From genotype to phenotype: content and activities of cytochromes P450 2A6 in human liver in vitro and predicted in vivo.

Unraveling the molecular mechanisms by which genetic variants of cytochrome P450 2A6 (CYP2A6) lead to different metabolic phenotypes remains a long-standing but important challenge. CYP2A6 is an enzyme involved in the metabolism of several clinical drugs as well as the metabolic activation of carcinogenic nitrosamines. Herein, CYP2A6 genotypes and phenotypes, as indicated by protein content (by LC-MS/MS) and metabolic activities (Vmax, CL), were determined for 90 human liver samples. We determined the median, range, and inter-individual and intra-individual variation of CYP2A6 content and activity at the microsomal, liver tissue, and whole liver level and predicted hepatic in vivo clearance by in vitro-in vivo extrapolation based on CYP2A6-mediated coumarin metabolism by each CYP2A6 genotype. These results reveal how different CYP2A6 genotypes yield different phenotypic traits in protein content and enzyme activity. For relative Vmax, CL and protein content, the intra-individual percentage coefficients of variation (ICVs) were 41.0% (18.8-125.1%), 28.5% (2.39-133.5%) and 27.8% (2.68-88.0%), respectively. The high ICVs implied large intra-individual variation at different levels, sometimes in a genotype-dependent manner. Inter-genotype analysis revealed that the CYP2A6*4 allele demonstrated the most obvious effect on phenotypic outcomes, both in protein content and in metabolic activity. Indeed, decreased CYP2A6 protein content with the CYP2A6*4 genotype might explain the decreased metabolic activity from the molecular to the organismal level. These findings may allow useful predictions for CYP2A6-mediated drug metabolism on an individual patient basis in accord with the goal of achieving personalized medicine. SIGNIFICANCE STATEMENT We provide the median, range, and inter-individual and intra-individual variation in CYP2A6 content at the microsomal, liver tissue, and whole liver level by LC-MS/MS as well as activities at the protein, microsomal, liver tissue, and whole liver level both in vitro and at the organismal level based on CYP2A6-mediated coumarin metabolism with each CYP2A6 genotype, thereby allowing us to elucidate how different CYP2A6 genotypes yield differing phenotypic traits (protein content and enzyme activity), facilitating the development of personalized medicine.