Reversible modulation of liver hydroxymethylglutaryl CoA reductase.

Abstract The activity of rat liver microsomal hydroxymethylglutaryl CoA reductase (HMG CoA reductase, E.C. 1.1.1.34) is severely diminished when microsomes are preincubated with both ATP and Mg 2+ before the enzyme is assayed. Sensitivity to MgATP is progressively lost when microsomes are extracted with buffers. The MgATP response is restored by adding back soluble microsomal extracts or protein fractions derived from the liver cytosol. The component(s) that permits reductase inactivation has been designated the I system. The activity of HMG CoA reductase that has been diminished by pretreatment with MgATP and the I system can be fully restored in a separate preincubation with a cytosolic protein system (after removal of ATP and/or Mg 2+ ). Reactivation of reductase is blocked with NaF. The cytosolic reactivating system has been purified extensively in several laboratories and seems to be a protein that is virtually identical to a 35,000 dalton phosphoprotein phosphatase. Consequently the evidence suggests that HMG CoA reductase can be modulated, in vitro , by covalent phosphorylation-dephosphorylation. The I system itself is inactivated by simply preincubating microsomes for several hours at 37°. The inactivation of I can be prevented by including NaF in the preincubation system. Addition of the purified 35,000 dalton phosphoprotein phosphatase (which is fluoride-sensitive) accelerates the inactivation of soluble I. Soluble, inactivated I can be restored to full activity with MgATP. A cAMP-insensitive protein kinase has been identified in the I extract that is distinguishable from the I activity itself. These data indicate that the I enzyme, like the HMG CoA reductase, is probably interconvertible through phosphorylation-dephosphorylation. The reductase is consequently subject to short-term control through a bicyclic system that should be quite sensitive to hormonal and effector signalling.