The interference of polychlorinated biphenyls (Aroclor 1254) with membrane regulation of the activities of cytochromes P-450C21 and P-450(17) alpha,lyase in guinea-pig adrenal microsomes.

Regulation of cytochromes P-450 21-hydroxylase (P-450C21) and P-450 17 alpha-hydroxylase/C17,20-lyase (P-450(17) alpha,lyase) activities and impairment of this regulation by Aroclor 1254 was studied in guinea-pig adrenal microsomes. In a membrane depleted system, a decrease in the normally predominant, P-450C21 activity and an increase in P-450(17) alpha,lyase activities was observed. The same deviations were observed in intact microsomes with increase in the reaction temperature (0-40 degrees C). Breaks in Arrhenius plots for activities of P-450C21 and P-450(17) alpha,lyase correlate with transition temperatures reported for the microsomal membrane. These results point to: (1) preference of a gel state membrane for catalytic expression of P-450C21 suggesting a clustered organization of this P-450 species with reductase; (2) preference of a fluid membrane for lyase activity suggesting a random collision mechanism for reduction of P-450(17) alpha,lyase. Aroclor 1254 introduced to reaction mixtures containing intact microsomes elicited basically the same changes as caused by depletion of the microsomal membrane or by increase in the incubation temperature. Lack of effect of Aroclor 1254 on P-450C21 and P-450(17) alpha,lyase activities in the membrane depleted system demonstrates that its interference with monooxygenase activities is mediated by the microsomal membrane. The similarities between altered cytochrome P-450 mediated activities in the presence of Aroclor 1254 and the deviations observed in the membrane depleted system or upon increase in the incubation temperature may suggest that this chemical exerts its impacts by influencing membrane fluidity.     

Postnatal development of the mixed function oxidase system in nestling barn owls and baby chicks and induction of this system in the mature forms by Aroclor 1254

Postnatal changes in content and activity of the mixed-function oxidase system in nestling barn owls and baby chicks showed the following: 1. Increase in liver weight in both. 2. Significantly higher cytochrome P-450 level in 1-day old chicks but lower in 1-day old owls than at any other age. 3. Ratio of cytochrome b5 to P-450 was lower than one in nestling owls but higher than one in chicks. 4. Aroclor 1254 (PCBs) increased the level and catalytic activity of cytochrome P-450 more in the owl than in the chick. 5. The ratio 455:430 nm characteristic of ethylisocyanide binding was not altered in the barn owl due to PCB treatment but changed significantly in the treated chicken.     

Differential time course of induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by Aroclor 1254

The time course of induction of rat liver microsomal cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase has been determined in immature male rats administered a single large dose [1500 mumol (500 mg)/kg body wt] of the polychlorinated biphenyl mixture Aroclor 1254. Differential regulation of these xenobiotic-metabolizing enzymes was indicated by their characteristic patterns of induction. The rate of induction of cytochrome P-450a and epoxide hydrolase was relatively slow, and steady-state levels of these enzymes were maintained from approximately Days 9 to 15 after Aroclor 1254 treatment. In contrast, cytochrome P-450c was maximally induced 2 days after Aroclor 1254 treatment and remained at a constant level through Day 15. Steady-state levels of cytochrome P-450d, beginning 1 week after Aroclor 1254 treatment, were preceded by a fairly rapid rate of induction and possibly by a small decline from maximal levels observed around Days 4 to 5. Like those of the other cytochrome P-450 isozymes and epoxide hydrolase, the levels of cytochromes P-450b + P-450e were constant from Day 9 to 15 after Aroclor 1254 treatment. However, an unexpected but reproducible decline (approximately 25%) in total cytochrome P-450 content observed between Days 4 and 9 after Aroclor 1254 treatment principally reflected a dramatic and totally unanticipated decrease (approximately 45%) in the level of cytochromes P-450b + P-450e. This transient decline in the level of cytochromes P-450b + P-450e was not due to an unusual effect of a mixture of polychlorinated biphenyls, since identical results were obtained with two individual congeners, namely 2,3,4,5,4'-penta- and 2,3,4,5,3',4'-hexachlorobiphenyl, that induced the same isozymes as Aroclor 1254. In contrast, when rats were treated with 2,4,5,2',4',5'-hexachlorobiphenyl, which induces cytochromes P-450a and P-450b + P-450e and epoxide hydrolase but not cytochromes P-450c or P-450d, maximal levels of cytochromes P-450b + P-450e were attained on Day 4 and no decrease was observed over the next 11 days. These results suggest that there may be an interaction in the regulation of induction of certain individual cytochrome P-450 isozymes.     

