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The nitric oxide-bound complexes of reduced yeast cytochrome c oxidase incorporated with [1,3-15N2]histidine have been investigated by EPR spectroscopy. The results of this study have allowed the unambiguous identification of histidine as the endogenous axial ligand to cytochrome alpha 3.
Large-angle X-ray diffuse scattering has been used for studying the conformational changes in cytochrome c during its transition from crystal into solution and during a change of the electron state of the heme. It has been found that the structure of cytochrome c in solution differs from its structure in crystal by a shift of the chain fragment in the region of 60-77 amino acid residues. The studies of the oxidized, reduced and cyanoforms of protein in solution have not revealed noticeable changes in the protein structure.
The effects of bile duct ligation (BDL) on the activity and content of individual hepatic mixed-function oxidases (MFOs) was examined. Five days after BDL, hepatic microsomal total cytochrome P450 (CYP) content and NADPH-cytochrome P450-reductase (P450-reductase) activity were reduced to 56% and 57% of control, respectively. MFO activities attributable to the sexually undifferentiated CYPs 1A, 2A1, 2C6, and 2E1 were decreased to 32% to 52% of control, but the activities of two male sex-specific CYPs, 2C11 and 3A2, were reduced to a significantly greater extent (P < .05). The microsomal contents of CYP proteins 2C6 and 2E1 were decreased after BDL to 61% and 63% of control, whereas 2C11 and 3A2 proteins were 21% and 45% of control. Corresponding reductions of the messenger RNA (mRNA) species for CYP 2C11 (9% of control) and 3A2 (37%) were detected, whereas there was no reduction of 2C6 mRNA. These findings are consistent with downregulation of the CYP 2C11 and 3A2 genes. Nuclear run-on studies performed 3 days after BDL showed that there was a generalized impairment of gene transcription after BDL, but a disproportionate reduction in transcription of CYPs 2C11 and 3A2. A possible explanation for downregulation of CYP 2C11 and 3A2 was provided by the observation that serum estradiol concentrations were threefold greater in BDL male rats, while serum testosterone was reduced; estradiol is known to downregulate CYPs 2C11 and 3A2. It is concluded that male sex-specific CYP enzymes are decreased to a greater extent than other microsomal proteins in BDL male rats.(ABSTRACT TRUNCATED AT 250 WORDS)
Endothelium is a single-cell layer lining blood vessels and constituting capillaries and could be a primary site of chemical effects in the cardiovasculature and systemically. Cytochrome P4501A1 (CYP1A1) is strongly inducible in vertebrate endothelium in vivo by aryl hydrocarbon receptor (AhR) agonists [Mol. Pharmacol. 36:723-729 (1989); Mol. Pharmacol. 41:1039-1046 (1992)]. We investigated CYP1A expression and activity in porcine aorta endothelial cells (PAEC) exposed in culture to the AhR agonists 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4'-tetrachlorobiphenyl (TCB), benzo[a]pyrene (BP), or beta-naphthoflavone (BNF). Immunoblotting with monoclonal anti-CYP1A1 and polyclonal anti-CYP1A1 and anti-CYP1A2 antibodies showed that CYP1A1 was induced in cultures exposed to TCDD, TCB, BP, or BNF but was not detectable in untreated or dimethylsulfoxide-exposed cultures. CYP1A1 was strongly induced at intermediate concentrations (0.1 microM or 1.0 microM) of TCB, BP, or BNF, but induction was suppressed by higher concentrations, a response not due to general toxicity; cell viability (trypan blue exclusion) was > 97% with BNF or TCB at up to 10 microM. CYP1A1 induction by TCDD was maximal at 0.3-1.0 nM. ED50 values for induction of CYP1A1 by TCDD, TCB, and BP were 0.016 nM, 3-10 nM, and 180 nM, respectively. Immunohistochemical analysis confirmed CYP1A1 induction in PAEC but also showed that only some cells in the cultures were induced. Subcellular fractionation, marker enzyme analysis, and immunoblot analysis showed that PAEC had a