THE EFFECT OF PRAVASTATIN ON HEPATIC 3-HYDROXY-3-METHYLGLUTARYL COA REDUCTASE OBTAINED FROM POLOXAMER 407-INDUCED HYPERLIPIDEMIC RATS

A single 300-mg intraperitoneal injection of poloxamer 407 (P-407) to rats produces a marked hypercholesterolemia for a minimum of 96 hours and increases the activity of hepatic 3-hydroxy-3-methylglutaryl coenz yme A (HMG-CoA) reductase compared with the enzyme activity found in m icrosomal homogenates of control animals. We attempted to determine wh ether inhibition of microsomal HMG-CoA reductase by pravastatin sodium would yield similar values for the maximum reaction in velocity (V-ma x) and the HMG-CoA reductase-pravastatin dissociation constant (K-i) w hen the enzyme was in the activated state compared with the control st ate. Knowledge of the respective values for V-max and K-i would allow us to determine whether P-407-induced hypercholesterolemia in the rat was refractory to pravastatin treatment. Over a pravastatin concentrat ion range of 0.5-50 nM, enzyme activity in vitro decreased as the drug 's concentration increased. A standard Dixon plot of mean values of re ciprocal reaction velocity versus pravastatin concentration yielded K- i of 3.7 and 4.1 nM for the control and activated states, respectively . The V-max for conversion of HMG-CoA to mevalonate in vitro in the pr esence of pravastatin was 3.5-fold greater when assayed in microsomal homogenates obtained from P-407-injected rats compared with control an imals. Dixon plot analysis of the data resulted in V-max of 58.1 and 2 02 pmol . min(-1). mg(-1) for the control and activated states, respec tively. These data suggest that whereas the V-max is affected, injecti on of P-407 to rats does not alter the binding affinity of pravastatin for receptor(s) contained in HMG-CoA reductase as reflected by simila r K-i values. This experimental animal model may be an additional scre en with which to rank order the relative potency of HMG-CoA reductase inhibitors by determining the drug's effectiveness when HMG-CoA reduct ase is in an activated state.