Allosteric and non-allosteric forms of rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase: differential inhibition of activity by adenosine 2'-monophospho-5'-diphosphoribose.

Adenosine 2'-monophospho-5'-diphosphoribose (P-ADP-Rib) is a structural analog of NADPH which was reported to competitively inhibit (Kiapp = 21.7 microM) solubilized rat liver 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (Tanazawa, K., and A. Endo. 1979. Eur. J. Biochem. 98: 195-201). However, microsomal HMG-CoA reductase, which at low thiol concentrations exhibits allosteric properties, is only poorly inhibited by P-ADP-Rib (Kiapp = 550 microM at 4.5 mM GSH). Gradual shift of the microsomal reductase towards a non-allosteric form by increasing glutathione (GSH) concentrations resulted in a higher inhibition by P-ADP-Rib. Under these conditions, Ki values for P-ADP-Rib were 165 microM and 53 microM at 9 mM and 27 mM GSH, respectively. The largest change in the degree of inhibition by P-ADP-Rib was observed within the 10 mM range of GSH. By contrast, freeze-thaw solubilized HMG-CoA reductase, which does not display allosteric properties, is readily inhibited by P-ADP-Rib, even when assayed at a low concentration of GSH (Kiapp = 50 microM at 4.5 mM GSH). Assaying the solubilized reductase in the presence of increased thiol concentration results in a minor decrease in the apparent Ki for P-ADP-Rib (22 microM at 27 mM GSH). Microsomal HMG-CoA reductase is allosterically activated by various nucleotides. When activated by NADH, the enzyme is effectively inhibited by P-ADP-Rib even at a 4.5-mM GSH concentration (Kiapp = 175 microM in the presence of 300 microM NADH).(ABSTRACT TRUNCATED AT 250 WORDS)