Induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by a series of 4′-substituted-2,3,4,5-tetrachlorobiphenyls

Abstract

We have shown previously that 2,3,4,5-tetrachlorobiphenyl is ineffective as an inducer of rat liver microsomal cytochrome P-450. Addition of a single para-chloro substituent in the otherwise unsubstituted phenyl ring, to give 2,3,4,4′,5-pentachlorobiphenyl, produces a potent cytochrome P-450 inducer with both phenobarbital- and 3-methylcholanthrene-type characteristics. In the present study, 2,3,4,5-tetrachlorobiphenyl was substituted in the para(4′) position with 12 other functional groups. The 4′-X-C 12H 5Cl 4 derivatives were tested as inducers of cytochromes P-450 a– P-450 e and epoxide hydrolase, by immunochemical analysis of liver microsomes prepared 4 days after a single treatment (500 μmol/kg) of 1-month-old- male Long Evans rats. When the para′ substituents was a halogen (F, Cl, Br or I), the derivative induced both cytochromes P-450 b and P-450 e, and cytochromes P-450 c and P-450 d, which are the major phenobarbital- and 3-methylcholanthrene-inducible isozymes, respectively. A similar type of induction was observed with a second group of derivatives substituted with CN, NO 2 or CF 3. However, a derivative containing CH 3CO—(which is also a meta-directing, ring-activating substituent) failed to induce cytochromes P-450 a– P-450 e at the dosage and time tested. Members of a third group of derivatives, which contained an orto/para-directing, ring-activating substituent) were either ineffective inducers (OH, CH 3, CH 3O—), or were inducers of cytochromes P-450 c and P-450 d (isopropyl or t-butyl. Hence, 4′-substitution with a bulky lipophilic substituent conferred 3-methylcholanthrene- but not phenobarbital-type characteristics on 2,3,4,5-tetrachlorobiphenyl. Some of the derivatives tested, namely those substituted with Cl, Br, I and CF 3, were remarkably effective inducers of cytochrome P-450 a, causing a 10–11-fold induction of this isozyme. Data on the induction of cytochrome P-450 c were analyzed by multiparameter linear regression in an attempt to correlate the biological activity of the 4′-X-C 12H 5Cl 4 derivatives with the physiochemical properties of the various substituents. From these results, and those reported recently, we propose that binding of the 4′-X-C 12H 5Cl 4 derivatives to the rat cytosolic Ah receptor is favored by increasing the electronegativity, lipophilicity and hydrogen bonding characteristics of the 4′ substituent, whereas enzyme induction (both in vivo and in cultured rat hepatoma cells) is also governed by a fourth characteristic, the STERIMOL factor, which gives a measure of the width of the substituent. The importance of the size of the substituent in enzyme induction, but not receptor binding, may reflect a need for the Ah receptor to undergo a ligand-induced conformational change to initiate the chain of events leading to enzyme induction.