Reversible Cold Inactivation of Microsomal 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase from Rat Liver

Soluble 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.34) from rat liver microsomes has been shown to be inactivated reversibly by cold temperature whereas microsomal enzyme did not show any cold sensitivity. At 0° the soluble enzyme lost 80 % of its activity within 30 min and warming at 37° for 20 min restored all the lost activity. Active enzyme preparations did not lose activity if quick frozen at -80°. The simplest explanation of these observations is the hypothesis that the enzyme can exist in two forms, i.e. a cold-inactivated and a heat-activated form. The enzyme had minimum activity between 0 and 19° and the specific activity was less than 0.1 nmole per min per mg. A 10-fold increase in activity was obtained by incubation at 37° for 20 min, but activity was destroyed at 42°. The original assay method employed for this solubilized enzyme involved incubation at 37° which does not differentiate between the two reactions (a) inactive enzyme → active enzyme and (b) substrate → product. However, more readily interpretable results were obtained either by activating the enzyme at 37° prior to assay or by conducting the assay at a temperature at which activation does not occur, i.e. between 16 and 19°. The Arrhenius plot of the activation of the enzyme by heat (log Vmax versus 1/T) showed apparent changes in slope at 19 and 28°. The rate of activation was found to be strongly protein concentration-dependent indicating an intermolecular reaction. The enzyme was stable in 40 to 120 mm buffer concentrations for at least 5 hours, but addition of 4 m KCl caused an irreversible loss in activity of cold-inactivated enzyme and did not prevent the cold inactivation of activated enzyme. Km for substrate d-3-hydroxy-3-methylglutaryl-CoA of the active soluble enzyme was estimated to be 2.2 x 10-5m, similar to that of the microsome-bound enzyme. HMG-CoA reductase is the first example of a microsomal enzyme that is reversibly cold-sensitive.