Background: Inter-individual variability in hepatic drug metabolizing enzyme (DME) activity is a major
contributor to heterogeneity in drug clearance and safety. Accurate data on expression levels and activities of DMEs
is an important prerequisite for in vitro-in vivo extrapolation and in silico based predictions. Characterization and
assessment of inter-correlations of the major DMEs cytochrome P450s (CYPs) and UDP-glucuronosyltransferases
(UGTs) have been extensively documented, but simultaneous quantification including other major DMEs has been
Objective: Assessment of inter-donor variability and inter-correlations of CYPs, UGTs, sulfotransferases (SULTs),
glutathione S-transferases (GSTs), NAD(P)H:quinone oxidoreductase 1 (NQO1) and NRH: quinone oxidoreductase
2 (NQO2) in a set of 20 individual liver homogenates.
Method: The main drug metabolizing isoforms of CYP and UGT have been reaction phenotype in individual liver
microsomes and NQO1, NQO2, GSTT1 and GSTT2 in corresponding cytosol. In addition, we assessed overall
SULT activity in liver cytosol using acetaminophen and 7-hydroxycoumarin as non-selective substrates and cytosolic
GST activity using the non-selective substrate 1-chloro-2,4-dinitrobenzene (CDNB). Expression of GST isoforms
was also assessed.
Results and Conclusion: While hepatic NQO1 activity was highly variable, NQO2 activity was more conserved. In
addition, we found that of the hepatic GST isoforms, the variation in GSTM3 levels, which is poorly studied, was
highest. The majority of significant correlations were found amongst CYP and UGT enzyme activities. The dataset
presented provides the absolute quantification of the largest number of hepatic DME activities so far and constitute
an essential resource for in silico toxicokinetic and metabolic modelling studies.