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A BSTR ACTThe nitric-oxide synthase (NOS; EC 1.14.13.39) reaction is formulated as a partially tetrahydrobiopterin (H 4 Bip)-dependent 5-electron oxidation of a terminal guanidino nitrogen of L-arginine (Arg) associated with stoichiometric consumption of dioxygen (O 2 ) and 1.5 mol of NADPH to form L-citrulline (Cit) and nitric oxide (⅐NO). Analysis of NOS activity has relied largely on indirect methods such as quantification of nitrite͞nitrate or the coproduct Cit; we therefore sought to directly quantify ⅐NO formation from purified NOS. However, by two independent methods, NOS did not yield detectable ⅐NO unless superoxide dismutase (SOD; EC 1.15.

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