title: Differential expression and function of three closely related phenobarbital-inducible cytochrome P-450 isozymes in untreated rat liver.
abstract: The levels of expression of cytochromes P-450b and P-450e (both inducible by phenobarbital (PB) and differing by only 14 of 491 amino acids) in liver microsomes from untreated male rats were separately quantitated by Western blotting with a polyclonal antibody raised against P-450b that is equally effective against P-450e (anti P-450b/e). A protein with mobility identical to P-450e was detected in all microsomal samples. Microsomes from uninduced livers of individual male rats from five different strains exhibited only minor interstrain and interindividual variability in the expression of P-450e (17 +/- 5 pmol P-450e/mg microsomal protein) with the exception of the Brown Norway strain (8.5 +/- 0.5 pmol P-450e/mg). Expression of P-450b varied widely from undetectable levels (less than 2 pmol/mg) in most Sprague-Dawley rats to about 50% of P-450e levels in Fischer and Brown Norway strains. Anti P-450b/e inhibited total metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) by uninduced microsomes, to an extent dependent on rat strain (15-30%), predominantly through inhibition of formation of 12-hydroxymethyl-7-methyl BA (12HOMMBA) (65-85%), the major metabolite of purified P-450e. A specific activity for P-450e-dependent DMBA metabolism was calculated from four sets of microsomes where the P-450b content was either undetectable or very low (0.7-1.0 nmol/nmol P-450e/min-1). Comparable calculated activities were, however, obtained from other untreated rat liver microsomes where P-450b levels were significant. Polymorphism in P-450b was detected but did not affect total P-450b expression or the sensitivity of DMBA metabolism to anti P-450b/e. A fourth band of greater mobility than P-450b (apparent Mr less than 50,000), was also recognized by anti P-450b/e. The intensity of this band did not vary among individual rats or among the different strains and therefore did not correlate with the sensitivity of microsomal DMBA metabolism to anti P-450b/e. A monoclonal antibody (MAb) against P-450b (2-66-3) recognized P-450's b, b2, and e on Western blots but did not react with this higher mobility band. MAb 2-66-3 and two other MAbs produced against P-450b inhibited 12-methylhydroxylation of DMBA by untreated rat liver microsomes to the same extent as anti P-450b/e. Following PB induction, P-450b was induced to about double the level of P-450e in most rat strains examined.(ABSTRACT TRUNCATED AT 400 WORDS)
mesh_terms: D015127:9,10-Dimethyl-1,2-benzanthracene / Q000378:metabolism; D000818:Animals; D000936:Antigen-Antibody Complex; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000378:metabolism*; D004790:Enzyme Induction; D007106:Immune Sera; D007527:Isoenzymes / Q000096:biosynthesis / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D010634:Phenobarbital / Q000494:pharmacology*; D051381:Rats; D011916:Rats, Inbred F344; D011919:Rats, Inbred Strains; D013045:Species Specificity
chemical_list: D000936:Antigen-Antibody Complex; D007106:Immune Sera; D007527:Isoenzymes; D015127:9,10-Dimethyl-1,2-benzanthracene; D003577:Cytochrome P-450 Enzyme System; D010634:Phenobarbital