate concentration and a linear function of product formation. A competitive inhibitor, 9,11-(methanoepoxy)-15(S)-hydroxy-prosta-5Z,13E-dienoic acid, protected the enzyme from inactivation. Prostaglandin H1, an endoperoxide which is not a substrate, inactivated the enzyme less effectively than prostaglandin H2. The differences between inactivation by prostaglandin H2 and H1, the protective effect of the competitive inhibitor, the quantitative similarity between Km and Ki, and the dependence on catalysis all suggest that inactivation originates primarily from a transition-state intermediate, not from malondialdehyde formed by hydrolysis of prostaglandin endoperoxides. Collectively, the data conform to criteria for a specific, mechanism-based process in which a common enzyme-substrate complex participates in two parallel reactions, one leading to turnover and the other to suicide inactivation. Inactivation accompanying catalysis by prostaglandin I synthase in intact endothelial cells was transient, consistent with the cellular capacity for de novo protein synthesis. Enzyme activity returned to the initial steady-state level within 15-20 min, suggesting that prostaglandin I synthase has a half-life < or = 5 min.
