The Mechanism of the Rhodanese-catalyzed Thiosulf atekipoate Reaction

The individual reactions comprising the rhodanesecatalyzed thiosulfate-lipoate reaction were demonstrated kinetically. The mechanism was shown to be a reaction sequence in which the enzyme forms kinetically significant binary complexes with both thiosulfate and dihydrolipoate substrates. Moreover, the first product (sulfite) is discharged, forming a sulfur-substituted enzyme intermediate, before the second substrate (dihydrolipoate) is attached. It was further shown that the mechanism includes mutually competitive inhibition reactions with both substrates. Extension of the analysis to several pH values permitted determination of the variation in kinetic coefficients with pH. The changes in these values over the pH range of 8 to 11 suggested that the enzyme exists in an association equilibrium between monomeric and dimeric species and that an enzymic sulfhydryl group is involved in lipoate binding.