typical complement of microsomal electron-transport components. NADPH-cytochrome P450 reductase showed comparable rates (approximately 40 nmol/min/mg) in induced and control cultures. Cultures maximally induced by 0.1 microM TCB had microsomal CYP1A1 [ethoxyresorufin-O-deethylase (EROD)] activity averaging 25 pmol/min/mg. Addition of purified rat reductase to PAEC microsomes increased the EROD rates 3-fold. EROD rates measured in intact cells maximally induced by BP, TCB, or TCDD ranged from 15 to 30 pmol/min/mg of whole-cell protein. Methoxyresorufin O-demethylase activity induced by TCDD was 2 pmol/min/mg, i.e., < 10% of the EROD activity. In cultures in which CYP1A1 was strongly induced, CYP1A2 was not detectably expressed. The CYP1A2 inducer acenaphthylene did not induce EROD or methoxyresorufin O-demethylase in intact cells. The results show that CYP1A1 but not CYP1A2 is strongly induced in mammalian endothelial cells in culture and that CYP1A1 is active in intact cells, although the catalytic rates are low.(ABSTRACT TRUNCATED AT 400 WORDS)
Using in vitro liver systems, we previously demonstrated that 3'-azido-3'-deoxythymidine (AZT) is reduced to a highly toxic metabolite, 3'-amino-3'-deoxythymidine (AMT) through a NADPH-dependent system. This pathway also occurs for other 3'-azido-2',3'-dideoxynucleosides (3'-azido ddNs), indicating that reduction to a 3'-amino metabolite is a general catabolic route of this class of compounds. This study was undertaken to understand the enzymatic reaction responsible for this catabolic pathway. Rat liver microsomes were exposed to 1 mM [3H]AZT or 1 mM [3H]AzddU, and incubated under various conditions. Reduction to the 3'-amino derivative was enhanced 5-fold by the addition of NADPH. When FAD or FMN was combined with NADPH, AMT and AMddU formation was enhanced 2-fold. Addition of equimolar FAD and FMN enhanced azido reducing activity by 3-fold and 5-fold when compared with NADPH alone for AZT and AzddU, respectively. Exposure to carbon monoxide inhibited 3'-amino formation approximately 60%, consistent with involvement of cytochrome P-450 (P-450). This inhibitory effect was not detected in the presence of combined flavin and NADPH; in control incubations that contained these cofactors but no microsomes, AMT or AMddU formation was not observed. This suggests that a flavoprotein, possibly NADPH-cytochrome P-450 reductase (P-450 reductase), is also involved in azido reduction. Preincubation with various P-450 ligands resulted in variable inhibition; reduction of AZT and AzddU was decreased approximately 20-80%.(ABSTRACT TRUNCATED AT 250 WORDS)
A decline in the ionic interactions in the medium with increasing ionic strength (decrease in the ionic activity coefficients) was accompanied by an increase in the fast phase rate constants of CYP2B4 and CYP1A2 reduction. The stimulations were observed both in reconstituted P450 systems and in microsomes. An increase in the ionic strength from 10 to 100 mM sodium phosphate resulted in a 7-fold decrease in the Km of CYP1A2 for NADPH-cytochrome P450 reductase, while the Vmax was unchanged. When ionic interactions were neutralized without changing the ionic strength by addition of charged oligopeptides (polylysine and polyglutamic acid), stimulations of CYP1A2 and CYP2B4 reduction were observed. Increase in the ionic strength also enhanced the rate of cytochrome P450 reduction in control and phenobarbital-induced rat liver microsomes and in reconstituted systems containing purified rat liver CYP2C6, CYP2C12, CYP2C13, and CYP2E1, and rat reductase. A method was devised for the quantification of the number of charges involved in protein-protein interactions based on the estimation of the ionic activity coefficients. Different numbers of charged residues are involved in the repulsion between different P450 forms and the reductase. The product of the number of charges involved in the interaction between rabbit reductase and CYP2B4 is 10.84 compared with the value of 6.64 for the reductase-CYP1A2 interaction.
