The "albumin effect" and in vitro-in vivo extrapolation: sequestration of long-chain unsaturated fatty acids enhances phenytoin hydroxylation by human liver microsomal and recombinant cytochrome P450 2C9.

This study characterized the mechanism by which bovine serum albumin (BSA) reduces the K(m) for phenytoin (PHY) hydroxylation and the implications of the "albumin effect" for in vitro-in vivo extrapolation of kinetic data for CYP2C9 substrates. BSA and essentially fatty acid-free human serum albumin (HSA-FAF) reduced the K(m) values for PHY hydroxylation (based on unbound substrate concentration) by human liver microsomes (HLMs) and recombinant CYP2C9 by approximately 75%, with only a minor effect on V(max). In contrast, crude human serum albumin increased the K(m) with both enzyme sources. Mass spectrometric analysis of incubations containing HLMs was consistent with the hypothesis that BSA sequesters long-chain unsaturated acids (arachidonic, linoleic, oleic) released from membranes. A mixture of arachidonic, linoleic and oleic acids, at a concentration corresponding to 1/20 of the content of HLMs, doubled the K(m) for PHY hydroxylation by CYP2C9, without affecting V(max). This effect was reversed by addition of BSA to incubations. K(i) values for arachidonic acid inhibition of human liver microsomal- and CYP2C9-catalyzed PHY hydroxylation were 3.8 and 1.6 microM, respectively. Similar effects were observed with heptadecanoic acid, the most abundant long-chain unsaturated acid present in Escherichia coli membranes. Extrapolation of intrinsic clearance (CL(int)) values for each enzyme source determined in the presence of BSA and HSA-FAF accurately predicted the known CL(int) for PHY hydroxylation in vivo. The results indicate that previously determined in vitro K(m) values for CYP2C9 substrates are almost certainly overestimates, and accurate in vitro-in vivo extrapolation of kinetic data for CYP2C9 substrates is achievable.