Chemical modification of the histidine residues of purified hepatic cytochrome P-450: influence on substrate binding and the haemoprotein spin state

A hepatic cytochrome P-450 isolated in an electrophoretically homogeneous form from phenobarbital-treated rats, exists predominantly in the low spin configuration (82% at 20 degrees C). The addition of saturating amounts of the substrate benzphetamine to this haemoprotein shifted the spin equilibrium to the high spin form, resulting in a doubling of the spin equilibrium constant from 0.220 to 0.539 at 20 degrees C. The histidine residues of this low spin, substrate-free cytochrome P-450 were modified in a time- and concentration-dependent manner with diethylpyrocarbonate, and progressive histidine modification resulted in a decrease of both the affinity and extent of substrate interaction with the haemoprotein. Although the histidine-modified haemoprotein maintained the capacity to undergo a temperature-dependent spin transition of the haem iron in the presence of saturating amounts of substrate, this capability was substantially decreased in comparison to the unmodified cytochrome. These results indicate that a histidine residue(s) is involved in the binding of substrate to cytochrome P-450 and hence interferes with the substrate-bound spin equilibrium. Our results further imply that histidine is probably not the sixth ligand of the substrate-free ferric form of the rat liver cytochrome P-450.     

Apparent lack of correlation between the loss of cytochrome P-450 in hepatic parenchymal cell culture and the stimulation of haem oxygenase activity

The hypothesis that the stimulation of haem oxygenase activity in cultured adult rat liver parenchymal cells is intimately associated with the accelerated breakdown of the haemoprotein cytochrome P-450 was examined. Even though the time course of the loss of cytochrome P-450 and the stimulation of haem oxygenase activity were found to be compatible with this hypothesis, further work however showed that high levels of cytochrome P-450 could be maintained in liver cell culture in the face of high haem oxygenase activities.     

Activity of NADPH-dependent monooxygenase hydroxylating system of rat liver under the effect of tetramethylthiuramdisulfide

Changes in the activity of a NADPH-dependent monooxygenase system of the rat liver are studied under the effect of tetramethylthiuramdisulphide. Under these conditions aniline hydroxylation is shown to be inhibited to a higher extent than amidopyrine demethylation. Besides a decrease in the level of cytochrome P-450, the central component of the microsomal system of hydroxylation, there appears cytochrome P-420--an inactivated form of cytochrome P-450.     

Effect of carbon source on the accumulation of cytochrome P-450 in the yeast Saccharomyces cerevisiae.

The appearance of cytochrome P-450 in the yeast Saccharomyces cerevisiae depended on the substrate supporting growth. Cytochrome P-450 was apparent in yeast cells grown on a strongly fermentable sugar such as D-glucose, D-fructose or sucrose. When yeast was grown on D-galactose, D-mannose or maltose, where fermentation and respiration occurred concomitantly, cytochrome P-450 was also formed. The cytochrome P-450 concentration was maximal at the beginning of the stationary phase of the culture. Thereafter the concentration decreased, reaching zero at a late-stationary phase. When the yeast was grown on a medium that contained lactose or pentoses (L-arabinose, L-rhamnose, D-ribose and D-xylose), cytochrome P-450 did not occur. When a non-fermentable energy source (glycerol, lactate or ethanol) was used, no cytochrome P-450 was detectable. Transfer of cells from D-glucose medium to ethanol medium caused a slow disappearance of cytochrome P-450, although the amount of the haemoprotein still continued to increase in the control cultures. Cytochrome P-450 appeared thus to accumulate in conditions where the rate of growth was fast and fermentation occurred. Occurrence of this haemoprotein is not necessarily linked, however, with the repression of mitochondrial haemoprotein synthesis.     

Prolonged induction of hepatic haem oxygenase and decreases in cytochrome P-450 content by organotin compounds.

