Mechanisms of electron transfer from sulfite to horseradish peroxidase-hydroperoxide compounds

Abstract

Using a rapid-scan spectrophotometer equipped with a stopped-flow apparatus, reactions of sulfite with compounds I and II of 2 horseradish peroxidase isoenzymes A and C were investigated. The direct 2-electron reduction of perioxidase compound I by sulfate occurred at acidic pH but the mechanism gradually changed to the 2-step reduction with the intermediate formation of compound II as the pH increased. The pH at which the 1- and 2-electron changes occurred at the same speed was 4.5 for peroxidase A and 7.7 for peroxidase C. A new peroxidase intermediate was found in the reaction between peroxidase compound II and sulfite. The sulfite compound showed a characteristic absorption band at 850 nm and the optical spectrum was similar to that of isoporphyrins but was quite different from that of sulfhemoproteins. The rate (k) of conversion from the sulfite-compound II complex to the sulfite compound was proportional to the concentration of H+ and the log k vs. pH plot for peroxidase A moved to the acidic side by 1.1 pH unit from that for peroxidase C.