Selective loss of pulmonary cytochrome P-450I in rabbits pretreated with polychlorinated biphenyls

The polychlorinated biphenyls mixture, Aroclor 1254, generally considered a powerful inducer of rat hepatic and pulmonary microsomal monooxygenases, caused a 70% decrease in ethylmorphine N-demethylase activity, a 31% decrease in benzo(a)pyrene hydroxylase activity, and a 42% decrease in cytochrome P-450 content in rabbit lung microsomes. When pulmonary cytochrome P-450 was solubilized and subjected to column chromatography, the elution profiles of the two forms of the hemeprotein showed a marked decrease in cytochrome P-450I in treated rabbits, with no significant alteration in cytochrome P-450II content. These data were confirmed by subjecting the two cytochromes to gel electrophoresis and staining the electrophoretic bands for protein and heme-associated peroxidase activity. Cytochromes P-450I and P-450II isolated from Aroclor 1254-treated rabbits showed differences in spectral properties as well as in their stabilities. The CO difference spectral determinations showed absorbance maxima at 452 and 450 nm for cytochromes P-450I and P-450II, respectively. At room temperature, cytochrome P-450II was much more stable than P-450I. The present studies provide evidence not only for species differences in the biological actions of the polychlorinated biphenyls but also demonstrate differential effects of the environmental pollutant on the two major forms of cytochrome P-450 and associated enzymic activities in rabbit lungs.     

Application of the SOS umu-test in detection of pollution using fish liver S9 fraction

1. The possibility of Aroclor 1254 and beta-naphthoflavone treated fish Mugil auratus and fish sampled in low and high polluted areas to convert some premutagens to active intermediers in the SOS umu-test have been investigated. 2. Genotoxicity of Aflatoxin b1 differed markedly upon activation with liver S9 fractions from fish with different pollution histories, with the highest activation potency in fish living near a fish cannery. 3. Inhibition of umu gene expression by 7,8-benzoflavone in vitro clearly demonstrates a cytochrome P-450 mediated activation of aflatoxin b1. 4. 2-Aminoanthracene and 2-aminofluorene were weakly activated to genotoxic products and the induction of umu gene expression could be detected only in the presence of S9 fractions from fish treated with beta-naphthoflavone and Aroclor 1254 in the laboratory. 5. The capability of S9 from fish living near a fish cannery to convert 2-aminoanthracene and 2-aminofluorene was not observed.     

Content of cytochrome P-450 and its induction capacity in primary liver tumors in rats

The content of cytochrome P-450 has been measured in primary hepatomas induced by diethylnitrosamine. As a rule, the enzyme content in hepatomas was decreased, as compared to normal liver and tumor-affected liver, but some hepatomas contained cytochrome P-450 in greater amount than normal tissue. Aroclor 1254 induced an increase in cytochrome P-450 content, which was identical in hepatomas, normal liver and tumor-affected liver. The dependence of hepatoma morphology on cytochrome P-450 content was not detected.     

Rat pulmonary microsomal cytochrome P-450 enzymes involved in the activation of procarcinogens.

