CATALYTIC AND IMMUNOCHEMICAL CHARACTERIZATION OF CYTOCHROME-P450 ISOZYME INDUCTION IN DOG LIVER

The purpose of this study was to characterize hepatic cytochrome P450 induction in the dog by phenobarbital, beta-naphthoflavone, dexamethas one, and isoniazid using catalytic activities and Western blots with a ntibodies prepared against rat cytochrome P450 isozymes. Male beagle d ogs were treated with phenobarbital (10 mg/kg for 2 days and 30 mg/kg for the following 5 days), beta-naphthoflavone (50 mg/kg for 5 days), or isoniazid (10 mg/kg for 2 days and 30 mg/kg for the following 5 day s). Female beagle dogs were treated with dexamethasone (50 mg/kg for 5 days). Increases in the liver/body weight ratio were observed after t reatment of dogs with phenobarbital (133% of control) and dexamethason e (153%). Total cytochrome P450 content was increased as a percentage of control after treatment with phenobarbital (264%) and beta-naphthof lavone (186%), while it slightly decreased after treatment with isonia zid (54%) and dexamethasone (71%). Dog liver microsomes hydroxylated t estosterone mainly at the 6 beta and 16 alpha positions but also at th e 6 alpha-, 15 beta-, 15 alpha-, 16 beta-, 18-, 2 beta-, and 17-positi ons. There were no sex differences in terms of regioselectivity of tes tosterone metabolism between control male and female dogs. Treatment o f dogs with phenobarbital produced increases in 6 beta- (184%), 16 alp ha- (379%), 16 beta- (210%), 18- (195%), and 2 beta-testosterone (203% ) hydroxylase and pentoxyresorufin O-dealkylase (651%) activities. On Western blots, phenobarbital treatment produced induction of P450 3A- and 2B1-related proteins. Although treatment with dexamethasone result ed in a large increase in liver weight, no significant increase in P45 0 3A-related protein or 6 beta-hydroxylase activity was detected. Howe ver, dexamethasone and isoniazid treatment produced slight increases i n chlorzoxazone hydroxylase activity. Treatment with isoniazid induced a P450 2E1-related protein. Treatment with beta-naphthoflavone produc ed increases that were 689 and 357% of control in ethoxyresorufin O-de ethylase and chlorzoxazone hydroxylase activities, respectively. beta- Naphthoflavone treatment increased the amount of two proteins immunoch emically related to the cytochrome P450 1A subfamily. Thus, although g enerally similar to other species, the response of the dog to cytochro me P450 inducers differs significantly from the rat and human in some cases. (C) 1996 Society of Toxicology