Catalytic and Immunochemical Characterization of Cytochrome P450 Isozyme Induction in Dog Liver

Abstract

The purpose of this study was to characterize hepatic cytochrome P450 induction in the dog by phenobarbital, beta-naphthoflavone, dexamethasone, and isoniazid using catalytic activities and Western blots with antibodies prepared against rat cytochrome P450 isozymes. Male beagle dogs 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 days). Female beagle dogs were treated with dexamethasone (50 mg/kg for 5 days). Increases in the liver/body weight ratio were observed after treatment of dogs with phenobarbital (133% of control) and dexamethasone (153%). Total cytochrome P450 content was increased as a percentage of control after treatment with phenobarbital (264%) and (3-naphthoflavone (186%), while it slightly decreased after treatment with isoniazid (54%) and dexamethasone (71%). Dog liver microsomes hydroxylated testosterone mainly at the 6-beta and 16-alpha positions but also at the 6-alpha-, 15-beta-, 15-alpha-, 16-beta-, 18-, 2-beta-, and 17-positions. There were no sex differences in terms of regio-selectivity of testosterone metabolism between control male and female dogs. Treatment of dogs with phenobarbital produced increases in 6-beta- (184%), 16-alpha- (379%), 16-beta- (210%), 18- (195%), and 2-beta-testosterone (203%) hydroxylase and pentoxyresorufin 0-dealkylase (651%) activities. On Western blots, phenobarbital treatment produced induction of P450 3A- and 2B1-related proteins. Although treatment with dexamethasone resulted in a large increase in liver weight, no significant increase in P450 3A-related protein or 6-beta-hydroxylase activity was detected. However, dexamethasone and isoniazid treatment produced slight increases in chlorzoxazone hydroxylase activity. Treatment with isoniazid induced a P450 2E1-related protein. Treatment with (beta-naphthoflavone produced increases that were 689 and 357% of control in ethoxyresorufin 0-deethylase and chlorzoxazone hydroxylase activities, respectively. Beta-Naphthoflavone treatment increased the amount of two proteins immunochemically related to the cytochrome P450 1A subfamily. Thus, although generally similar to other species, the response of the dog to cytochrome P450 inducers differs significantly from the rat and human in some cases.

Effect of oral administration of low doses of pentobarbital on the induction of cytochrome P450 isoforms and cytochrome P450-mediated reactions in immature Beagles

“…24-26 These 2 isoforms have distinct substrate specificities 25 and differ by approximately 1.5 kd. Dogs possess 2 distinct members of the CYP3A family, CYP3A12 and CYP3A26, which are induced by phenobarbital.…”

Cytochromes P450 and experimental models of drug metabolism

“…Two CYP3A forms are expressed in dog, CYP3A12 and CYP3A26. However, dexamethasone, a typical inducer of 3A enzymes, does not induce these forms in dog [18,30]. CYP2D15 is the major CYP form in dog with enzymatic activities similar to the human CYP2D6 [31].…”

“…The antibodies against human CYP1A were shown to influence in dog microsomes the 6-hydroxylation of chlorzoxazone -a typical activity of CYP2E1 [29]. Moreover, beta naphthoflavone, a typical inducer of CYP1A forms in dog, has been shown to induce the same activity in dog microsomes [30]. Hence, the CYP1A and 2E1 forms in dog are not good models of the respective human CYP enzymes.…”

Seasonal emaciation causes tissue redistribution and an increased potential for toxicity of lipophilic pollutants in farmed arctic fox (Vulpes lagopus)

“…The hepatic enzyme analyses revealed EROD activity as well as 6b-, 2b-, 16a-OHT, and androstenedione. Although no studies have been performed to characterize CYP isoforms in the Arctic fox, it is reasonable to assume, based on previous studies on laboratory animals (including dogs) and in seals [15,[34][35][36][37][38], that these activities reflect the presence of CYP1A, CYP3A, CYP2B, and CYP2C enzymes also in the Arctic fox.…”