Closely related isozymes of alcohol dehydrogenase Carboxymethylation: γ1γ1 differs widely from both β1β1 and its equine equivalence EE

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Abstract
Human γ1γ1 alcohol dehydrogenase is quite insensitive to inactivation by iodoacetate, its equine counterpart EE highly sensitive, and the human β1β1 form intermediately sensitive. Imidazole hardly influences the iodoacetate inactivation of γ1γ1, enhances that of EE and decreases that of β1β1. In all isozymes metal‐binding Cys residues are the most reactive, but the patterns for those binding the active site zinc atom differ. In phosphate, Cys‐46 is most sensitive in EE and γ1γ1, Cys‐174 in β1β1. This difference appears to correlate with the absence or presence, respectively, of an extra methyl group in the side‐chain at position 48 (Ser in EE and γ1γ1, Thr in β1β1). In imidazole, the reactivity in β1β1 is shifted to Cys‐46, while the specificity is enhanced in EE and decreased in γ1γ1. Thus, the inactivations illustrate large defferences among structures closely related.
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