Tp. Johnston et Wk. Palmer, THE EFFECT OF PRAVASTATIN ON HEPATIC 3-HYDROXY-3-METHYLGLUTARYL COA REDUCTASE OBTAINED FROM POLOXAMER 407-INDUCED HYPERLIPIDEMIC RATS, Pharmacotherapy, 17(2), 1997, pp. 342-347
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.