Activation of low and null activity isozymes of maize alcohol dehydrogenase by antibodies.

Antisera were raised against several purified, high specific activity isozymes of maize alcohol dehydrogenase (ADH1). The various antisera had different effects on the activity of immunoprecipitated ADH. One antiserum completely inactivated maize ADH. This inactivation could be blocked by preincubation of the enzyme with NAD+, its cofactor, or with NADP. The different antisera were used to analyze variant forms of ADH1. Isozymes having lowered specific activity were activated to wild-type levels by precipitation of the enzymes with noninactivating antisera. Isozymes having no detectable ADH activity (CRM+ nulls) were activated by immunoprecipitation with noninactivating antisera when preincubated with NAD+ or NADP. All of the CRM+ nulls were shown to be unable to bind NAD+, a flaw which can account for their lack of activity. The results indicate that a conformational equilibrium between active and inactive forms of maize ADH in solution controls the specific activity of the various isozymes. Both NAD+ and antibodies raised against high specific activity enzymes can interact with low activity isozymes to shift the balance of the equilibrium toward the active form, thus increasing their specific activity.