We characterized six novel missense mutations in mitochondrial cytochrome b (C133Y, W142R, S206L, M221K, L282F, and G340E) which impair the respiratory growth of yeast and which have differential effects on the functioning and assembly of the bc1 complex. The mutations have been mapped genetically in exons of the mitochondrial gene coding for apocytochrome b and their nucleotide sequence established. The mutants help to better define the topographical and primary sequence location of the ubiquinol oxidase (center P) and ubiquinone reductase (center N) sites on cytochrome b. Two mutants (C133Y and S206L) resulted in an active assembled complex, with selective disturbances of heme 565 and heme 562, respectively, which is consistent with the assignment of the axial ligands of these hemes; the C133Y mutation induced myxothiazol resistance, whereas the S206L did not modify the antimycin binding site, although perturbing the center N. These two amino acid replacements, along with those described elsewhere (Tron, T., and Lemesle-Meunier, D. (1990) Curr. Genet. 18, 413-419), constitute a novel class of mutants exhibiting appreciable electron transfer activity, despite their impaired ability to grow on respiratory substrates, raising the possibility that these mutants carry alleles which result in "decoupling" of proton translocation from electron transfer. Mutants W142R and M221K had an inactive but well assembled bc1 complex, whereas the G34OE and L282F mutations impaired the assembly of the bc1 complex.
High level Escherichia coli expression of cytochromes P450 3A12 and 3A6 has facilitated the characterization of proteins which exhibit limited activity as purified hepatic enzymes in reconstituted systems. Three 3A12 and two 3A6 constructs modified at the 5'-end to encode the bovine 17 alpha-sequence (Barnes et al., Proc. Natl. Acad. Sci. U.S.A. 88: 5597-5601, 1991), or related sequences, exhibited expression levels ranging from 2 to 89 nmol of cytochrome P450 liter-1. Recombinant canine 3A12 catalyzed steroid 6 beta-hydroxylation and erythromycin demethylation at rates comparable to those obtained in phenobarbital-induced canine liver microsomes. In contrast, 3A12 troleandomycin demethylase activity (2.5 nmol/min/nmol) was significantly lower than that of canine phenobarbital-induced liver microsomes (6.6 nmol/min/nmol). This difference in activity suggests that at least two 3A forms, which may differ functionally, are present within the canine liver. Purification of recombinant rabbit 3A6 revealed that homogeneous and E. coli-solubilized membrane preparations of 3A6 exhibit similar metabolic rates and identical substrate specificities; 3A activity was modulated by 25 microM alpha-naphthoflavone, which stimulated an unidentified progesterone metabolite 9-fold in 3A6 reconstituted systems in contrast to the 4-fold stimulation of 3A12. Furthermore, 25 microM alpha-naphthoflavone inhibited erythromycin demethylation 64 and 33% by purified recombinant 3A6- or 3A6-solubilized membrane fractions, respectively; 3A12-mediated erythromycin demethylation in solubilized membrane fractions was resistant to flavonoid inhibition. These results indicate that, although 3A substrate specificities are highly conserved between species, functional differences exist between canine 3A12 and rabbit 3A6, which may be utilized to better understand 3A structure-function relationships.