The administration of organotin compounds to rats in single doses causes a significant and prolonged induction of haem oxygenase and a sustained decrease in haemoprotein content in the liver. The extent of induction of hepatic haem oxygenase varied between 3 and 5-fold at 72h after a single injection of water-insoluble organotins of differing structure. The alterations in haem metabolism produced by tricyclohexyltin hydroxide were studied in detail. The effects were dose-dependent, with doses as low as 3.75 mg/kg body wt. resulting in significant induction of haem oxygenase and a decrease in cytochrome P-450 and cytochrome b5 contents at 72h in the liver. The effects with time of a single dose of tricyclohexyltin on various parameters of liver haem metabolism were also examined. The organotin produced a substantial and very prolonged induction of haem oxygenase accompanied by a steady decline in cytochrome P-450 content for periods up to 8 days. The long duration of action of these organotins with respect to induction of haem oxygenase and depletion of cellular haemoprotein content provides a highly sensitive metabolic system with which to define further the toxic potential of organometals as well as to study the adaptive responses in liver to long-term perturbations of haem metabolism by foreign chemicals.     

Inhibition of protein synthesis: a basis for tunicamycin-induced decrease in rat liver cytochrome P-450

Tunicamycin caused a dose and time dependent decrease in cytochrome P-450 in rat liver. A dose of 50 micrograms/kg caused a decrease of about 50% in 72 hours. A similar decrease in the activities of rat liver microsomal aniline hydroxylase, aminopyrine N-demethylase and ethoxycoumarin O-deethylase were also seen after the tunicamycin treatment. Tunicamycin also suppressed food and water intake but the decrease in cytochrome P-450 was not related to these effects. NADPH cytochrome c reductase was not markedly decreased by tunicamycin. A decrease in cytochrome P-450 was also observed in cultured rat hepatocytes treated with tunicamycin. It decreased incorporation of [35S]-methionine into total proteins as well as into various cytochrome P-450 isozymes of rat hepatocytes. This indicates that a decrease in protein synthesis may be responsible for the tunicamycin-induced decrease in cytochrome P-450 and drug metabolism.     

The role of heme in the regulation of tryptophan-2,3-dioxygenase activity and content of cytochrome P-450 in rat liver

The effects of exogenous heme on the activity of delta-aminolevulinate synthase, heme oxygenase, tryptophan-2.3-dioxygenase and microsomal cytochrome content in rat liver were studied. It was shown that hemin chloride diminishes the delta-aminolevulinate synthase activity and provokes heme oxygenase induction. This is paralleled with the induction of the tryptophan 2.3-dioxygenase apoenzyme and an increase in the saturation of the enzyme with heme. The cytochrome b5 content does not change thereby, whereas that of cytochrome P-450 shows a decrease. Upon combined administration of actinomycin D and hemin the cytochrome P-450 level is markedly increased. Actinomycin D by itself has no effect on the hemoprotein concentration. It is concluded that the increase in the cytochrome P-450 level results from the activation of heme-induced mRNA translation.     

The heterogeneity of arylhydrocarbon hydroxylase in fetal liver microsomes of rats

The characteristic of arylhydrocarbon hydroxylase system in fetal liver microsomes of rat was investigated. NADH-synergistic effect on NADPH-dependent arylhydrocarbon hydroxylase was observed in fetal liver microsomes of rat but not in maternal liver microsomes. NADH-synergistic effect decreased in parallel with the decrease of the ratio of cytochrome b5/cytochrome P-450 in liver microsomes. The cytochrome P-450 in arylhydrocarbon hydroxylase system in fetal liver microsomes of rat seemed to be different from that in offspring liver microsomes in respect of its dependency on cytochrome b5 system for its maximum activity.     

Relation between the 7-ethoxyresorufin-O-deethylase activity of the liver microsomal monooxygenase system and the level of methylcholanthrene-induced form of cytochrome P-450

Changes in the metabolic activity of 7-ethoxyresorufin in rat liver microsomes containing different amounts of cytochrome P-450 induced by 3-methylcholanthrene and other polycyclic hydrocarbons (P-450c) were studied. Using antibodies to cytochrome P-450c for the determination of the cytochrome P-450c content and its metabolic role, it was demonstrated that 7-ethoxyresorufin O-deethylation by the liver microsomal monooxygenase system is catalyzed exclusively by cytochrome P-450c. The rate of the substrate metabolism is correlated with the cytochrome P-450c content in microsomal membranes; the cytochrome P-450c activity does not depend on the cytochrome P-450c/NADPH-cytochrome P-450 reductase ratio. The experimental results suggest that the level of 7-ethoxyresorufin metabolism in liver microsomes can be regarded as a measure of the cytochrome P-450c content, whose function is associated with the stimulation of potential carcinogenic and toxic substances.     

