Spectral evidence for weak ligand in sixth position of hepatic microsomal cytochrome P-450 low spin ferric iron in vivo

Effects of sulphydryl reagents on the formation of the aniline metabolite 4-aminophenol and its sulphate and glucuronide conjugates in primary cultures of rat hepatocytes

1. The effects of various sulphydryl-blocking reagents on aniline biotransformation and cytochrome P-450 levels were studied in cultured rat hepatocytes. 2. Exposure of aniline-metabolizing hepatocytes to p-chloro-mercuribenzoate (PCMB) or p-chloromercuribenzenesulphonic acid (PCMBS) resulted in decreased levels of cytochrome P-450, decreased glucuronidation of 4-aminophenol and increased levels of free 4-aminophenol. 3. Incubation of aniline-metabolizing hepatocytes with disulfiram resulted in decreased formation of 4-aminophenol, but this was not associated with impaired glucuronidation or cytochrome P-450 levels. 4. Exposure of aniline-metabolizing hepatocytes to mersalyl, 2,2'-dithiodipyridine (DTP), 6,6'-carboxydipyridine disulphide (CPDS) or N-ethylmaleimide did not affect the biotransformation of aniline or cytochrome P-450 levels. 5. Metyrapone prevented degradation of cytochrome P-450. Exposure of cells to SKF-525 A inhibited aniline biotransformation without altering cytochrome P-450 levels. 6. PCMB and PCMBS inhibited aniline metabolism, probably by binding to a cysteinyl-SH residue in cytochrome P-450 apoenzyme and 'active sites' of UDP-glucuronyl transferases. Disulfiram inhibited aniline biotransformation, probably indirectly by diminishing NADPH.

Dependence of hydrogen peroxide formation in rat liver microsomes on the molecular structure of cytochrome P-450 substrates: a study with barbiturates and beta-adrenoceptor antagonists

In the present study, the molecular structure of xenobiotics has been successfully linked to their effect on the oxidase activity of cytochrome P-450, determined as microsomal hydrogen peroxide formation. A homologous series of 5-alkyl-5-ethyl barbiturates and a heterologous series of beta-adrenoceptor antagonists was used. The logarithm of the rate of microsomal hydrogen peroxide formation could be correlated with the logarithm of the apparent partition (n-octanol/buffer) coefficient for the barbiturate derivatives according to a parabolic function. The statistics of the correlation improved considerably by applying a bilinear model in order to fit the data. This probably indicates that both transport of the substrate to cytochrome P-450 and interaction with hydrophobic substrate binding sites of cytochrome P-450 are involved in the modulating effect of substrates on the oxidase function of cytochrome P-450. With the series of beta-adrenoceptor antagonists no clear-cut structure activity relationship with regard to the oxidase activity was apparent at first sight. However, when the inhibitory effect of the beta-antagonists on the 'cytochrome P-450 metabolic intermediate (MI) complex' formation that occurs during the microsomal biotransformation of 33 microM tofenacine was studied a relationship with the lipophilicity could be demonstrated. It is known that MI complex formation occurs with specific subforms of cytochrome P-450. By using this inhibitory activity of the beta-adrenoceptor antagonists, the interaction of the compounds becomes restricted to these specific subforms of cytochrome P-450. In both the oxidase activity as well as the MI complex formation phenobarbital induced cytochrome P-450 is involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Modifications of hepatic microsomal 9-oxidation of N-2-fluorenylacetamide in response to gonadectomy and treatment of rats with phenobarbital or diethylnitrosamine

1. The hepatic microsomal 9-hydroxylation of N-2-fluorenylacetamide (2-FAA) is greater in the presence of male, or absence of female, hormones. Thus, 9-hydroxy-2-FAA was the major microsomal metabolite of male rats, which formed 6-fold greater amounts than did female rats. One week after gonadectomy, the amount of 9-hydroxy-2-FAA formed by male rats was decreased by 61%, whereas that formed by female rats was increased 1.4-fold. 2. Treatment of rats with phenobarbital (PB) increased 2- to 3-fold the capacities of hepatic microsomes of both sexes (normal and gonadectomized) for 9-hydroxylation of 2-FAA. 3. Hepatic microsomes of male rats also had greater capacities to form 9-oxo-2-FAA, the metabolite of 9-hydroxy-2-FAA, and 6-hydroxy-2-FAA, a newly identified microsomal metabolite of 2-FAA. These metabolites were also decreased by orchidectomy and induced by PB. 4. 9-Hydroxy-2-FAA was a poor substrate for hepatic microsomal UDP-glucuronyltransferase, and conjugation was not induced by treatment of rats with PB. This indicated retention of 9-hydroxy-2-FAA in the liver and/or further metabolism (e.g. to 9-oxo-2-FAA). 5. The formation of 9-oxo-2-FAA from 2-FAA or 9-hydroxy-2-FAA was increased (1.5-fold) two weeks after treatment of male rats with a single i.p. dose of diethylnitrosamine (200 mg/kg), an initiator of hepatocarcinogenesis. 6. Based on the data we suggest that 9-oxidized metabolites of 2-FAA, the preferential formation of which coincides with the susceptibility of the rat to hepatocarcinogenesis, are promoters.

