Determination of the michaelis-menten constant for beta-hydroxy-beta-methylglutaryl coenzyme A reductase. Demonstration of a substrate affinity 10-fold greater than previously reported.

This paper presents evidence that earlier kinetic determinations of rat liver beta-hydroxy-beta-methylglutaryl coenzyme A (HMG-CoA) reductase, the reported rate-determining enzyme in cholesterol biogenesis, may have substantially underestimated the affinity of this enzyme for HMG-CoA. Nonlinear conversion of this substrate to mevalonic acid, with time, is shown under the conditions of a previous study, and it was concluded that velocities were limited at the lower substrate levels tested by an insufficient quantity of substrate. For this present study, conditions were established under which linearity of time versus product formation was observed, and the extent of conversion did not exceed 6% of the initial quantity of D isomer. These conditions gave a Michaelis-Menten constant of 1.01 +/- 0.12 muM for the D isomer, which is 3.5 to 40 times lower than those previously reported.