DIFFERENCES IN THE INTERACTIONS OF R AND S WARFARIN WITH HEPATIC MICROSOMAL CYTOCHROME P-450

Abstract

The enantiomers of the anticoagulant, warfarin, are metabolized to a series of monohydroxylated products by rat hepatic cytochrome P-450 (P-450). The patterns of products vary for R and S warfarin and following induction of P-450 by phenobarbital (PB) and cytochrome P-448 (P-448) by 3-methylcholanthrene (MC). We studied the binding of R and S warfarin to cytochrome P-450 by difference spectrophotometry to determine how the product patterns produced are influenced by the heterogeneity of P-450. Uninduced P-450 yielded modified type II difference spectra with R and S warfarin with binding constants, Ks = 1.60 mM. PB induced microsomes also yielded modified type II spectra which produced biphasic plots with Ks(S) = 0.25, 0.07 mM and Ks(R) = 0.83, 0.11 mM. MC induction with R warfarin yielded type I spectra with Ks = 0.23 mM. With S warfarin modified type II spectra were observed with Ks = 0.12 mM. In all cases prior binding of R or S warfarin influenced the binding of the opposite enantiomer. We conclude that R and S warfarin bind to two separate forms of cytochrome P-450 and two separate forms of cytochrome P-448, all of which differ from uninduced cytochrome P-450. The variety of monohydroxylated metabolites of R and S warfarin are a consequence of the interactions with these different forms of cytochrome P-450.