Dual ligand binding of pyridylalkanamides to microsomal cytochrome P-450

Twelve homologous and regioisomeric pyridylalkanamides were examined spectrally for their binding affinity to cytochrome P-450 in phenobarbital- and 3-methylcholanthrene-induced rat liver microsomes. The pKs values were calculated by the Lineweaver-Burk method and by non-linear analysis using both a one ligand-one acceptor and a one ligand-two acceptor model. The latter model best fits most of the data, confirming that two pKs values exist for most derivatives in the 3-pyridyl and 4-pyridyl series. Structure-binding relationships are discussed. The two binding constants are hypothesized to arise from a dual mode of binding to the ferric ion. At low ligand concentrations, binding to hexacoordinated cytochrome P-450 occurs and involves displacement of an endogenous 6th ligand; at higher concentrations, the ligands bind to the pentacoordinated P-450, resulting in a high-to-low spin shift.

Steroid interactions with cytochrome P-450 from testis microsomes

The cytochrome P-450 of gonadal microsomes is an integral component of the steroid converting enzymes, 17 alpha-hydroxylase and 17,20-lyase. Interaction of the steroid substrates with this cytochrome results in a shift in the Soret band as measured by difference spectroscopy. In these studies it is shown that in contrast to placental microsomal cytochrome P-450 which binds C19 steroids, testis microsomal cytochrome P-450 primarily binds C21 steroids. However, addition of a 17 alpha- methyl, 17 beta-acetate or a 17 beta-benzoate group to testosterone permits interaction. The addition of hydroxyl or methyl groups to other positions does not affect binding. The presence of multiple oxygen functions on C21 steroids, as in cortisol and corticosterone, precludes interaction. At least one oxygen function seems necessary for binding as 5 alpha- and 5 beta-pregnane do not bind whereas 20-deoxypregnenolone (5-pregnen-3 beta-ol) does bind. These findings indicate that factors in addition to hydrophobic interactions dictate the binding of steroid substrates to testis microsomal cytochrome P-450.

Changes in cytochrome P-450 iron spin state as a function of castration, testosterone or estrogen treatment, vigorous exercise, or starvation

3-Methylcholanthrene treatment induced large increases in high-spin P-450 form(s) in intact microsomal membranes from New Zealand White rabbit or C57BL/6N mouse liver but not in rabbit lung, kidney or intestine. Testosterone enhanced the high-spin/low-spin ratio of P-450 in female C57BL/6N liver to the same degree as 3-methylcholanthrene treatment; there was no additive increase in the ratio with treatment of testosterone plus 3-methylcholanthrene. The same effect was seen in ovariectomized females. In the liver of normal or castrated C57BL/6N mice, estradiol-17 beta pretreatment caused no significant change in the high-spin/low-spin ratio. Vigorous homogenization of the microsomes can decrease the high-spin/low-spin ratio, more so in control than in 3-methylcholanthrene-induced C57BL/6N microsomes. Vigorous exercise (20-min swim) and fasting for 2 days significantly enhanced the high-spin/low-spin ratio in C57BL/6N liver. Temperature studies showed that the low-spin electron paramagnetic resonance signal intensity, as a function of absolute temperature in the range of 16 to 70 degrees K, was consistent with a simple Boltzmann dependence within the Kramers doublet giving rise to the microwave transition. We conclude that, within the limits of measurement, the low-spin state acts like an isolated state and is not in thermal equilibrium with the high-spin form of P-450.

Inhibition of lipid peroxide decomposition by compounds which bind with cytochrome p-450

The kinetics of lipid peroxide decomposition catalysed by microsomal enzymes and inhibited by SKF-525 A, hexobarbital, phenobarbital and aniline were investigated. The results indicate that the in vitro interaction of hexobarbital and SKF-525 A (type I binding compounds) with microsomal cytochrome p-450 inhibits the peroxidase activity while the in vitro interaction of aniline (type II binding compound) only slightly affect the peroxidase activity. It is suggested that LAHPO and type I binding compounds are competing for the hydrophobic binding site on cytochrome p-450, while type II binding compounds such as aniline negate electron transfer non-competitively by combining with the heme.

