Prevention of cold inactivation of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase by NADPH.

Solubilized 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) from rat liver microsomes has been reported to be reversibly inactivated by temperatures below 19 degrees C. Cold inactivation has now been found to be completely prevented by NADPH and by NADP+ at a concentration of 3 mM. NADPH, however, was more active than NADP+ at lower concentrations and prevented 50% of the cold inactivation at 0.2 mM, whereas a 1.1 mM NADPH+ without effect and the substrate 3-hydroxy-3-methylglutaryl coenzyme A prevented only 30% of the cold inactivation at a concentration 50 times greater than the Km value.