PT  - JOURNAL ARTICLE
AU  - Bichara, N
AU  - Ching, M S
AU  - Blake, C L
AU  - Ghabrial, H
AU  - Smallwood, R A
TI  - Propranolol hydroxylation and N-desisopropylation by cytochrome P4502D6: studies using the yeast-expressed enzyme and NADPH/O2 and cumene hydroperoxide-supported reactions.
DP  - 1996 Jan 01
TA  - Drug Metabolism and Disposition
PG  - 112--118
VI  - 24
IP  - 1
4099  - http://dmd.aspetjournals.org/content/24/1/112.short
4100  - http://dmd.aspetjournals.org/content/24/1/112.full
SO  - Drug Metab Dispos1996 Jan 01; 24
AB  - We have studied the enantioselectivity and regioselectivity of ring-hydroxylation and N-desisopropylation of R(+)- and S(-)-propranolol in microsomes from yeast expressing cytochrome P4502D6 (CYP2D6), using both NADPH and molecular oxygen (NADPH/O2) and cumene hydroperoxide-supported reactions. With NADPH/O2-supported reactions, CYP2D6 catalyzed 4- and 5-ring-hydroxylation, as well as N-desisopropylation of propranolol, although Vmax was considerably greater for ring-hydroxylation, compared with N-desisopropylation. The R/S ratios for KM and Vmax were less than unity for all three pathways. In contrast, using cumene hydroperoxide-supported reactions, CYP2D6 catalyzed 4- and 5-ring-hydroxylation, and there was negligible N-desisopropylation of propranolol. The R/S ratio for KM was less than unity, but the R/S ratio for Vmax was close to unity. The cumyl group of cumene hydroperoxide did not seem to be a selective inhibitor of N-desisopropylation, because i) cumyl alcohol (a nonalkylhydroperoxide analog of cumene hydroperoxide) did not inhibit N-desisopropylation in NADPH/O2-supported reactions, and ii) the use of t-butyl hydroperoxide (a noncumyl alkylhydroperoxide) to support CYP2D6 catalysis resulted in ring-hydroxylation, but not N-desisopropylation. At a propranolol concentration near KM, quinidine inhibited both ring-hydroxylation and N-desisopropylation in an equipotent manner in NADPH/O2-supported reactions. However, in cumene hydroperoxide-supported reactions, the IC50 of inhibition of ring-hydroxylation by quinidine was an order of magnitude less potent than in NADPH/O2-supported reactions. Our study shows that recombinant CYP2D6 cannot only catalyze 4- and 5-ring-hydroxylation of propranolol, but also N-desisopropylation. The lack of propranolol N-desisopropylation observed in cumene hydroperoxide-supported reactions highlights the need for caution when using alkyhydroperoxides to study CYP2D6 catalysis.