Nitric oxide synthase: involvement of oxygen radicals in conversion of L-arginine to nitric oxide.

The objective of this study was to determine the role of superoxide ion in the formation of nitric oxide by brain NO synthase. NO synthase activity was detected by activation of guanylate cyclase in broken cell preparations. NO synthase activity was dependent on NADPH and was inhibited by EGTA, hemoglobin, Nw-methyl-L-arginine and nitroblue tetrazolium. While the addition of exogenous superoxide dismutase significantly enhanced NO synthase activity, bovine liver catalase completely abolished NO formation. None of these NO synthase modulators, however, altered NO-dependent stimulation of guanylate cyclase activity. These observations indicate that catalytic conversion of L-arginine to nitric oxide by cytosolic, isoform of brain NO synthase requires superoxide ion, hydrogen peroxide and possibly hydroxyl radical.