Rat lung microsomal cytochrome P-450 (P-450) enzymes have been characterized with regard to their catalytic specificities towards activation of several procarcinogens to genotoxic metabolites in Salmonella typhimurium TA1535/pSK1002. We first examined the roles of rat liver microsomal P-450 enzymes in the activation of benzo[a]pyrene and its 7,8-diol enantiomers to genotoxic products, and found that P-450 1A1 is a major catalyst for the activation of these potential procarcinogens in rat livers. Using lung microsomes isolated from rats treated with various P-450 inducers we obtained evidence that at least three P-450 enzymes are involved in the activation of several procarcinogens. Immunoinhibition studies support the view that benzo[a]pyrene and its 7,8-diol derivatives, other dihydrodiol derivatives of polycyclic aromatic hydrocarbons, and 3-amino-1-methyl-5H-pyrido[4,3-b]indole are activated to genotoxins mainly by rat P-450 1A1, which is inducible in rat lungs by 5,6-benzoflavone and the polychlorinated biphenyl mixture Aroclor 1254. Activation of 2-amino-3,5-dimethylimidazo[4,5-f]quinoline and 2-amino-3-methylimidazo[4,5-f]quinoline may be catalyzed by another P-450 enzyme because the activities were not induced by treatment with 5,6-benzoflavone or Aroclor 1254. The observation that both activities were inhibited by antibodies raised against P-450 1A2 and by 7,8-benzoflavone suggests a role for an enzyme of P-450 1A family, probably P-450 1A2, in rat lung microsomes. The activation of aflatoxin B1 and sterigmatocystin appears to be catalyzed by other P-450 enzyme(s) rather than the P-450 1A family as judged by the different responses of activities to the P-450 inducers and the specific antibodies in rat lung microsomes. Interestingly, lung microsomal activation of several procarcinogens was found to be suppressed in rats treated with isosafrole and pregnenolone 16 alpha-carbonitrile. Thus, the results support the roles of different P-450 enzymes in the activation of procarcinogens in rat lung microsomes.     

In vitro metabolism of [14C]4-chlorobiphenyl and [14C]2,2',5,5'-tetrachlorobiphenyl by hepatic microsomes from rats and pigeons. Evidence against an obligatory arene oxide in aromatic hydroxylation reactions.

1. The catalytic activities of cytochromes P-450IA1 and P-450IIB1 in control and Aroclor 1254 treated rats and pigeons (1 mmol/kg) were assessed using [14C]4-chloro- and [14C]2,2',5,5'-tetrachlorobiphenyl as substrates. Treatment of rats resulted in increases of the total amount of chloroform-extractable metabolites of [14C]4-chlorobiphenyl from 37.2 (control) to 199.4 and 221.6 nmol/hr per mg microsomal protein at 48 and 120 hr post treatment. The portion of [14C]4-chloro-3',4'-dihydroxybiphenyl (M4) and of a second unidentified dihydroxylated metabolite (M3) increased during these incubations from 13.7% for controls to 53.5% at 48 hr and 69.12% at 120 hr post treatment. 2. [14C]4-chloro-3'-hydroxybiphenyl (M1) and [14C]4-chloro-4'-hydroxybiphenyl (M2) were the major metabolites formed by pigeon hepatic microsomes; however, the amounts formed were 38.7- and 29.3-fold less, respectively, than in untreated rats. Treatment of pigeons with Aroclor 1254 increased the metabolite formation from 1.0 (control) to 13.6 and 22.4 nmol/hr per mg microsomal protein at 48 hr and 120 hr post treatment respectively; however, only small amounts of metabolites M3 (0.5 nmol/hr per mg protein) and M4 (2.0 nmol/hr per mg protein) were detected. 3. Treatment of rats with Aroclor 1254 resulted in an approximately two-fold increase in the rate of metabolism of [14C]2,2',5,5'-tetrachlorobiphenyl, and the ratio of 3- to 4-hydroxylation increased from 0.45 (control) to 0.6 and 0.8 at 48 hr and 120 hr post treatment respectively. The rate of metabolism of [14C]2,2',5,5'-tetrachlorobiphenyl by control and Aroclor 1254 treated pigeons was up to 23-fold lower than in rats and there was no evidence for the formation of the diol metabolite M3. However, as with rats, the ratio of meta- to para-carbon atom hydroxylation increased from 0.58 (controls) to 0.72 at 120 hr post treatment. 4. From the evidence presented, it is suggested that cytochromes P-450IA1 and P-450IIB1 may not metabolize PCB-congeneric substrates via an obligatory arene oxide intermediate.     

