ENZYME INACTIVATION THROUGH SULFHYDRYL OXIDATION BY PHYSIOLOGICAL NO-CARRIERS

Nitric oxide (NO) is a pluripotent regulatory molecule, yet the molecu lar mechanisms by which it exerts its effects are largely unknown. Few physiologic target molecules of NO have been identified, and even for these, the modifications caused by NO remain uncharacterized. Human g lutathione reductase (hGR), a central enzyme of cellular antioxidant d efense, is inhibited by S-nitrosoglutathione (GSNO) and by diglutathio nyl-dinitroso-iron (DNIC-[GSH](2)), two in vivo transport forms of NO. Here, crystal structures of hGR inactivated by GSNO and DNIC-[GSH](2) at 1.7 Angstrom resolution provide the first picture of enzyme inacti vation by NO-carriers: in GSNO-modified hGR, the active site residue C ys 63 is oxidized to an unusually stable cysteine sulfenic acid (R-SOH ), whereas modification with DNIC-[GSH](2) oxidizes Cys 63 to a cystei ne sulfinic acid (R-SO2H). Our results illustrate that various forms o f NO can mediate distinct chemistry, and that sulfhydryl oxidation mus t be considered as a major mechanism of NO action.