Hepatic Sulfite Oxidase

Abstract

Abstract Sulfite oxidase (EC 1.8.3.1) has been purified 1000-fold from acetone powders of bovine liver. This enzyme was capable of transmitting electrons, abstracted from sulfite, to oxygen, to 2,6-dichlorophenol indophenol, and to methylene blue as well as to cytochrome c and ferricyanide. The oxidase activity of this enzyme was shown to involve a 2-electron reduction of oxygen to hydrogen peroxide without the formation of any detectable radical intermediates. The kinetic behavior of sulfite oxidase was completely accountable in terms of a simple ping-pong model. The Ks for sulfite was found to be 1.4 x 10-4 m and that for molecular oxygen was 5.8 x 10-4 m. The pH optimum was 8.6. The ratio of the activity of the enzyme toward oxygen and toward 1-electron acceptors was constant throughout the purification procedure. Furthermore, the oxidase activity and the cytochrome c reductase activities were equally susceptible to thermal inactivation; and sulfate protected both activities to the same degree against this inactivation. Anions were noncompetitive inhibitors of the reduction of 1-electron acceptors by sulfite oxidase but had no effect on the reduction of 2-electron acceptors. Divalent anions were more effective in this respect than were monovalent anions. Because ferricyanide was a more efficient acceptor of electrons from the enzyme than was oxygen; ferricyanide inhibited the reduction of oxygen by sulfite oxidase plus sulfite; and anions, by virtue of their selective inhibition of the reduction of 1-electron acceptors, were able to eliminate this action of ferricyanide.