Immunoreactive cytochrome P-450(17 alpha) in rat and guinea-pig gonads, adrenal glands and brain.

The cytochrome P-450(17 alpha)-hydroxylase, 17----20 lyase (P-450(17 alpha)) is the key enzyme responsible for the biosynthesis of androgens in steroidogenic organs. Its cellular localization has been examined with an immunohistochemical technique. In immature rat ovary, P-450(17 alpha) was first detected in sparse interstitial cells on postnatal Day 8. The number of immunoreactive interstitial cells increased thereafter and the intensity of P-450(17 alpha) staining in these cells was highest at 3 weeks of age. The intensity of staining then started to decline and was very faint at Day 35. From 6 weeks on, the distribution of immunoreactive P-450(17 alpha) was of the adult type: it was detected exclusively in the thecal cells of the large antral, preovulatory, follicles. P-450(17 alpha) was not detectable during pregnancy except on the day of parturition, when thecal cells were transiently immunoreactive. The staining had vanished 24 h after delivery. Human chorionic gonadotrophin (hCG), injected into immature females on Days 24 to 26, induced P-450(17 alpha) prematurely in thecal cells. When injected on Days 12 to 14 of pregnancy, hCG also induced P-450(17 alpha) in the thecal cells surrounding the largest follicles, whereas the interstitial and luteal cells were not immunostained. The antiprogestin RU486, injected on Day 16 of pregnancy, reinstated P-450(17 alpha) (and P-450scc) immunoreactivity in the thecal cells. Oestradiol selectively suppressed P-450(17 alpha) expression in the thecal cells of RU486-treated females. In immature guinea-pig ovary, P-450(17 alpha) was immunostained in thecal cells, not in interstitial cells, although the interstitial cells expressed the delta 5-3 beta-hydroxysteroid dehydrogenase. P-450(17 alpha) was also immunolocalized in the Leydig cells of rat and guinea-pig testes, and in the guinea-pig adrenal cortex (zonae fasciculata and reticularis), but not in the rat adrenal cortex. P-450(17 alpha) was not detectable in the brain of either rat or guinea-pig.     

Cytochrome P-450 isoforms in the liver of rats treated with phenobarbital, 3-methylcholanthrene and Aroclor 1254 studied by monoclonal antibodies

Seven types of monoclonal antibodies to cytochrome P-450 were obtained from the rat liver. Liver microsomal samples from intact rats and those pretreated with phenobarbital, 3-methylcholanthrene, Aroclor 1254, pregnenalone carbonitrile, beta-naphthoflavone and imidazole were stained with these antibodies using immunoblotting technique. The study made it possible to draw the following conclusions. Firstly, two types of these antibodies react with two cytochrome P-450 isoforms, P-450b and P-450 PB/PCN-E. Secondly, two types of antibodies react with three cytochrome P-450 isoforms: P-450a, P-450b and P-450PB/PCN-E. Antibodies of the latter three clones react with two cytochrome P-450 isoforms: P-450c and P-450d. Antibodies of all seven clones can be used for immunomorphological identification of cytochrome P-450 on rat liver paraffin sections.     

