Abstract
Dihydralazine induces immunoallergic hepatitis and anti-LM autoantibodies found in the serum of the patients have been reported to react with CYPIA2. It is thus suggested that a reactive metabolite of dihydralazine covalently binds to the P450 protein. We investigated the selectivity of inactivation of P450 enzymes during the metabolism of dihydralazine to evaluate the target protein of its reactive metabolite. Preincubation of microsomes of β-naphthoflavone-treated rats with dihydralazine in the presence of NADPH resulted in time-dependent loss of phenacetin O-deethylase activity (POD, an indicator of CYPIA2 activity), showing inactivation of CYPIA2 during the dihydralazine metabolism. The preincubation was less effective on ethoxyresorufin O-deethylase activity in microsomes of β-naphthoflavone-treated rats (CYPIAI) and pentoxyresorufin O-depentylase activity in microsomes of phenobarbital-treated rats (CYP2B). Dihydralazine metabolism in microsomes of untreated rats caused time-dependent loss of testosterone 2α-, 16α- (CYP2C11) and 6β- (CYP3A) hydroxylase activities. Preincubation of human liver microsomes with dihydralazine caused a marked loss of testosterone 6β- hydroxylase activity in addition to that of POD. These results demonstrated that dihydralazine was metabolically activated by CYPIA2, and the chemically reactive metabolite bound to the enzyme itself and inactivated it, whereas CYP2C and 3A enzymes were also suggested to be the enzymes that activate dihydralazine and lead to the target of the reactive intermediates. On the other hand, carbamazepine induces hypersensitivity reactions and autoantibodies found in the serum of the patients have been reported to react with CYP2C and 3A. However, present preincubation studies demonstrated that CYP2D (bunitrolol 4-hydroxylase) but not CYP2C or 3A was inactivated during the metabolism of the drug.
