Abstract

Glucuronidation of digitoxigenin-monodigitoxoside (DT1), a metabolite of the cardiac glycoside digitoxin, is mediated by the microsomal isozymes, UDP-glucuronosyltransferase(s) (UDP-GT). The present studies examined the activation and induction properties of UDP-GT activity toward DT1 in hepatic microsomes of rats and mice. When compared to enzyme activity present in native (latent) microsomes of the rat (0.104 +/- 0.010 nmol/min/mg of protein), the activity toward digitoxigenin-monodigitoxoside in mouse native microsomes was 3.5-fold higher (0.379 + 0.44 mumol/min/mg of protein). After treatment with ionic (sodium cholate), zwitterionic [3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS)], or nonionic (Emulgen 911, Triton X-100) detergents, or with UDP-N-acetylglucosamine, enzyme activity in rat microsomes remained unchanged. In contrast, UDP-GT activity (DT1) in mouse liver microsomes treated with detergents or with the nucleotide was increased 2-3-fold above native enzyme activity. Pretreatment of rats with the microsomal enzyme inducers, 3-methylcholanthrene and phenobarbital, had no effect on this enzyme activity, whereas pretreatment with pregnenolone-16 alpha-carbonitrile (PCN) and dexamethasone (DEX) increased enzyme activity toward DT1 800 and 380%, respectively. These findings support the hypothesis that PCN and DEX induce a unique form of UDP-GT in the rat that selectively glucuronidates DT1. In marked contrast, the activity of this enzyme in mouse liver was not affected by pretreatment with any of the microsomal inducers, including PCN and DEX. In both rat and mouse, the P-450p-dependent N-ethylmorphine demethylase activity was increased 10-15-fold in PCN-pretreated animals.(ABSTRACT TRUNCATED AT 250 WORDS)