ENZYMATIC BASIS FOR THE NONLINEARITY OF HEPATIC ELIMINATION OF PROPRANOLOL IN THE ISOLATED PERFUSED-RAT-LIVER

Propranolol (PL) metabolism was studied in the isolated perfused rat liver under single-pass and steady-state conditions. An attempt was made to predict the data observed in the isolated rat liver perfusion at PL infusion rates of 89-1317 nmol/min using the microsomal kinetic parameters obtained in our previous paper (Ishida et al., Biochem Pharmacol 43: 2489-2492, 1992) and the unbound PL fractions in rat liver microsomes and the perfusion medium. The values of kinetic parameters obtained in rat liver microsomes were corrected for the whole liver. Two groups of cytochrome P450 isozymes having high (K(m) < 0.5 muM)- and low (K(m) > 20 muM)-affinities participate in the metabolism of PL and sudan III pretreatment induces the low-affinity enzymes rather than the high-affinity enzymes in control rats. Of high-affinity isozyme(s) PL 4-hydroxylase and 7-hydroxylase made a major contribution to the overall activity, while for low-affinity isozymes PL 4-hydroxylase and N-desisopropylase did. A nonlinear relationship between the PL concentrations entering and leaving the liver was predicted from these corrected kinetic parameters using the venous equilibrium model. The outflow concentrations and the metabolic rates of PL for the predicted curves were over-estimated at higher inflow PL concentrations and under-estimated at higher substrate concentrations, respectively. On the other hand, the prediction for them was successfully carried out for the livers whose intrinsic clearance was altered due to the induction of low-affinity enzymes in PL metabolism by sudan III pretreatment. The outflow rates of 4-hydroxypropranolol showed a downward curvature at lower substrate concentrations, followed a linear rise in the livers from control rats, while the outflow rates of 5- and 7-hydroxypropranolol exhibited their respective limiting values. The outflow rates of 4-hydroxypropranolol and N-desisopropylpropranolol were enhanced markedly with increasing the outflow unbound concentration of PL by sudan III pretreatment. These results indicate that non-linear PL first-pass metabolism is due to the saturation of the reactions for the high-affinity enzymes among enzymes engaging in PL ring hydroxylations.