Reversible inactivation of 3-hydroxy-3-methylglutaryl coenzyme A reductase: reductase kinase and mevalonate kinase are separate enzymes

Reductase kinase and mevalonate kinase are separated by: a) ammonium sulfate fractionation; b) chromatography on agarose-Procion Red HE3B; and c) chromatography on DEAE-Sephacel. Fractions containing only reductase kinase reversibly inactivated microsomal or homogeneous HMG-CoA reductase. Fractions containing only mevalonate kinase revealed artifactual reductase kinase activity in the absence of EDTA or mevalonic acid; however, addition of EDTA or mevalonate before reductase assay completely blocked any apparent decline in HMG-CoA reductase activity. Under these conditions no dephosphorylation (reactivation) was observed by phosphatase. The combined results demonstrate unequivocally that reductase kinase and mevalonate kinase are two different enzymes and inactivation of HMG-CoA reductase is catalyzed by ATP-Mg-dependent reductase kinase.