The objectives of this study were to quantitatively define alterations in the epithelial populations and expression of cytochrome P450 monooxygenases in distal airways which are associated with the tolerance resulting from repeated exposure of mice to the acute Clara cell toxicant, naphthalene. Bronchiolar epithelium of mice treated daily for 7 days with an acutely cytotoxic dose of naphthalene (200 mg/kg/day) quantitatively resembles the bronchiolar epithelium of control animals in terms of proportions of ciliated and nonciliated cells and nuclear and cytoplasmic volumes. Subsequent treatment of tolerant animals with higher doses (300 mg/kg) does not produce the same Clara cell injury observed in previously untreated mice after a single treatment with 300 mg/kg. After repeated exposures to naphthalene, cellular expression of immunoreactive cytochrome P450 monooxygenases 2B and 2F, P450 reductase and Clara cell secretory protein significantly decreased in bronchiolar epithelium. Although metabolism of naphthalene to the 1R,2S-naphthalene oxide is depressed in microsomes derived from whole-lung homogenates of tolerant animals, metabolism of naphthalene in distal airways isolated by microdissection is unchanged from the controls at saturating substrate concentrations. When substrate concentrations are less than the apparent K(m) for naphthalene (< 0.05 mM), the rate of naphthalene metabolite production is markedly lower in distal airways of tolerant mice compared with controls. Covalent binding of reactive naphthalene metabolites in lungs of tolerant mice is also unchanged from control. In conclusion, after repeated exposure to short-term cytotoxic doses of naphthalene distal bronchioles of tolerant mice 1) are lined by epithelium which resembles that of controls; 2) express lower levels of P450 proteins; 3) have reduced levels of naphthalene monooxygenase activity, but only at less than saturating concentrations and 4) have no decrease in covalent binding of reactive metabolites to proteins.
V79 Chinese hamster cell lines were genetically engineered for the stable expression of human NADPH-cytochrome P450 oxidoreductase (CYPOR) alone or for the combined expression of CYPOR and human cytochrome P450 1A2 (CYP1A2). As determined by immunoblotting, the expression level of CYP1A2 in the latter cell line was found to be the same as in a previously constructed V79 cell line expressing CYP1A2 only. The heterologous expression of CYPOR in V79 cells resulted in increased sensitivity to quinone-type cytotoxins, e.g. duroquinone and menadione, that exert their toxicity primarily through the production of reactive oxygen species during redox cycling. The metabolic properties of the cell line expressing both CYPOR and CYP1A2 were characterized regarding dealkylation and deethylation of 7-alkoxyresorufins and sulfoxidation of the triazine derivatives ametryne and terbutryne, in comparison with the cell line expressing only CYP1A2. Increased CYPOR activity impaired the CYP1A2-dependent fluorometric resorufin assay, presumably by conversion of the 7-alkoxyresorufins and resorufin to their one-electron-reduced semiquinoneimine forms. The CYP1A2-dependent metabolism of the triazine derivatives ametryne and terbutryne was moderately enhanced by increased CYPOR activity. Interestingly, with CYPOR overexpression sulfoxidation was increased 2-3-fold, compared with N-deethylation, with a 1.3-1.9-fold increase. Thus, the level of CYPOR not only had an influence on CYP1A2 activity rates but also affected the relative proportions of metabolites in CYP1A2-specific metabolite profiles.
Two members of the canine cytochrome P4502C subfamily [CYP2C21 and CYP2C41 (sequence has been submitted to Genbank with accession number AF016248)] were cloned from three beagle liver cDNA libraries. The two canine CYP2C cDNAs exhibited 70% nucleotide and amino acid identity as well as 74-83% nucleotide and 67-76% amino acid identity with the human CYP2Cs. Canine CYP2C41 is more homologous to the human CYP2Cs than CYP2C21. The two canine CYP2C cDNAs exhibited a slightly lower nucleotide and amino acid identity (66-77%) with the rat P450CYPs, 2C11 and 2C12. Reverse transcription-polymerase chain reaction-based restriction enzyme tests for CYP2C21 and 2C41 mRNAs as well as polymerase chain reaction-based tests for genomic DNA were developed. CYP2C21 cDNA was present in the livers of all dogs tested (N = 9), but CYP2C41 was present in only 1 of the 9 (11%). Genomic tests found that the gene coding for CYP2C21 was also present in all dogs tested (N = 25), of which 15 were beagles and 10 mixed breeds. In contrast, the gene coding for CYP2C41 was present in only 16% (4 out of 25) of the dogs. An even distribution of the CYP2C41 gene was found between the sexes and between beagles and mixed breeds. This unique polymorphism in the canine CYP2C subfamily may be a source of variability in the metabolic clearance in dogs of xenobiotics that are metabolized by the cytochrome P450 2C subfamily of enzymes.
