Kinetic studies on a genetically engineered fused enzyme between rat cytochrome P4501A1 and yeast NADPH-P450 reductase.

Abstract

Three expression plasmids, pAMC1 for rat P4501A1, pAMR2 for P4501A1 and yeast NADPH-P450 reductase, and pAFCR1 for a fused enzyme between P4501A1 and the reductase, were constructed, and each was introduced into Saccharomyces cerevisiae AH22 cells. The microsomal fraction prepared from the recombinant yeast cells was subjected to kinetic studies of zoxazolamine 6-hydroxylation at 10 degrees C. The apparent Km and Vmax values for hydroxylation by the fused enzyme in AH22/pAFCR1 microsomes were 0.38 mM and 0.42 s-1, respectively. The rate constant for reduction of the fused enzyme with NADPH in the presence of 1 mM zoxazolamine was larger than 50 s-1 using a dual-wavelength stopped-flow spectrometer, indicating that electrons are rapidly transferred from NADPH through FAD and FMN to the heme iron of the fused enzyme. The rate constant kon for substrate binding to the fused enzyme was 25 mM-1.s-1, which is not much different from that of nonfused P4501A1. These results together with spectral data measured during the hydroxylation reaction in the steady state suggest that the rate-limiting step of the reaction by the fused enzyme might be the release of product. On the other hand, the apparent Km and Vmax values for the hydroxylation of P4501A1 in AH22/pAMC1 and AH22/pAMR2 microsomes were 0.32 and 0.33 mM, and 0.015 and 0.29 s-1, respectively. The rate constants for the reduction of P4501A1 were 0.025 and 0.40 s-1, respectively, for AH22/pAMC1 and AH22/pAMR2 microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)