Determination of phase I metabolic enzyme activities in liver microsomes of Mrp2 deficient TR − and EHBR rats

Abstract

The canalicular multispecific organic anion transporter/multidrug resistance protein 2 (cMOAT/Mrp2) plays a major role in the transport of anionic xenobiotics across the bile canalicular membrane. Transport deficient rats (TR −) and Eisai-hyperbilirubinemic rats (EHBR), defective in Mrp2, are mutants of Wistar and Sprague Dawley (SD) rats, respectively. In this study, Phase I metabolic enzyme activities in liver microsomes prepared from these mutant male and female rats were compared to their corresponding non-mutant rats. The total cytochrome P450 contents and NADPH-cytochrome P450 reductase activity in male and female TR - rats were significantly higher than in Wistar rats. In male TR − rats, ethoxyresorufin O-deethylation (EROD), pentoxyresorufin O-deethylation (PROD), testosterone 2α, 7α and 16 α-hydroxylase activities were higher, but testosterone 6β-hydroxylase activity and the rate of androstenedione formation were lower than in Wistar rats. Female TR − rats had higher 7α-hydroxylase activity, but EROD activity was lower in female Wistar rats. Similar studies conducted in EHBR versus SD rats demonstrated increased total cytochrome P450 content in male and female EHBR rats; NADPH-cytochrome P450 reductase activity was not significantly affected. Decreased PROD activity and the rate of androstenedione formation were observed in male and female EHBR rats. Furthermore, testosterone 6β-hydroxylase activity was lower in male EHBR rats than in male SD rats while testosterone 7α-hydroxylase activity was significantly higher in male and female EHBR rats. Thus, in addition to Mrp2 deficiency, differential expression of CYP isoforms and their potential impact on the metabolism and pharmacokinetics of compounds should be considered when interpreting data from these rat strains.