Separation and characterization of highly purified forms of liver microsomal cytochrome P-450 from rats treated with polychlorinated biphenyls, phenobarbital, and 3-methylcholanthrene

pH-dependent interaction of microsomal cytochrome P-450 with substrates. I. Effect of pH upon the interaction of exogenous substrates with membrane-bound cytochrome P-450

The interaction of microsomal membrane-bound cytochrome P-450 with substrates was studied spectrophotometrically at various pH-values. The binding of type I compounds, hexobarbital and androstanedione, with cytochrome P-450, as determined by the magnitude of the type I spectral change of microsomes, was markedly enhanced at alkaline pH compared to that at acid pH. The pH-dependent spectral change could be reversed by changing the pH. The maximum absorption change (delta Amax) increased with increasing the pH, while the spectral dissociation constant (Ks) decreased. A similar pH-dependent binding reaction was also observed using a non-dissociative type I compound, cyclohexane. On the contrary, the absorbance magnitude between peak and trough in the aniline- or alcohol-induced difference spectrum of microsomes was enhanced by decreasing the pH, indicating easy complex formation of type II and reverse type I compounds with cytochrome P-450 in the acid rather than the alkaline region.

Modification of the mutagenicity and teratogenicity of cyclophosphamide in rats with inducers of the cytochromes P-450

Cyclophosphamide must be enzymatically activated to be either mutagenic or teratogenic. This activation is thought to be catalyzed by the cytochrome P-450 monooxygenase system. To study the relationship between the mutagenic and teratogenic metabolites of cyclophosphamide, the mutagenicity and teratogenicity of this drug were compared after activation by rats pretreated with chemicals (phenobarbital and beta-naphthoflavone) inducing different cytochromes P-450. Activation of cyclophosphamide to mutagenic metabolites by enzyme fractions from rats on day 13 of gestation was measured with the Ames test using Salmonella typhimurium TA1535. Teratogenicity was assessed in vivo by treatment of rats with cyclophosphamide on day 13 of gestation. Cyclophosphamide was activated to mutagenic metabolites to the same extent (on a tissue wet weight basis) by enzyme fractions from maternal liver, kidney and placenta, despite differences in cytochrome P-450 content. Fetal homogenates did not activate cyclophosphamide. Phenobarbital pretreatment increased the activation of cyclophosphamide to mutagenic metabolites by maternal liver microsomes 10-fold and liver cytochrome P-450 content 1.8 fold; however, this drug did not alter the activation of cyclophosphamide by maternal kidney, by placenta or by the fetus. Phenobarbital pretreatment increased the teratogenicity of cyclophosphamide in rats on day 13 of gestation (increased incidence of malformed embryos, decreased fetal weight). Pretreatment with beta-naphthoflavone did not induce liver cytochrome P-450 in the pregnant rat and did not change the activation of cyclophosphamide to mutagenic metabolites by liver, kidney, placenta or the fetus. Pretreatment with this polycyclic aromatic hydrocarbon had no effect or decreased the teratogenicity of cyclophosphamide. Thus, these experiments suggest that the mother, rather than the fetus, is the site of activation of cyclophosphamide; after phenobarbital pretreatment the predominant site of cyclophosphamide activation is the maternal liver. There appears to be a correlation between the teratogenicity and mutagenicity of cyclophosphamide after induction of the cytochromes P-450. We can speculate that the "proximate teratogen" of cyclophosphamide may also be the "proximate mutagen".

Dose-dependent induction of rat liver microsomal aryl hydrocarbon monooxygenase by benzo[k]fluoranthene

The environmentally widespread polycyclic aromatic hydrocarbon (PAH) benzo[k]fluoranthene is a potent AHH inducer. This has been proven by recording the benz[a]anthracene metabolite profile in the rat liver by means of gas chromatography/mass spectrometry (GC/MS) technique. Even a total dose of 3 times 50 micrograms/kg body wt increases the metabolism of benz[a]anthracene by a factor of 2. The formation of the 8,9- as well as the 5,6-dihydrodiol is stimulated to about the same extent, whereas the formation of the 10,11-dihydrodiol is suppressed. After comparatively low doses of the inducer, a metabolite is formed which corresponds in all parameters with the postulated ultimate carcinogen 3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenz[a]anthracene. This metabolite and a number of other primary and secondary oxidation products could be identified after incubation with induced but not with normal microsomes. Therefore, it should be emphasised that metabolite profiles have to be recorded instead of measuring brutto conversions of PAH substrates to evaluate inducing effects.