Differential induction of peroxisomal and microsomal fatty-acid-oxidising enzymes by peroxisome proliferators in rat liver and kidney. Characterisation of a renal cytochrome P-450 and implications for peroxisome proliferation

The induction of renal fatty-acid-oxidising enzymes has been investigated following short-term exposure to a group of structurally diverse peroxisome proliferators and compared to the more extensively documented hepatic responses in the rat. There was a marked compound dependence on induction of both cytochrome P-450-IVA1-dependent omega-hydroxylation of lauric acid and enzymes of the peroxisomal fatty acid beta-oxidation pathway (measured as cyanide-insensitive palmitoyl-CoA oxidation and enoyl-CoA hydratase). Cytochrome P-450 IVA1 (or a very closely related isoenzyme in the same gene family) was a major constitutive haemoprotein in rat kidney microsomes and actively supported the omega-hydroxylation of lauric acid. This activity was induced 2-3-fold by peroxisome proliferators such as clofibrate, di-(2-ethylhexyl)phthalate, bezafibrate and nafenopin. By using a cDNA probe to the cytochrome P-450 IVA1 gene in Northern blot analysis, we have shown that increased renal and hepatic omega-hydroxylation of lauric acid, after treatment with peroxisome proliferators is a consequences of a substantial increase in the mRNA coding for this haemoprotein. In addition, programming of an in vitro rabbit reticulocyte translation system with both renal and hepatic RNA resulted in the synthesis of similar (if not identical) cytochrome-P-450-IVA1-related polypeptides. Furthermore, we have provided Western blot evidence that both rat liver and kidney microsomes contain two closely related cytochrome P-450 IVA1 polypeptides, the major one characterised by a monomeric molecular mass of 51.5 kDa (identical to authentic, purified hepatic cytochrome P-450 IVA1) and a minor one of 52 kDa. The kidney-supported fatty acid omega-hydroxylase activity was refractory to inhibition by a polyclonal antibody to liver cytochrome P-450 IVA1, which may be related to the existence of two closely related (but immunochemically distinct) fatty acid hydroxylases in this tissue. Our studies have also demonstrated that certain of the compounds tested (including clofibrate, bezafibrate and nafenopin) induced renal fatty acid beta-oxidation, mirroring the increased omega-hydroxylase activity in the endoplasmic reticulum. Our studies have also indicated that the kidney was more refractory to induction of the endoplasmic reticulum and peroxisomal fatty-acid-oxidising enzymes than the liver. Taken collectively, our data is strongly suggestive of a possible linkage of the renal fatty acid oxidative enzymes in these two organelles, a situation that also occurs in the liver. In addition, our studies have provided a possible conceptual framework that may rationalise the decreased susceptibility of the k     

Effect of prior starvation on the development of acute thioacetamide-induced liver damage in rats

The effect of previous fasting on the liver morphological changes and microsomal cytochrome P-450 and b5 content was studied in thioacetamide-induced (100 mg/kg) rat liver necrosis. Starvation for 48 hours immediately before thioacetamide administration aggravates the dystrophic and necrotic processes, as revealed by histology, electron microscopic investigations and serum aminotransferase activity. The liver microsomal cytochrome P-450 concentration tended to decrease after thioacetamide challenge, with fasting resulting in a more significant loss of cytochrome P-450. Cytochrome b5 content, however, was found to increase in acute liver necrosis induced by thioacetamide.     