We have recently reported that beta-ionone induces cytochrome P450 (P450) 2B1 in rats. Effects of beta-ionone on the expression of other P450 isozymes and NADPH-P450 reductase were further investigated in Sprague Dawley rats. Administration of beta-ionone subcutaneously 72 and 48 h before sacrificing the animals not only significantly induced the liver microsomal activities of P450-associated enzymes and NADPH-P450 reductase, but also clearly increased in the level of P450 1A1/2, P450 2C, and NADPH-P450 reductase proteins. The induction of P450 1A1/2 and 2C by beta-ionone was much greater in male than in female as measured by western immunoblotting. Reverse transcriptase-polymerase chain reactions showed that, in addition to P450 2B1 and 2B2 mRNAs, P450 1A2, 2C6 and NADPH-P450 reductase mRNAs were increased when beta-ionone was administered. Our previous and present results indicated that beta-ionone may induce several P450s and NADPH-P450 reductase by the accumulation of their corresponding mRNAs.
Cytochrome P450 (P450) 2D6 was first identified as the polymorphic human debrisoquine hydroxylase and subsequently shown to catalyze the oxidation of a variety of drugs containing a basic nitrogen. Differences in the regioselectivity of oxidation products formed in systems containing NADPH-P450 reductase/NADPH and the model oxidant cumene hydroperoxide have been proposed by others to be due to an allosteric influence of the reductase on P450 2D6 (Modi, S., Gilham, D. E., Sutcliffe, M. J., Lian, L.-Y., Primrose, W. U., Wolf, C. R., and Roberts, G. C. K. (1997) Biochemistry 36, 4461-4470). We examined the differences in the formation of oxidation products of N-methyl-4-phenyl-1,2,5,6-tetrahydropyridine, metoprolol, and bufuralol between reductase-, cumene hydroperoxide-, and iodosylbenzene-supported systems. Catalytic regioselectivity was not influenced by the presence of the reductase in any of the systems supported by model oxidants, ruling out allosteric influences. The presence of the reductase had little effect on the affinity of P450 2D6 for any of these three substrates. The addition of the reaction remnants of the model oxidants (cumyl alcohol and iodobenzene) to the reductase-supported system did not affect reaction patterns, arguing against steric influences of these products on catalytic regioselectivity. Label from H(2)18O was quantitatively incorporated into 1'-hydroxybufuralol in the iodosylbenzene- but not in the reductase- or cumene hydroperoxide-supported reactions. We conclude that the P450 systems utilizing NADPH-P450 reductase, cumene hydroperoxide, and iodosylbenzene use similar but distinct chemical mechanisms. These differences are the basis for the variable product distributions, not an allosteric influence of the reductase.
To clarify the involvement of cytochrome P4503A (CYP3A) and CYP2C19 in the metabolism of alprazolam, the effects of pretreatment with erythromycin, which is an inhibitor of CYP3A and S-mephenytoin 4-hydroxylation capacity on the single-oral dose kinetics of alprazolam were studied in 12 healthy male volunteers. Six each were extensive metabolizers (EMs) and poor metabolizers (PMs) of S-mephenytoin 4-hydroxylation. Each subject received erythromycin 1,200 mg/day or placebo for 10 days in a double-blind, crossover manner. At 9 AM of day 8 alprazolam 0.8 mg was given orally, and blood samplings were performed up to 48 hours postdosing. Erythromycin treatment compared with placebo treatment increased significantly plasma alprazolam concentrations from 6 to 48 hours postdosing. Erythromycin significantly decreased the apparent oral clearance and prolonged the elimination half-life of alprazolam. No significant difference was found in plasma concentrations and pharmacokinetic parameters of alprazolam between the EM and PM groups. The present study thus suggests that CYP3A, but not CYP2C19, is involved in the metabolism of alprazolam.
High levels of dietary fat caused a significant reduction in HMG CoA reductase activity in the liver of germ-free rats whereas significantly elevated small intestinal enzyme activity was observed. Dietary fat had no significant effect on HMG CoA reductase activity in any tissue studied in the conventional rat. No significant change in colonic HMG CoA reductase activity was observed between any of the experimental groups. Rats fed a high-fat diet tended to exhibit higher cytochrome P450 levels in all tissues studied, regardless of the presence of intestinal microflora.
