Sulfite oxidase has been purified 100-fold from extracts of Thiobacillus neapolitanus. The enzymatic activity, measured as the rate of oxygen consumption, was stimulated some 1.5--2-fold by 3.3 mM AMP throughout the course of purification, indicating that a single enzyme is responsible for the oxidation of sulfite in the presence and absence of AMP. Reduced glutathione inhibited activity and prevented the AMP-dependent rate stimulation. The rate of sulfite oxidation in the presence and absence of AMP was first order with respect to the sulfite concentration.
The thiobacilli oxidize thiosulfate to sulfate through sulfite as a probable intermediate. P~CK (1960) has suggested that the pathway of sulfite oxidation proceeds through the formation of adenosine 5'-phosphosulfate (APS) as intermediate in Thiobacillus thioparus extracts in the presence of AMP. AMP, however, was not essential for sulfite oxidation with ferricyanide as electron acceptor (P~cK, 1961), although its presence increased the rate of sulfite oxidation some five-fold. Sulfite oxidase preparations from other thiobacilli have been described (CHARLV, S and SuzvKT, 1965;BOWEN et al., 1966; A])M~, 1966), each with different properties with regard to stimulation by AMP and ability to transfer reducing equivalents to oxygen. This communication contains a description of the purification of sulfite oxidase from Thiobacillus neapolitanus and provides evidence that * Dedicated to Prof. C. B. VAn NI~L on the occasion of his 70th birthday. ** Part of a dissertation submitted by W. P. HE~eFnINO to Yale University in partial fulfillment of the requirements for the Ph.D. degree, 1964.