Multifold Activation of CYP3A4 Caused by Alcohol-Induced Increase in the Content of CYP2E1 in Human Liver Microsomes

In order to probe the effect of alcohol-induced increase in the content of CYP2E1 in human liver microsomes (HLM) on the function of other P450 species we established a model that implements enrichment of HLM samples with CYP2E1 through membrane incorporation of the purified protein. Enrichment of HLM with CYP2E1 results in a considerably increased rate of metabolism of 7-benzyloxyquinoline (BQ) and eliminates the homotropic cooperativity observed with this CYP3A-specific substrate. Furthermore, incorporation of CYP2E1 also eliminates the activating effect alpha-Naphthoflavone (ANF) on BQ metabolism seen in untreated HLM. To probe the physiological relevance of these effects we compared three pulled preparations of HLM from normal donors (HLM-N) with a preparation obtained from heavy alcohol consumers (HLM-A). The composition of the P450 pool in all four samples was characterized with mass-spectrometric determination of isoform-specific peptides of 11 cytochrome P450 species. The molar content of CYP2E1 in HLM-A samples was from 2.5 to 3.3 times higher than that found in HLM-N preparations. In contrast, the content of CYP3A4 in HLM-A was the lowest among all four studied HLM samples. Despite the lower content of CYP3A4, HLM-A exhibited much higher rate of metabolism and lower degree of homotropic cooperativity with BQ, similar to that observed in CYP2E1-enriched HLM-N. Our results demonstrate that the catalytic activity and allosteric properties of CYP3A4 are fundamentally dependent on the composition of the cytochrome P450 ensemble in human liver and imply a profound impact of chronic alcohol exposure on the pharmacokinetics of drugs metabolized by CYP3A4.