Stereoselective propranolol metabolism in two drug induced rat hepatic microsomes

AIM:To study the influence of inducers BNF and PB on the stereoselective metabolism of propranolol in rat hepatic microsomes.METHODS:Phase I metabolism of propranolol was studied by using the microsomes induced by BNF and PB and the non induced microsome as the control.The enzymatic kinetic parameters of propranolol enantiomers were calculated by regression analysis of Lineweaver-Burk plots. Propranolol concentrations were assayed by HPLC.RESULTS:A RP-HPLC method was developed to determine propranolol concentration in rat hepatic microsomes. The linearity equations for R(+)propranolol and S(-) propranolol were A = 705.7C+311.2C (R = 0.9987) and a = 697.2C+311.4C (R = 0.9970) respectively. Recoveries of each enantiomer were 98.9%, 99.5%, 101.0% at 60&mgr;mol/L, 120&mgr;mol/L, 240&mgr;mol/L respectively. At the concentration level of 120&mgr;mol/L, propranolol enantiomers were metabolized at different rates in different microsomes. The concentration ratio R(+)/S(-) of control and PB induced microsomes increased with time, whereas that of microsome induced by BNF decreased. The assayed enzyme parameters were: 1. Km. Control group: R(+)30 plus minus 8, S(-)18 plus minus 5; BNF group: R(+)34 plus minus 3, S(-)39 plus minus 7; PB group: R(+)38 plus minus 17, S(-)36 plus minus 10. 2. Vmax. Control group: R(+)1.5 plus minus 0.2, S(-)2.9 plus minus 0.3; BNF group: R(+)3.8 plus minus 0.3, S(-)3.3 plus minus 0.5; PB group: R(+)0.07 plus minus 0.03, S(-)1.94 plus minus 0.07. 3. Clint. Control group: R(+)60 plus minus 3, S(-)170 plus minus 30; BNF group: R(+)111.0 plus minus 1, S(-) 84 plus minus 5; PB group: R(+)2.0 plus minus 2, S(-)56.0 plus minus 1. The enzyme parameters compared with unpaired t tests showed that no stereoselectivity was observed in enzymatic affinity of three microsomes to enantiomers and their catalytic abilities were quite different and had stereoselectivities.Compared with the control, microsome induced by BNF enhanced enzyme activity to propranolol R(+)enantiomer, and microsome induced by PB showed less enzyme activity to propranolol S(-)enantiomer which remains the same stereoselectivities as that of the control.CONCLUSION:Enzyme activity centers of the microsome were changed in composition and regioselectivity after the induction of BNF and PB, and the stereoselectivities of propranolol cytochrome P450 metabolism in rat hepatic microsomes were likely due to the stereoselectivities of the catalyzing function in enzyme.CYP1A subfamily induced by BNF exhibited pronounced contribution to propranolol metabolism with stereoselectivity to R(+)enantiomer.CYP2B subfamily induced by PB exhibited moderate contribution to propranolol metabolism, but still had the stereoselectivity of S(-)enantiomer.