Antibody preference for the catalytically active form of beta-hydroxy-beta-methylglutaryl coenzyme A reductase.

The catalytically inactivating subset within rabbit serum polyclonal antibody to the solubilized, purified 55,000 to 60,000 dalton active fragment of rat liver microsomal beta-hydroxy-beta-methylglutaryl coenzyme A reductase immunoinactivates this enzyme with little or no diminution of effect by enzyme catalytically inactivated by incubation of microsomes with ATP,Mg++. Reactivation of inactive enzyme with ethanol-treated rat liver phosphatase restores antibody affinity showing that the catalytically inactivating subset of antibody exhibits marked or complete affinity for the active enzyme over the ATP,Mg++- inactivated form. This means that immunoinactivation using this antibody is not a valid way of measuring changes in the specific activity of the enzyme via phosphorylation-dephosphorylation. Preference for the active enzyme has not been obvious because when different amounts of enzyme activity are used in immunotitrations of samples of low activity, apparent differences in specific activity are observed when none actually exist. If precautions are not taken, results are obtained supporting phosphorylation by using an antibody that is not capable of distinguishing it.