Alternative fusion protein strategies to express recalcitrant QconCAT proteins for quantitative proteomics of human drug metabolizing enzymes and transporters.

QconCAT is a tool for quantitative proteomics, consisting of an artificial protein, expressed from an artificial gene, made up of a concatenated string of proteotypic peptides selected from the proteins under study. Isotopically labeled QconCAT (usually containing (13)C6-arginine and (13)C6-lysine) provides a standard for each proteotypic peptide included in its sequence. In practice, some QconCAT proteins fail to express at sufficient levels for the purpose of quantitative analysis. Two complementary methods are presented to express recalcitrant QconCAT proteins intended to quantify human hepatic enzymes and transporters.