Liver cytochrome P-450 induction in rats by drug preparations and the body vitamin A allowance

The content of P-450 cytochrome and vitamin A was determined in the liver of mature male rats who received for a month daily parenteral administrations of phenobarbital solutions (Pb; 40 mg/kg), rheopyrine (a mixture of equal aliquots of amidopyrine and butadione; 200 mg/kg), amidopyrine (100 mg/kg) or water (control). The animals were kept on a semisynthetic diet, receiving once, every week 400 IU of retinol-palmitate per rat. Pb administration markedly (more than threefold) increased P-450 cytochrome content in the liver. Rheopyrine and amidopyrine also elevated its level, but to a lesser extent than Pb. Pb and rheopyrine also depressed vitamin A levels in the liver and caused either a trend towards its decrease (Pb) or a significant decrease (rheopyrine) of its overall content in the liver. The effect of amidopyrine on the concentration and storage of retinol in the liver was less pronounced. The data obtained suggest that the drugs inducing P-450 cytochrome are capable of disturbing vitamin A content in the body.     

A model for the regulation of δ-aminolaevulinate synthetase induction in rat liver

A reciprocal relationship exists between the cytochrome P-450 content and delta-aminolaevulinate synthetase activity in adult rats. In young rats the basal delta-aminolaevulinate synthetase activity is higher and the cytochrome P-450 content is lower compared with the adult rat liver. Administration of allylisopropylacetamide neither induces the enzyme nor causes degradation of cytochrome P-450 in the young rat liver, unlike adult rat liver. Allylisopropylacetamide fails to induce delta-aminolaevulinate synthetase in adrenalectomized-ovariectomized animals or intact animals pretreated with successive doses of the drug, in the absence of cortisol. The cortisol-mediated induction of the enzyme is sensitive to actinomycin D. Allylisopropylacetamide administration degrades microsomal haem but not nuclear haem. Haem does not counteract the decrease in cytochrome P-450 content caused by allylisopropylacetamide administration, but there is evidence for the formation of drug-resistant protein-bound haem in liver microsomal material under these conditions. Phenobarbital induces delta-aminolaevulinate synthetase under conditions when there is no breakdown of cytochrome P-450. On the basis of these results and those already published, a model is proposed for the regulation of delta-aminolaevulinate synthetase induction in rat liver.     

Studies on the mechanism of hepatic microsomal N-oxide formation. The role of cytochrome P-450 and mixed-function amine oxidase in the N-oxidation of NN-dimethylaniline.

Evidence is established for the existence of alternative metabolic routes of N-oxidation of NN-dimethylaniline in rabbit liver microsomal fraction. One pathway involves the participation of two types of cytochrome P-450 with different sensitivities towards heat. Both types may represent distinct haemoprotein species or two physical forms of a single pigment. The other pathway is represented by the mixed-function amine oxidase. The enzyme lacks NADPH dehydrogenase activity and is insensitive to treatment with 2-bromo-4'-nitroacetophenone and steapsin: it catalyses N-oxidation of imipramine, trimethylamine and NN-dimethylaniline in molar proportions considerably different from those of the cytochrome P-450-supported reactions. Cytochrome P-450 is estimated to account for the formation of at least 50-60% of the total NN-dimethylaniline N-oxide formed in the intact rabbit liver microsomal fraction, the remainder arising from the action of the mixed-function amine oxidase.     

Hormonal and developmental regulation of expression of the hepatic microsomal steroid 16 alpha-hydroxylase cytochrome P-450 apoprotein in the rat

The hormonal regulation of the sexually differentiated cytochrome P-450 isozyme which catalyzes 16 alpha-hydroxylation of testosterone and 4-androstene-3,17-dione in male rat liver (P-450(16) alpha) was investigated. Estradiol valerate injection of male rats caused a decrease in P-450(16) alpha levels to almost the female level, while methyltrienolone injection had the reverse effect in female animals. Hypophysectomy abolished the sex difference in P-450(16) alpha levels. Human growth hormone infusion into male rats, mimicking the female pattern of growth hormone secretion, caused a feminization of P-450(16) alpha levels. The same effect was also seen in hypophysectomized rats of both sexes. In contrast, a different administration schedule involving 12 h injections of human growth hormone, mimicking the male pattern of growth hormone secretion, caused a masculinization of P-450(16) alpha levels in hypophysectomized rats, at a daily dose which causes feminization when given by infusion. Thus, the level of expression of P-450(16) alpha in the liver is dependent on the temporal pattern of blood growth hormone levels. While infusion of rat growth hormone into male rats also feminized the P-450(16) alpha levels, infusion of ovine prolactin had no effect. Ontogenic studies showed that the developmental pattern of P-450(16) alpha expression in the liver coincided with the known pattern of development of the sexual differentiation of hepatic steroid 16 alpha-hydroxylase activity and of the diurnal pattern of growth hormone secretion.     

