Properties of latent and thiol-activated rat hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase and regulation of enzyme activity

Abstract

The effect of the thiols glutathione (GSH), dithiothreitol (DTT), and dithioerythritol (DTE) on the conversion of an inactive, latent form (E 1) of rat liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.34) to a catalyticaly active form (E a) is examined. Latent hepatic microsomal HMG-CoA reductase is activated to a similar degree of activation by DTT and DTE and to a lower extent by GSH. All three thiols affect both K m and V max values of the enzyme toward HMG-CoA and NADPH. Studies of the effect of DTT on the affinity binding of HMG-CoA reductase to agarose-hexane-HMG-CoA (AG-HMG-CoA) resin shows that thiols are necessary for the binding of the enzyme to the resin. Removal of DTT from AG-HMG-CoA-bound soluble E a (active enzyme) does not cause dissociation of the enzyme from the resin at low salt concentrations. Substitution of DTT by NADPH does not promote binding of soluble E 1 (latent enzyme) to AG-HMG-CoA. The enzymatic activity of E a in the presence of DTT and GSH indicates that these thiols compete for the same binding site on the enzyme. Diethylene glycol disulfide (ESSE) and glutathione disulfide (GSSG) inhibit the activity of E a. ESSE is more effective for the inhibition of E a than GSSG, causing a higher degree of maximal inhibition and affecting the enzymatic activity at lower concentrations. A method is described for the rapid conversion of soluble purified E a to E 1 using gel-filtration chromatography on Bio-Gel P-4 columns. These combined results point to the importance of the thiol/disulfide ratio for the modulation of hepatic HMG-CoA reductase activity.