Studies on biosynthesis of cholesterol. 8. The effect of Mg2+ and Mn2+ ions and of adenosine triphosphate on enzymic activity of microsomes in sterol biosynthesis from mevalonic acid

Abstract

The enzyme system from rat liver, consisting of a soluble protein fraction and microsomes, and which synthesizes cholesterol from mevalonic acid, was investigated for stability of the microsomal enzymes. Microsomes kept frozen at -15[degree] for a few weeks to a few months are unable to catalyze sterol synthesis when combined with the soluble enzymes under conditions optimum for an enzyme system containing fresh microsomes. Inactivation of fresh microsomes can be induced rapidly by incubation at 37[degree]. Partial reactivation of microsomes stored at -15[degree], or of those "aged" at 37[degree], can be achieved by the addition of Mn++ ions (m[image]) or by an increased concentration of Mg++ ions (5 m[image]). The sterol-synthesizing system has an absolute requirement for Mg++ or Mn++ ions. Enzyme systems containing old preparations of microsomes also have a requirement for adenosine triphosphate higher than those containing fresh microsomes. Sodium fluoride, which has no effect on fresh preparations, enhances the stimulating effects of high-concentrations of adenosine triphosphate on old preparations of microsomes. During ageing, substances inhibiting sterol synthesis appear in the microsomal preparations. Microsome preparations as old as 4 months can be fully reactivated by a suitable combination of Mg++ ions, adenosine triphosphate and sodium fluoride.