CYTOCHROME-P450 ENZYMES INVOLVED IN THE ENHANCEMENT OF PROPRANOLOL N-DESISOPROPYLATION AFTER REPEATED ADMINISTRATION OF PROPRANOLOL IN RATS

Repeated oral administration of propranolol (PL, 100 mg/kg daily, for 5, 10 and 15 days) to male Wistar rats increased PL N-desisopropylase and decreased PL 4-,5- and 7-hydroxylase activities in liver microsome s. The increase was highest al the 10 day time point whereas the decre ase was relatively constant over the 15 day treatment period. There we re no significant changes in the total content of cytochromes P450 (P4 50) or cytochrome b(5) or in NADPH-cytochrome c reductase activity dur ing the PL treatment. The enhanced N-desisopropylase activities were m arkedly inhibited by alpha-naphthoflavone (a P450-1A1/2 inhibitor), an d moderately by triacetyloleandomycin (a P450-3A1/2 inhibitor) and die thyldithiocarbamate (a P450-2E1 inhibitor). Phenacelin O-deethylase ac tivity, an index of P450-1A2, was significantly increased on day 5, 10 and 15 of the treatment, whereas p-nitrophenol hydroxylase activity w as elevated on day 10 only. The PL N-desisopropylation showed a strong and significant correlation with phenacetin O-deethylation, and a wea ker but significant correlation with p-nitrophenol hydroxylation. Immu noblot analysis revealed that a protein band corresponding to P450-1A2 was increased by PL pretreatment, and protein band corresponding to P 450-3A tended to be increased slightly, but other protein band corresp onding to the subfamily of P450-2B, -2C, or -2E was not changed. Pretr eatment of rats with P450 inducers (beta-naphthoflavone, phenobarbital , acetone and dexamethasone) increased PL N-dealkylase activity in liv er microsomes. Furthermore, antibodies raised against P450-1A and -3A enzymes suppressed PL N-desisopropylation in a concentration-dependent manner, but P450-2E antibody did not. Reconstitution studies showed t hat P450-1A1, -1A2, -2E1 and -3A2 exhibited catalytic activities for P L N-dealkylation. These results suggest that P450-1A2 is a major PL N- desisopropylase in the PL-treated rats, and P450-3A related enzyme(s) and P450-2E1 as a moderate or minor enzyme are also involved in PL N-d ealkylation in native and PL-treated rats.