Hepatic sulfite oxidase effect of anions on interaction with cytochrome c

Abstract

The one-electron transfer reactions of chicken liver sulfite oxidase (EC 1.8.3.1), with either ferricytochrome c or ferricyanide as the electron acceptor, are inhibited by inorganic anions. Inhibition by smaller monovalent anions is competitive with respect to cytochrome c and mixed non-competitive with respect to SO 3 2−. Inhibition by larger monovalent anions or by divalent anions is mixed non-competitive with respect to either substrate. In contrast, oxidation of SO 3 2− in the presence of O 2 as electron acceptor is not inhibited by the anions. The electron paramagnetic resonance (EPR) spectrum of Mo(V) in the enzyme is markedly perturbed by the presence of anions, with the change in line shape being characteristic of the anion used. These characteristic changes in shape are also seen in the EPR spectrum of sulfite oxidase in mitochondria in the presence of some anions but not others. The effects of any two anions on the EPR signal of purified enzyme show a competitive behavior. These results demonstrate an interaction of anions with the molybdenum center of sulfite oxidase, which may be the basis of the inhibitory effects.