The Salmonella/mammalian microsome mutagenicity test: comparison of human and rat livers as activating systems

The mutagenicity of several test compounds was verified by the Salmonella/microsome mutagenicity test (Ames test), using both human liver and rat liver (untreated or pretreated with Aroclor 1254) S9 under identical experimental conditions. Aflatoxin B1, 3-methylcholanthrene, and cigarette-smoke condensate were less mutagenic in the presence of human-liver S9 than in the presence of rat-liver S9 (particularly after treatment with Aroclor 1254). The opposite was observed with 2-aminonanthracene and to a lesser degree with 2-aminofluorene; correlation studies indicate that the two compounds were activated by the same or by very similar enzymes, probably cytochrome P-450s. These results clearly indicate that human-liver S9, as an activating system, behaves differently than rat-liver S9; therefore, it may constitute a useful, additional tool for the study of mutagenicity and probably, carcinogenicity in man.     

Purification and characterization of liver microsomal cytochromes p-450: electrophoretic, spectral, catalytic, and immunochemical properties and inducibility of eight isozymes isolated from rats treated with phenobarbital or beta-naphthoflavone

Eight different forms of cytochrome P-450 (P-450) were purified to electrophoretic homogeneity by a common procedure from liver microsomes of rats treated with phenobarbital or beta-naphthoflavone. Antibodies were prepared to seven of these forms in rabbits. The eight P-450s were distinguished by spectral properties of the ferric, ferrous, and ferrous carbonyl forms, apparent monomeric molecular weights, peptide mapping, immunological reactivity as discerned by double-diffusion immunoprecipitin analysis and crossed immunoelectrophoresis, and catalytic activities toward the substrates acetanilide, aminopyrine, aniline, benzo[a]-pyrene, d-benzphetamine, N,N-dimethylnitrosamine, 7-ethoxycoumarin, 7-ethoxyresorufin, ethylmorphine, p-nitroanisole, testosterone, and (R)- and (S)-warfarin. Crossed sodium dodecyl sulfate-polyacrylamide gel immunoelectrophoresis was used to estimate the levels of each of the eight forms of P-450 present in the liver microsomes of untreated rats and rats treated with phenobarbital, 5,6-benzoflavone, pregnenolone-16 alpha-carbonitrile, isosafrole, or the polychlorinated biphenyl mixture Aroclor 1254. In each situation, the sum of the levels of these eight P-450s was at least as high as the spectrally determined P-450 content. The results clearly demonstrate that individual forms of P-450 can be induced by different compounds and that a single compound can lower the level of one form of P-450 while inducing one or more other forms of P-450. Catalytic activities toward each of the substrates observed with microsomal preparations are compared to rates predicted on the basis of the content of each of the eight P-450s. These studies provide a basis for further studies on the regulation of individual P-450s, the physical properties of the different P-450s, and the metabolic consequences of changes in the forms of P-450 in rat liver models.     

Evidence for the induction of cytochrome P-452 in rat liver by Aroclor 1254, a commercial mixture of polychlorinated biphenyls

We have examined the ability of a commercial mixture of polychlorinated biphenyls (Aroclor 1254) to induce hepatic cytochrome P-452-linked enzyme activities in rat and pigeon liver five days after its intraperitoneal injection. The results provide evidence that, at the doses used, Aroclor 1254 induces cytochrome P-452-linked enzyme activities in rats, but not in pigeons. This inductive effect was previously regarded as being specific for hypolipidemic drugs and phthalate ester plasticisers.     

Circulating thyroid hormone levels and iodothyronine deiodinase activities in Nile tilapia (Oreochromis niloticus) following dietary exposure to Endosulfan and Aroclor 1254

