A Probable Intermediate in the Enzymic Reduction of 3-Hydroxy-3-methylglutaryl Coenzyme A

1 The exhange between 3-hydroxy-3-methylglutaryl-CoA and [14C]CoASH is not catalysed by hydroxymethylglutaryl-CoA reductase. 2 The addition compound of (RS)-mevaldate and CoASH, most probably a diastereomeric mixture of hemithioacetals, is a good substrate for the reductase. Km and Vmax for the new substrate were determined and the possibility of its intermediate nature in the hydroxymethyl-glutaryl-CoA reductase reaction discussed.