Yeast Alcohol Dehydrogenase: –SH Groups, Disulfide Groups, Quaternary Structure, and Reactivation by Reductive Cleavage of Disulfide Groups

Abstract

Storage of yeast alcohol dehydrogenase in crystal suspension causes a decrease of the enzymatic activity which is strongly related to the number of free –SH groups. With a rate slower than the rate of inactivation the enzyme molecule (molecular weight reinvestigated from hydrodynamic measurements to be 141000) dissociates into four polypeptide chains (molecular weight 35000) which are enzymatically inactive. By sulfitolysis in 4 M urea and amperometric titration it was shown that free –SH groups are oxidized mainly to disulfide groups during inactivation. The formation of disulfide bonds does not occur between –SH groups of different polypeptide chains. It is assumed that the inactivation process from the fully active enzyme molecule to the inactive polypeptide chains is not an all‐or‐none process but passes through three stages: 141000 (fully active) → 141000 (partially active) → 141000 (inactive) → 4 → 35000 (inactive). The more inactive the alcohol dehydrogenase, the more sensitive it is to heat denaturation and peptidase attack. By treatment with mercaptoethanol or Cleland's reagent the disulfide bonds could be reduced and the enzymatic activity was recovered partially. The undissociated inactive molecules could be fully reactivated. With one of the investigated enzyme preparations which contains a peptidase it was also possible to reactivate and reassociate alcohol dehydrogenase from the polypeptide chains.