We evaluated the effects of two organochlorinated environmental contaminants, Endosulfan and Aroclor 1254 on peripheral thyroid hormone metabolism and thyroid hormone plasma levels in Nile tilapia (Oreochromis niloticus). Tilapia were exposed through diet to 0.1 and 0.5 microg g(-1) of Endosulfan and 0.5 microg g(-1) of Aroclor 1254 for 21 and 35 days. Decreased plasma T4 and rT3 levels were observed in tilapia exposed to the lower dose of Endosulfan, while treatment with a higher dose and Aroclor 1254 produced no changes. Plasma T3 levels were not affected by these compounds. Hepatic type I deiodinase (D1) activity was depressed by a lower dose of Endosulfan and hepatic type III (D3) activity was increased following 35 days of exposure to the lower dose of Endosulfan and following 21 and 35 days of exposure to Aroclor 1254; while type II (D2) remained unchanged in liver as well as in all other organs analysed. Apart from hepatic D3 activity, Endosulfan and Aroclor 1254 also increased D3 activity in gill, but not in other tested organs. It is concluded that dietary exposure of tilapia to Endosulfan or Aroclor 1254 can lead to changes in circulating thyroid hormone levels and/or in peripheral thyroid hormone metabolism. The changes in hormone metabolism differ between tissues, eventually reflecting tissue-specific differences in adaptation.     

Drug metabolism in adult white leghorn hens--response to enzyme inducers

1. Adult hens were given a mixture of polychlorinated biphenyls (0.5 and 2 g/kg Aroclor 1254), 3-methylcholanthrene (80 mg/kg) or beta-napthoflavone (80 mg/kg). 2. beta-Napthoflavone elevated activities of both microsomal and nonmicrosomal enzymes 48 hr after dosing, with cytochrome P-450 p-nitroanisole O-demethylase, aniline hydroxylase, and glutathione S-epoxytransferase at 319% +/- 25, 157% +/- 12, 410% +/- 26 and 120% +/- 3 of control values, respectively. 3. Aroclor 1254 and 3-methylcholanthrane also elevated microsomal enzyme activities, but did not increase the activity of glutathione S-epoxytransferase. 4. Drug metabolizing capability in control and experimental hens differed from that in rats and mice.     

Multiple constitutive but phenobarbital-inducible rat hepatic nuclear RNA species homologous to a novel cytochrome P-450 cDNA

A cDNA clone, pPB8, representing partial information for a phenobarbital-inducible rat hepatic cytochrome P-450, immunochemically related to cytochrome P-450b and/or P-450e, hybridized to multiple hepatic nuclear RNA species. In addition to the 3.7 +/- 0.2 kb mRNA encoding this novel cytochrome P-450 isozyme, pPB8 hybridized to nuclear RNAs of 4.9 +/- 0.3, 5.4, 5.7 +/- 0.2, and 6.3 +/- 0.1 kb. These nuclear RNAs were constitutively expressed and were inducible to various extents by phenobarbital administration. The time course of induction of these nuclear RNA components suggested product-precursor relationships. A "full-length" cDNA clone, pPB8/7, synthesized from poly(A)+ RNA homologous to pPB8, detected two mRNA species of 4.6 and 1.8 kb. The 4.6 kb nuclear RNA was inducible by 3-methylcholanthrene, Aroclor 1254, and phenobarbital, while the 1.8 kb nuclear RNA was not appreciably affected. It is suggested that pPB8 and pPB8/7 were synthesized from distinct mRNAs that share homology in their 3' regions.     

Regulation of cytochrome P-450j, a high-affinity N-nitrosodimethylamine demethylase, in rat hepatic microsomes

