In vivo regulation of rat liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase: enzyme phosphorylation as an early regulatory response after intragastric administration of mevalonolactone.

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Abstract
Although 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase [EC 1.1.1.34] is apparently the major regulatory enzyme in cholesterol biosynthesis, the molecular events involved in the in vivo regulation of this enzyme have remained obscure. To study this problem, rats were given a single 100-mg dose of mevalonolactone by intragastric tube. The rats were sacrificed 20 or 60 min later and liver microsomes were prepared by ultracentrifugation. Two phases of inhibition of microsomal HMG-CoA reductase were observed. The 1st phase of inhibition, observed 20 min after mevalonolactone administration, was completely reversed by preincubation of the microsomes with purified phosphoprotein phosphatase. The 2nd phase of inhibition, observed 60 min after mevalonolactone administration, was not reversed by phosphoprotein phosphatase. The reactivation of liver microsomal HMG-CoA reductase by phosphoprotein phosphatase was blocked by KF or by phosphoprotein phosphatase inhibitor. Immunotitration also showed that microsomal HMG-CoA reductase obtained from animals killed 20 min after mevalonolactone administration was significantly activated by phosphoprotein phosphatase treatment of the microsomes. Phosphorylation of rat liver HMG-CoA reductase is an early in vivo regulatory response after intragastric administration of mevalonolactone.

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