Influence of N,N-dimethylaniline on the association of phenobarbital-induced cytochrome P-450 and NADPH-cytochrome c(P-450) reductase in a reconstituted rabbit liver microsomal enzyme system

N,N-Dimethylaniline when added to reaction mixtures provokes deviation from Michaelis-Menten law of the interaction kinetics of NADPH-cytochrome c(P-450) reductase (NADPH:ferrihaemoprotein oxidoreductase, EC 1.6.2.4) with highly purified phenobarbital-induced rabbit liver microsomal cytochrome P-450 (P-450LM2). This phenomenon is not associated with the low-to-high spin transition in the iron-coordination sphere of the haemoprotein, as elicited by the arylamine. Substrate-triggered departure from linearity of the kinetics is abolished by inclusion into the assay media of p-chloromercuribenzoate, hinting at a vital role in the process of thiols. Similarly, the parabolic progress curve (nH = 1.7) is transformed to a straight line (nH = 1.01) when the N-terminal reductase-binding domain in the P-450LM2 molecule is selectively blocked through covalent attachment of fluorescein isothiocyanate (FITC); such a modification does not alter the affinity of the haemoprotein for the amine substrate. Steady-state fluorescence polarization measurements reveal that N,N-dimethylaniline perturbs the motional properties of the fluorophore-bearing reductase-binding region, suggesting the induction of a conformational change. Summarizing these results, the data possibly indicate N,N-dimethylaniline-induced cooperativity in the association of reductase with P-450LM2.     

The accumulation of distinct mRNAs for the immunochemically related cytochromes P-450c and P-450d in rat liver following 3-methylcholanthrene treatment

Treatment of rats with 3-methylcholanthrene leads not only to a marked accumulation in the liver of translatable mRNA coding for a 56-kilodalton polypeptide representing cytochrome P-450c, the major 3-methylcholanthrene-induced cytochrome P-450 of rat liver, but also to the accumulation of comparable amounts of mRNA encoding a 52-kilodalton polypeptide which is immunoprecipitated with antibodies prepared against rat liver cytochrome P-450c. Further electrophoretic and immunochemical characterization of the latter translation product demonstrates that it corresponds to cytochrome P-450d, the major isosafrole-induced form of rat liver cytochrome P-450. The mRNAs for cytochromes P-450c and P-450d can be completely separated by electrophoresis in denaturing agarose gels and have chain lengths of approximately 4000 and 2000 nucleotides, respectively. These two mRNAs do not show detectable sequence homology to the mRNAs coding for the major phenobarbital-induced forms of cytochrome P-450 (P-450b and P-450e) since in Northern blotting experiments they fail to hybridize under conditions of low to moderate stringency to cloned probes for the latter mRNAs.     

Effect of cytochrome b5 on the functional activity and conformational state of cytochrome P-450

Microsomal cytochromes P-450 and b5 were shown to form mixed complexes with the association constant of 0.24 microM in water solution. Such complex formation stabilizes cytochrome P-450 in the catalytically active conformational state characterized by increased conformational rigidity and temperature stability. This stabilization results in acceleration of the cumene hydroperoxide-dependent oxidation of p-nitroanisol catalyzed by cytochrome P-450. The thermodynamic parameters of O-demethylation of p-nitroanisol catalyzed by cytochrome P-450 and mixed haemoprotein complexes measured in water solution and in a membrane-bound state were found to be different.