Polyclonal antibodies were produced in rabbits against purified cytochrome P-450j isolated from isoniazid-treated adult male rats. The monospecificity of immunoadsorbed antibody to cytochrome P-450j was demonstrated by Ouchterlony double diffusion analyses, enzyme-linked immunosorbent assays, and immunoblots. Immunoquantitation results indicated that rat liver microsomal cytochrome P-450j content decreases between 3 and 6 weeks of age in both the male and female animal. Several xenobiotics, such as Aroclor 1254, mirex, and 3-methylcholanthrene, repressed cytochrome P-450j levels when administered to male rats. Isoniazid, dimethyl sulfoxide, pyrazole, 4-methylpyrazole, and ethanol were inducers of cytochrome P-450j in rat liver although these compounds showed different inducing potencies. Microsomes from adult male rats with chemically induced diabetes also contained elevated levels of cytochrome P-450j compared to untreated animals. Cytochrome P-450j levels were measurable in kidney, whereas this isozyme was barely detectable in lung, ovaries, and testes; however, extrahepatic cytochrome P-450j was inducible by isoniazid. Approximately 80-90% of microsomal N-nitrosodimethylamine demethylation was inhibited by antibody to cytochrome P-450j whether the microsomes were isolated from untreated rats or animals administered inducers or repressors of cytochrome P-450j. The residual catalytic activity resistant to antibody inhibition may be a reflection of the inaccessibility of a certain amount of cytochrome P-450j due to interference by NADPH-cytochrome P-450 reductase based on results obtained with the reconstituted system. There was a good correlation (r2 = 0.87) between cytochrome P-450j content and N-nitrosodimethylamine demethylase activity in microsomes from rats of different ages and treated with various xenobiotics. The evidence presented indicates that cytochrome P-450j is the primary, and perhaps sole, microsomal catalyst of N-nitrosodimethylamine demethylation at substrate concentrations relevant to hepatocarcinogenesis induced by N-nitrosodimethylamine.     

Evidence for the induction of fatty acid desaturation in proliferating hepatic endoplasmic reticulum in response to treatment with polychlorinated biphenyls. Are fatty acid desaturases cytochrome P-450-dependent monooxygenases?

1. Polychlorinated biphenyls (PCBs) are abundant and persistent pollutants in the ecosystem which accumulate in biological systems. 2. We have shown previously (Borlakoglu et al., 1990; Eur. J. Biochem. 118, 327-332) that 120 hr after treating pigeons and rats with 1.5 mmol Aroclor 1254/kg body weight, hepatic microsomal membranes showed significant increases in the proportion of arachidonate (20:4,5, 8,11,14), in the concentration of cytochrome P-450 and in the activities of a wide range of cytochrome P-450-dependent enzymes involved in the metabolism of drugs and other xenobiotics. 3. After treating pigeons and rats in vivo with Aroclor 1254, linoleate desaturases activity increased significantly 3.35-, 4.35-, 5.83- and 8.61-fold 24, 48, 68 and 120 hr for pigeons and 2- and 7-fold for rats respectively 48 and 120 hr post treatment. The total activity of linoleate desaturases in the whole liver of pigeons and rats increased 40- and 10-fold respectively. 4. There were excellent correlations between the concentrations of cytochrome b5 and cytochrome P-450 and the activity of pigeon linoleate desaturases. Extrapolation of the concentration of cytochrome P-450 to zero is coincident with zero linoleate desaturase activity. 5. Evidence is presented to suggest the novel concept that linoleate desaturation is dependent upon the catalytic cycle of these monooxygenases.     

Interaction of 9-hydroxyellipticine with two forms of partially purified rabbit lung cytochrome P-450. Inhibition of lung microsomal benzo[a]pyrene hydroxylase.

9-Hydroxyellipticine (9-OHE), a potent inhibitor of rat liver monooxygenase activities, binds to the various forms of partially purified lung cytochromes P-450 from untreated and 3-methylcholanthrene (3-MC)-treated rabbits. The spectral data (lambda max: 428 nm (ox.), 447 nm (red.), Ks: 10 microM and 5 muM for cytochrome I and cytochrome II from 3-MC-treated rabbits respectively) resemble those obtained with cytochrome P-450 purified from liver of Aroclor 1254-pretreated rats (lambda max: 428 nm (ox.), 445 nm (red.), Ks: 8 microM). 9-OHE has been shown to inhibit the benzo[a]pyrene hydroxylase activity of rat and rabbit lung microsomes. The inhibitory effect was higher towards the 3-MC-induced lung microsomes than with the control microsomes. However, the lung microsomes, as well as the liver microsomes of rabbits were less sensitive to inhibition by 9-OHE than the corresponding microsomes from rats. These results suggest that rabbit and rat cytochromes P-450 have subtle structural differences.