Transient-state kinetics of the reactions of 1-methoxy-4-(methylthio)benzene with horseradish peroxidase compounds I and II.

Transient-state reactions of horseradish peroxidase compounds I and II with 1-methoxy-4-(methylthio)benzene (a para-substituted thioanisole) were studied over the pH range from 3.4 to 10.5. The pH-jump technique was applied to the compound II reactions at pH values below 8.6. The reactions of both compound I and compound II with the para-substituted thioanisole consisted predominantly of an initial burst. The burst was followed by a steady-state phase that became more obvious at lower concentrations of the thioanisoles. The burst phase for both compounds I and II can be explained in terms of two independent transient-state reactions with 1-methoxy-4-(methylthio)benzene as follows: (i) a single reaction of compound I (or compound II) with the substrate and (ii) the formation of a complex between compound I or II and the substrate followed by reaction of the productive complex with another molecule of sulfide. The overall rate of reaction path ii is faster than that of path i. The preference for path i or ii is highly dependent upon the concentration of sulfide with step ii favored at higher sulfide concentrations. The experimental results obtained on the overall reaction under both pseudo-first-order and single-turnover conditions indicate that compound II reacts competitively with both the organic sulfide substrate and the sulfur cation radical produced from compound I oxidation of sulfide.