Functional Characterization of Cytochrome P450 2B6 Allelic Variants

Abstract

ABSTRACT:Cytochrome P450 (P450) 2B6 is a hepatic enzyme of potential importance for the metabolism of clinically used drugs and environmental or abused toxicants. Genetic polymorphisms of CYP2B6 (CYP2B6*2, CYP2B6*3, CYP2B6*4, CYP2B6*5, CYP2B6*6 and CYP2B6*7; wild-type, CYP2B6*1) were found previously in white and Japanese populations. In the present study, the goal was to investigate the effects of amino acid substitutions on CYP2B6 function. Wild-type (CYP2B6.1) and all of the known variants of CYP2B6 (CYP2B6.2, CYP2B6.3, CYP2B6.4, CYP2B6.5, CYP2B6.6, and CYP2B6.7) were transiently expressed in COS-1 cells, and their 7-ethoxy-4-trifluoromethylcoumarin O-deethylation activities were determined. The levels of the variant CYP2B6 proteins were relatively low compared with that of CYP2B6.1, although the differences were not significant. The activities of 7-ethoxy-4-trifluoromethylcoumarin O-deethylation on the basis of the CYP2B6 protein level at low (0.5 M) and high (50 M) substrate concentrations varied among wild-type and variant CYP2B6 proteins. All CYP2B6 enzymes showed typical Michaelis-Menten kinetics. The K m value of CYP2B6.6 was significantly higher than that of CYP2B6.1. Those CYP2B6 variants having a Lys262Arg substitution (CYP2B6.4, CYP2B6.6, and CYP2B6.7) showed increased values for V max and V max /K m , whereas the kinetic parameters of CYP2B6.2 and CYP2B6.3 were not affected by the corresponding amino acid substitution. These results may mean that Lys262 in combination with other amino acid residues such as Gln172 and Arg487 is associated with the CYP2B6 function and that the genetic polymorphism of CYP2B6 leads to interindividual differences in xenobiotic metabolism.

Influence of pharmacogenetics on response and toxicity in breast cancer patients treated with doxorubicin and cyclophosphamide

“…CYP2B6 variants have been associated with both decreased (Lang et al, 2001), (Jinno et al, 2003) and unchanged (Xie et al, 2003) protein expression compared with the wild-type gene. Increased specific activity associated with the variants has also been observed (Lang et al, 2001;Jinno et al, 2003).…”

“…CYP2B6 variants have been associated with both decreased (Lang et al, 2001), (Jinno et al, 2003) and unchanged (Xie et al, 2003) protein expression compared with the wild-type gene. Increased specific activity associated with the variants has also been observed (Lang et al, 2001;Jinno et al, 2003). Similarly, though there are reports that CYP2B6 variants confer differences in the pharmacokinetics of cyclophosphamide (Xie et al, 2006), other studies have failed to show a statistically significant impact (Timm et al, 2005;Nakajima et al, 2007;Ekhart et al, 2008).…”

Polymorphisms of drug‐metabolizing enzymes (GST, CYP2B6 and CYP3A) affect the pharmacokinetics of thiotepa and tepa

“…In vitro studies have shown that protein levels of variant CYP2B6 were less than that of wild-type CYP2B6 [21,33]. The activity of the CYP2B6 variants A785G and G516T was significantly higher than that of wild-type CYP2B6 [33,34].…”

“…Five of these polymorphisms cause amino acid substitutions in exons 1, 4, 5 and 9. Polymorphisms in the CYP2B6 gene have been shown to result in altered protein expression and activity [33].…”

“…The CYP2B6 gene has been mapped to chromosome 19 between 19q12 and 19q13.2, and is composed of nine exons [33]. Lang et al [21] identified nine point mutations of the CYP2B6 gene in a White population.…”