Immunological comparison of rat, rabbit, and human microsomal cytochromes P-450

Antibodies were raised in rabbits to electrophoretically homogeneous cytochromes P-450 isolated from rat and human liver microsomes. These antibodies were used to compare various forms of rat, rabbit, and human cytochromes P-450 present in microsomes and in purified preparations by using double-diffusion analysis, immunoelectrophoresis, quantitative microcomplement fixation, competitive radioimmune assay and inhibition of enzyme activity toward d-benzphetamine and benzo[a]pyrene. The results indicate that (1) at least some forms of cytochrome P-450 from the three species share certain common immunological determinants, (2) there are immunological differences between cytochromes P-450 isolated from the three species, (3) some immunological differences exist between cytochromes P-450 isolated from rats of different strains, (4) immunologically distinguishable forms of cytochrome P-450 exist within individual human liver samples, and (5) human liver samples obtained from different individuals contain immunologically different forms of cytochrome P-450. Quantitative microcomplement fixation techniques were used to assign immunological distances to different form of rat, rabbit, and human liver microsomal cytochrome P-450. Cross-reactivity was observed in all systems tested, and the extent of immunological similarity was dependent upon the particular assay used.

Synthesis and insertion, both in vivo and in vitro, of rat-liver cytochrome P-450 and epoxide hydratase into Xenopus laevis membranes

We described whole cell and cell-free systems capable of inserting into membranes cytochrome P-450 and epoxide hydratase made under the direction of rat liver RNA. The systems have been used to study the pathways followed by newly made secretory and integral membrane proteins. The cell-free system contains Xenopus laevis embryo membranes, and demonstrates competition for a common receptor between cytochrome P-450 and epoxide hydratase, and normal secretory proteins: evidence is provided for differential membrane receptor affinity. Thus, synthesis of secretory and membrane proteins appears to involve a common initial pathway. Microinjection of rat liver RNA into whole oocytes suggests that membrane insertion is neither cell type nor species specific, because functional rat liver enzymes are found inserted in the endoplasmic reticulum of the frog cell. Nonetheless, insertion is highly selective since albumin and several other proteins made under the direction of the injected liver RNA are sequestered within membrane vesicles and are then secreted by the oocyte, whilst epoxide hydratase and cytochrome P-450 are inserted into membranes but are not secreted.

Neonatal imprinting and hepatic cytochrome P-450. II. Partial purification of a sex-dependent and neonatally imprinted form(s) of cytochrome P-450

A new form of cytochrome P-450 was partially purified from hepatic microsomes of neonatally imprinted rats (adult male and adult male castrated at four weeks of age). This new form of cytochrome P-450 appears to have an apparent molecular weight of approximately 50,000 daltons as judged by sodium dodecyl sulfate polyacrylamide gel electrophoresis. It appears that this form of cytochrome P-450 is either absent or present in low concentrations in cytochrome P-450 preparations isolated from neonatally nonimprinted rats (adult female and adult male castrated at birth). Reconstitution of testosterone hydroxylase and benzphetamine N-demethylase activities of this partially purified cytochrome P-450 revealed that the presence of testosterone 16 alpha-hydroxylase activity, an imprintable microsomal enzyme, was in parallel with the imprinting status of the animals; a significantly higher activity was detected in the neonatally imprinted than that of the nonimprinted animals. This was in contrast to the nonimprintable benzphetamine N-demethylase, testosterone 7 alpha- and 6 beta-hydroxylase activities which exhibited no correlation with the imprinting status of the animals. We have prepared antisera from rabbits using the partially purified cytochrome P-450 preparations from adult male rats as antigens. These antisera inhibited microsomal testosterone 16 alpha- and 7 alpha-hydroxylase activities in a concentration-dependent manner, without impairing 6 beta-hydroxylase activity. These data suggest that the partially purified cytochrome P-450 from adult male rats consists of both imprintable (16 alpha-) and nonimprintable (7 alpha-) testosterone hydroxylase activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental aspects of 2-acetamidofluorene metabolism and mutagenic activation in the chick

1. Oxidative metabolism and mutagenic activation of 2-acetamidofluorene (AAF) in the Salmonella test system were studied with liver subfractions from untreated and beta-naphthoflavone (BNF)-pretreated chicks during the first 10 days after hatching. 2. Newly hatched chicks had high liver microsomal mono-oxygenase activities, and these were markedly increased by BNF-pretreatment. 3. The mutagenic activation of AAF with control liver subfractions was highest on day 1 after hatching and declined towards day 10. 4. BNF-pretreatment caused decreases in the mutagenicity of AAF at high protein concentrations in the test system, but led to increases in mutagenic activity when low protein concentrations were applied. 5. N-Hydroxy-2-acetylaminofluorene was activated to a mutagen by microsomal metabolism, and this reaction was blocked by the deacetylase inhibitor paraoxon. 6. SDS polyacrylamide gel electrophoresis of liver microsomes from BNF-pretreated chicks showed an increase in a band with mol. wt. of approx. 55,000. The increase in the 55,000 mol. wt. band following BNF-pretreatment was associated with increases in benzo[a]pyrene hydroxylase and AAF N-hydroxylase activities in the chick liver microsomes.

