Rapid reactions of peroxynitrite with heme-thiolate proteins as the basis for protection of prostacyclin synthase from inactivation by nitration.

Prostacyclin (PGI(2)) synthase is a heme-thiolate (P450) protein which reacts with low levels of peroxynitrite (PN) under tyrosine nitration and inactivation. Studying heme proteins as models, we have found the heme-thiolate protein NADH-NO reductase (P450(NOR)) to be highly efficient in decomposing PN under concomitant nitration of phenol. The present study investigates two other P450 proteins, P450(BM-3) and chloroperoxidase, in order to test for the specific role of the thiolate ligand in the reaction with PN. A comparison with horseradish peroxidase and microperoxidase gives evidence of kinetic differences that classify heme-thiolate proteins, but not other heme proteins, as effective inhibitors of PGI(2) synthase nitration and inactivation. P450(BM-3) with PN catalyzes phenol nitration and nitration of its own tyrosine below 10 microM PN, whereas chloroperoxidase and P450(NOR) at such concentrations also nitrate phenol but not enzyme-bound tyrosine residues. We conclude that heme-thiolate proteins in general exhibit high reactivity with PN and turnover, probably due to the special electronic structure of the presumed thiolate-ferryl intermediate. Copyright 2000 Academic Press.