Valinomycin/K+ is shown to selectively interact at sub-micromolar concentrations with the cytochrome bf complex in thylakoid membranes, inducing a red shift of the ferrohaem b absorbance alpha-band, a slow down of post-illumination b-reoxidation and a corresponding increase of b-reduction level in continuous light. These effects of valinomycin/K+ are not related to its field dissipating action, as they are not affected by nonactin. Presence of K+ is required. Phenomenologically the valinomycin/K+ effects are similar to those caused by 10 times higher MOA stilbene nonactin. Presence of K+ is required. Phenomenologically the valinomycin/K+ effects are similar to those caused by 10 times higher MOA stilbene concentrations. However, synergism is observed between the two inhibitors, suggesting different modes of action. When both inhibitors are combined more than one haem b can be reduced by illumination.
The mutagenic activation of various promutagens by liver microsomes from dogs, monkeys and humans was investigated. Dog liver microsomes efficiently catalyzed the mutagenic activation of Trp-P-2 and Glu-P-1 followed by IQ and AAF. Monkey liver microsomes were most active in the activation of IQ followed by Glu-P-1, AAF and Trp-P-2. Although there were remarkable individual differences, human liver microsomes were found to be most active in the mutagenic activation of IQ followed by Trp-P-2, Glu-P-1 and AAF. Antibodies against rat P-448-H inhibited the mutagenic activation of Glu-P-1, Trp-P-2 and IQ in rat and dog liver microsomes, and Glu-P-1 and Trp-P-2 in monkey liver microsomes. The activation of Glu-P-1 and IQ in human liver microsomes was also strongly inhibited by anti-P-448-H antibodies. The amounts of cytochrome P-450 cross-reactive with anti-P-448-H antibodies in human liver microsomes highly correlated with the capacity to activate Glu-P-1, Trp-P-2 and IQ but not AAF.
OBJECTIVES: Cytochrome P450 (CYP) 2C19 and CYP3A4 are the major enzymes responsible for voriconazole elimination. Because the activity of CYP2C19 is under genetic control, the extent of inhibition with a CYP3A4 inhibitor was expected to be modulated by the CYP2C19 metabolizer status. This study thus assessed the effect of the potent CYP3A4 inhibitor ritonavir after short-term administration on voriconazole pharmacokinetics in extensive metabolizers (EMs) and poor metabolizers (PMs) of CYP2C19. METHODS: In a randomized, placebo-controlled crossover study, 20 healthy participants who were stratified according to CYP2C19 genotype received oral ritonavir (300 mg twice daily) or placebo for 2 days. Together with the first ritonavir or placebo dose, a single oral dose of 400 mg voriconazole was administered. Voriconazole was determined in plasma and urine by liquid chromatography-mass spectrometry, and pharmacokinetic parameters were estimated by noncompartmental analysis. RESULTS: When given alone, the apparent oral clearance of voriconazole after single oral dosing was 26%+/-16% (P > .05) lower in CYP2C19*1/*2 individuals and 66%+/-14% (P < .01) lower in CYP2C19 PMs. The addition of ritonavir caused a major reduction in voriconazole apparent oral clearance (354+/-173 mL/min versus 202+/-139 mL/min, P = .0001). This reduction occurred in all CYP2C19 genotypes (463+/-168 mL/min versus 305+/-112 mL/min [P = .023] for *1/*1, 343+/-127 mL/min versus 190+/-93 mL/min [P = .008] for *1/*2, and 158+/-54 mL/min versus 22+/-11 mL/min for *2/*2) and is probably caused by inhibition of CYP3A4-mediated voriconazole metabolism. CONCLUSIONS: Coadministration of a potent CYP3A4 inhibitor leads to a higher and prolonged exposure with voriconazole that might increase the risk of the development of adverse drug reactions on a short-term basis, particularly in CYP2C19 PM patients.