Effects of methylbenzenes on microsomal enzymes in rat liver, kidney and lung

The effects of pretreatment with toluene, o-, m-, p-xylene and mesitylene were investigated on the microsomal enzymes of liver, kidney and lung in rats. The activities of aminopyrine N-demethylase, aryl hydrocarbon hydroxylase, aniline hydroxylase, NADPH-cytochrome c reductase, as well as the concentrations of cytochrome P-450 and cytochrome b5 were determined. The effects were most marked in the liver, where toluene caused increase in aniline hydroxylase and cytochrome P-450; o-xylene in aminopyrine N-demethylase and cytochrome b5; m-xylene and mesitylene in all the enzymes investigated. In kidneys, all the compounds increased the activity of aniline hydroxylase; m-xylene induced cytochrome P-450 and b5 as well as NADPH-cytochrome c reductase; p-xylene induced cytochrome P-450, and mesitylene cytochrome P-450 and b5. Aminopyrine N-demethylase activity was decreased by toluene. In lungs, only mesitylene caused any significant differences from the controls: increase in aminopyrine N-demethylase and aryl hydrocarbon hydroxylase, decrease in aniline hydroxylase. The methylbenzenes tested induced the microsomal enzymes in a rough correlation to the number of their methyl groups and their hydrophobic properties.

The N-hydroxylation and ring-hydroxylation of 4-aminobiphenyl in vitro by hepatic mono-oxygenases from rat, mouse, hamster, rabbit and guinea-pig

1. An analytical h.p.l.c. method has been developed which permits the separation and quantification of the in vitro metabolites of 4-aminobiphenyl (4-ABP). The method employs gradient elution from a reverse phase column. 2. The major metabolite in vitro of 4-ABP in liver fractions from rat, mouse, guinea-pig, rabbit and hamster was N-hydroxy-4-aminobiphenyl. 3. The observation that liver fractions from the guinea-pig are very effective in the n-hydroxylation of 4-ABP is in excellent agreement with the 1966 report from Kiese's laboratory, showing that the N-hydroxylation of 4-ABP is an important metabolic pathway in vivo in this species. 4. The ortho-phenol, 3-hydroxy-4-aminobiphenyl was also an important metabolite in each species except guinea-pig and rabbit. 5. Hydroxylation at the 4' and 2' positions was a minor pathway in all species studied. 6. Aroclor 1254 was a potent inducer of N-hydroxylation in rat, mouse and guinea-pig but not hamster and rabbit. Phenobarbital induced N-hydroxylation in rabbit and guinea-pig but not rat, while methylcholanthrene induced in rat and guinea-pig but not rabbit.

Polychlorinated biphenyls as inducers of hepatic microsomal enzymes: structure-activity rules

A number of highly purified polychlorinated biphenyl (PCB) isomers and congeners were synthesized and administered to male Wistar rats at dosage levels of 30 and 150 mumol . kg-1. The effects of this in vivo treatment on the drug-metabolizing enzymes were determined by measuring the microsomal benzo[a]pyrene (B[a]P) hydroxylase, dimethylaminoantipyrine (DMAP) N-demethylase and NADPH-cytochrome c reductase enzyme activities, the cytochrome b5 content and the relative peak intensities and spectral shifts of the reduced microsomal cytochrome P-450 CO and ethylisocyanide (EIC) binding difference spectra. The results were compared to the effects of administering phenobarbitone (PB), 3-methylcholanthrene (MC) and PB plus MC (coadministered) to the test animals. The synthetic PCB congeners used in this study included 3,4,4',5-tetrachlorobiphenyl (TCBP-1), 2,3',4,4'-tetrachlorobiphenyl (TCBP-2), 2,3',4,4',5'-pentachlorobiphenyl (PCBP-1), 2,3,4,-4',5-pentachlorobiphenyl (PCBP-2), 2,3,3',4,4',5-hexachlorobiphenyl (HCBP-1) 2,3',4',5,6-hexachlorobiphenyl (HCBP-2), 2,3,3',5,5',6-hexachlorobiphenyl (HCBP-3), 2,2',3,5,5',6-hexachlorobiphenyl (HCBP-4) and 2,3,3',4,5,5'-hexachlorobiphenyl (HCBP-5) and were used to reappraise the structure-activity rules for PCBs as hepatic microsomal enzyme inducers. The results suggested that (a) PCBs which induce MC or mixed-type activity must be substituted at both para positions, at least two meta positions but not necessarily on the same phenyl ring and can also contain one ortho chloro substituent; (b) due to the considerable structural diversity of the PB-type inducers the rules for induction of this activity by PCB congeners are not readily defined.

Aryl hydrocarbon hydroxylase is inhibited by antibody to rat liver cytochrome P-450

Antibody to the major purified cytochrome P-450 induced by 3-methylcholanthrene in rat liver strongly inhibits aryl hydrocarbon hydroxylase activity of uninduced and benz[a]anthracene-induced human monocytes and lymphocytes. Antibody to the cytochrome P-450 induced by phenobarbital has relatively little or no effect on the aryl hydrocarbon hydroxylase activity of the same human cells.