A rapid, biphasic inhibition of rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase (mevalonate:NADP+ oxidoreductase (CoA-acylating), EC 1.1.1.34) was induced by intragastric administration of R,S-mevalonolactone. The initial phase had a t1/2 of 5.3 min. 30 min after drug administration the inhibition could be reversed in vitro by cytosol or a partially purified cytosolic activator. The reactivation was prevented by 50mM NaF. Thus the initial inhibition appeared to be the result of reversible inactivation possibly by phosphorylation of the enzyme. Consistent with this was the finding that the net reductase activator (phosphatase) activity present in cytosol was decreased 64% in these animals. The rapid reversible inhibition could not be reproduced in vitro by incubating microsomes or postmitochondrial supernatants with mevalonate suggesting the intact cell was necessary for expression of the effect. The second phase of inhibition due to mevalonate administration had a t1/2 of 1.3 h and was not reversible. It was attributed to inhibition of synthesis of reductase probably as the result of sterol accumulation in the cell. Perfusion of 25-hydroxycholesterol through livers isolated from animals at the circadian peak of cholesterol biosynthesis resulted in a rapid, 75-80% inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase. This inhibition was not reversed by incubation with cytosol or partially purified activator. Further, there was no apparent change in net activator levels in cytosol from the livers perfused with 25-hydroxycholesterol. This suggests the effect of this sterol on reductase does not involve reversible phosphorylation-dephosphorylation. On the basis of this study it is postulated that there are at least two mechanisms by which 3-hydroxy-3-methylglutaryl coenzyme A reductase activity can be rapidly suppressed in the intact liver. One is reversible and appears to be the result of alteration in the reductase kinase-phosphatase system. The second is irreversible and may be due to acceleration of the normal degradation system.
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