CYP1A2 and CYP2D6 4-hydroxylate propranolol and both reactions exhibit racial differences

We have previously shown racial differences in propranolol kinetics, with t he largest differences appearing to be in its 4-hydroxylation. The purpose of this study was to identify and confirm the cytochrome P450 enzymes (CYP) with propranolol 4-hydroxylase activity, describe their enzyme kinetics, a nd determine whether there were racial differences in their catalytic activ ity. Eleven human recombinant, expressed CYPs were screened, but only CYP1A 2 and CYP2D6 possessed propranolol 4-hydroxylase activity. Subsequent studi es were conducted in human liver microsomes, including correlation, inhibit ion, enzyme kinetics, and racial comparison studies. Significant correlatio ns were noted between propranolol 4-hydroxylation and ethoxyresorufin-O-dee thylation (marker of CYP1A2 activity), with marked improvement in the corre lations when CYP2D6-mediated propranolol 4-hydroxylation was inhibited with quinidine. Inhibition studies showed that quinidine inhibited approximatel y 55% of propranolol 4-hydroxylation and furaphylline (CYP1A2-selective inh ibitor) inhibited about 45% of propranolol 4-hydroxylation. Median (range) parameter estimates of (S)-4-hydroxypropranolol [(S)-HOP] formation were a V-max value of 307 (165-2397) and 721 (84-1975) pmol/mg of protein/60 min f or CYP1A2 and CYP2D6, respectively, and a K-m value of 21.2 (8.9-77.5) and 8.5 (5.9-31.9) mu M for CYP1A2 and CYP2D6, respectively. CYP1A2- and CYP2D6 -mediated propranolol 4-hydroxylation was about 70 and 100% higher (P < .05 for both), respectively, in African-Americans compared with Caucasians. In summary, we found that both CYP1A2 and CYP2D6 catalyze formation of 4-hydr oxypropranolol and that both enzymes exhibited racial differences in this r eaction. The observed racial differences in drug metabolism may have releva nce to drug efficacy, toxicity, or carcinogen activation for CYP1A2 or CYP2 D6 substrates.