Nitric oxide stabilizes the Mo(V) oxidation state of dimethyl sulfoxide reductase from Rhodobacter sphaeroides without inhibiting enzyme activity.

Dimethyl sulfoxide reductase from Rhodobacter sphaeroides is isolated in an oxidized, Mo(VI) containing form. Both nitric oxide and reduced ascorbate carried out a one electron reduction of the enzyme with formation of stoichiometric amounts of EPR active Mo(V). Nitric oxide also caused a one electron oxidation of reduced, Mo(IV) enzyme. Mo(V) formation was accompanied by appearance of absorbance peaks at 387 and 528 nm. Neither nitric oxide nor ascorbate inhibited the enzyme nor did either compound support enzyme turnover. Both nitrite plus ascorbate and nitroxyl anion (NO-) induced a previously reported rhombic EPR signal (g1 = 1.994, g2 = 1.982, g3 = 1.966) which exhibited superhyperfine coupling to an exchangeable proton (A1 = 1.25 mT, A2 = 0.85 mT, and A3 = 1.0 mT). On the other hand, NO(g) induced an axial signal with g perpendicular = 1.982 and g parallel = 1.961 in which there is no evidence of superhyperfine coupling. Thus, ascorbate, nitric oxide, and nitric oxide donors induce and stabilize Mo(V) formation in dimethyl sulfoxide reductase without inhibiting enzyme activity. The resemblance between NO and the natural N-oxide substrates of this enzyme suggest that the Mo(V)-NO complex may be a transition state analog of the enzyme-substrate complex.