Comparison of the effects of inhibitors of cytochrome P-450-mediated reactions on human platelet aggregation and arachidonic acid metabolism

Metyrapone and SKF-525A, together with amphenone B, a structural analogue of metyrapone, which are all inhibitors of cytochrome P-450-mediated reactions, were shown to inhibit the arachidonic acid-induced aggregation of human platelets. Amphenone B, like metyrapone, exhibited a type II (ligand) binding spectrum with rat liver microsomal cytochrome P-450, in contrast to SKF 525A which is a type I (substrate) binding agent. Independently of their type of binding spectra and of their maximum spectral change, however, the affinity of the three compounds for rat liver cytochrome P-450 showed a close proportional correlation with their platelet aggregation inhibitory potency. All three compounds inhibited the formation of [1-14C]thromboxane B2 from [1-14C]arachidonic acid by human platelets aggregated with collagen. The effect of metyrapone on the remaining labelled products suggested that it is a selective thromboxane synthesis inhibitor, while amphenone B exhibited activity reminiscent of cyclo-oxygenase inhibitors. SKF 525A produced complex effects possibly attributable to cyclo-oxygenase inhibition and enhanced lipid peroxidation, since it also enhanced platelet malonaldehyde formation, which the other two compounds inhibited. These data provide further support for a role of cytochrome P-450 in thromboxane synthesis and platelet aggregation.

Inhibition by norharman of metabolism of benzo[a]pyrene by the microsomal mixed function oxidase of rat liver

The effect of norharman on the metabolism of ethyl acetate-soluble metabolic intermediates of benzo[a]pyrene (BP), 9,10-dihydro-9,10-dihydroxybenzo[a]pyrene (9,10-diol), 4,5-dihydro-4,5-dihydroxybenzo[a]pyrene (4,5-diol), 7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (7,8-diol), benzo[a]pyrene diones, 3-hydroxybenzo[a]pyrene (3-OH-BP) and 9-hydroxybenzo[a]pyrene (9-OH-BP), were studied. These metabolic intermediates were converted by microsomal enzymes to other more polar ethyl acetate-soluble metabolites and then finally to the water-soluble metabolites. Norharman inhibited markedly the disappearance of each metabolite added as a substrate. With high-pressure liquid chromatographic (HPLC) separation it was revealed that formation of more polar metabolite was more efficiently inhibited by norharman than the formation of less polar metabolite. Formation of water-soluble metabolite was most efficiently inhibited by norharman. The mechanisms of the inhibitory effect of norharman on BP metabolism were studied by difference spectroscopy. On the addition of norharman, microsomes showed a type II difference spectrum, while on the addition of BP, they showed a type I difference spectrum. 3-OH-BP and 4,5-diol also gave a type I spectrum. Thus both BP and its metabolites bind to the active center of P-450, whereas norharman binds to the sixth ligand position of the iron ion of P-450. Kinetic studies showed that the Km-value of microsomes for BP was 6.25 microM in the presence and absence of norharman. This indicated that norharman inhibits the metabolism of BP non-competitively.

Biotransformation of xenobiotics in Drosophila melanogaster and its relevance for mutagenicity testing

Biotransformation of lipophilic xenobiotics may lead to formation of reactive intermediates which can give rise to irreversible toxic events such as carcinogenesis, mutagenesis, teratogenesis, and tissue necrosis. In recent years considerable attention has been paid to the problem of testing for these effects. Short-term mutagenicity tests have been shown to have value for predicting the occurrence of delayed toxic effects in mammals following administration of indirectly acting harmful xenobiotics. In any test system the capacity to bioactivate the compound under test is a necessary prerequisite, and in most short-term test assays this is provided for by adding a metabolic activation system generally consisting of the 9,000 g supernatant fraction of a rat liver homogenate supplied with cofactors. The fruitfly Drosophila melanogaster constitutes an organism well-suited for mutagenicity testing, and it was shown that a number of precarcinogens evoke mutagenic effects in this species. Thus Drosophila is apparently able to metabolize xenobiotics to reactive intermediates, which in turn induce mutagenicity. However, knowledge about the presence and characteristics of the xenobiotic-metabolizing enzymes involved is lacking. Since knowledge of these enzymes contributes to the evaluation and interpretation of observed mutagenic events, this paper described studies concerning some important xenobiotic-metabolizing enzymes of Drosophila. Files were homogenized and subcellular fractions were investigated with respect to enzymatic activities. It was possible to demonstrate cytochrome P-450 and some related mixed-function oxidase activities. Cytochrome b5, epoxide hydrolase, and glutathione S-transferase are also present, while preliminary experiments suggest the presence of UDP-glucosyltransferase and phosphotransferase activities. The enzymes which have been found are discussed with respect to their similarities with rat liver enzymes and their relevance for mutagenicity testing with Drosophila melanogaster.