Interaction of plasma proteins with cytochromes P450 mediated metabolic reactions: inhibition by human serum albumin and alpha-globulins of the debrisoquine 4-hydroxylation (CYP2D) in liver microsomes of human, hamster and rat

The effect of human serum albumin (HSA), alpha1-acid glycoprotein (alpha1-A GP), and alpha- and gamma -globulins on the in vitro metabolism of debrisoq uine in human, hamster and rat liver microsomes was studied. Interaction of albumin with cytochrome P450 mediated phenytoin metabolism has been report ed. Since plasma protein binding of phenytoin is high, in the present study a weakly protein bound drug, debrisoquine, was studied. Debrisoquine is a substrate of CYP2D6. The debrisoquine 4-hydroxylation was measured using a radio-TLC method. Among the four plasma proteins, alpha -globulins had the strongest inhibitory effect on the debrisoquine 4-hydroxylase activity. The inhibition with 2% alpha -globulins was 42 +/- 18% for human and higher fo r hamster and rat liver microsomes (65-71%). HSA had less effect than alpha -globulins. In the presence of HSA, the decrease in activity was between 1 8 and 35% for all liver microsomes studied. The debrisoquine 4-hydroxylase activity was not significantly changed by alpha1-AGP or gamma -globulins. U sing an ultra-filtration method, the protein binding of debrisoquine to 4% HSA, 0.5% al-AGP, 2% a-globulins and 2% gamma -globulins was found to be 22 , 20, 22 and 5%, respectively. Since the observed inhibition is inconsisten t with level of protein binding, it appears, particularly in the case of al pha -globulins, that the plasma proteins interact with CYP2D directly. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.