The roles of cytochrome b5 in a reconstituted N-demethylase system containing cytochrome P-450.

Compound 102804 isolated from $\text{Bacillus cereus}$ has been found to be a potent inhibitor of the N⁵-methyltetrahydrofolate-homocysteine transmethylase isolated from $\text{Escherichia coli}$ B. This inhibition was noted when 102804 was added to the enzyme reaction mixture after the reaction started or concurrently with the preparation of the mixture. Chemically inactivated 102804 has no activity as an inhibitor of this enzyme system.     

Production of hydroxyl radicals and their role in the oxidation of ethanol by a reconstituted microsomal system containing cytochrome P-450 purified from phenobarbital-treated rats

Ethanol oxidation by a reconstituted system composed of cytochrome P-450 purified from liver microsomes of phenobarbital-treated rats, NADPH-cytochrome c reductase, phospholipid and NADPH was inhibited by a series of hydroxyl radical scavenging agents. Inhibition was competitive with respect to ethanol and was specific in the sense that the metabolism of aminopyrine or benzphetamine by the reconstituted system was not affected by the scavengers. The generation of ethylene gas from 2-keto-4-thiomethylbutyric acid in an ethanol-sensitive manner provided chemical evidence consistent with the ability of the reconstituted system to generate hydroxyl radicals. These results suggest that the oxidation of ethanol by the reconstituted system reflects the interaction of ethanol with hydroxyl radicals generated during NADPH oxidation.     

Aldosterone biosynthesis by a reconstituted cytochrome P-45011 beta system

[3H]Corticosterone was incubated with cytochrome P-45011 beta purified to electrophoretic homogeneity from bovine adrenocortical mitochondria, and the reaction products were analyzed by high performance liquid chromatography. The production of aldosterone (21.2 pmol/nmol P-450/min) and 18-hydroxycorticosterone (1.17 nmol/nmol P-450/min) was observed. When lipidic extracts from mitochondria of bovine adrenocortical zona glomerulosa were added to the reaction mixture, the rate of production of aldosterone was increased 28-fold. When [3H]18-hydroxycorticosterone was incubated with cytochrome P-45011 beta, the amount of aldosterone produced was 55.7 pmol/nmol P-450/min in the absence of the lipidic extracts and the enhancing effect of the lipidic extracts was 4-fold.     

Retinoic acid metabolism by a system reconstituted with cytochrome P-450

Feeding rats with a diet containing a hundred times the normal amount of vitamin A resulted, within 2 to 3 weeks, in an increase in total hepatic microsomal cytochrome P-450 content. This was associated, in isolated microsomes, with an enhanced conversion of all-trans-retinoic acid to polar metabolites, including a two- to threefold increased production of 4-hydroxy- and 4-oxo-retinoic acid, whether expressed per microsomal protein or per cytochrome P-450. Unlike effects of other inducers (e.g., phenobarbital or methylcholanthrene), activities of benzphetamine, aminopyrine, and ethylmorphine demethylases or benzopyrene hydroxylase were not increased. Furthermore, the CO-reduced difference spectral peak was shifted towards 449 nm. On sodium dodecyl sulfate-gel electrophoresis, one band was increased with electrophoretic mobility identical to that of cytochrome P-450f, a recently isolated new form which has a CO-reduced difference spectral peak at 448 nm. In a system reconstituted with NADPH-cytochrome P-450 reductase, NADPH, and phospholipid, purified cytochromes P-450f and b were discovered to promote conversion of retinoic acid to polar metabolites, including 4-hydroxy-retinoic acid.     

The 14alpha-demethylation of lanosterol by a reconstituted cytochrome P-450 system from yeast microsomes.

Native and zinc reconstituted carboxypeptidase B were nitrated with tetranitromethane. The inactivation of the reconstituted enzyme was faster than that of the native enzyme and was accompanied by the formation of a considerable amount of enzyme dimers. The inactivation and dimerization reflected changes in the reactivity of active site tyrosine residue(s), thus indicating microenvironmental changes which occur during metal substitution. The change in tyrosine reactivity could be correlated with the residence of the enzyme in the metal-free state.     

Enzymatic oxidation of disulfides and thiolsulfinates by both rabbit liver microsomes and a reconstituted system with purified cytochrome P-450.

Both rabbit liver microsomes and reconstituted system with purified cytochrome P-450 and cofactors enzymatically oxidized o-dithiane (1, 2-dithiane), 3-methyl-o-dithiane, thiane and 2-methylthiane to the corresponding mono-oxygenated products; sulfides or disulfides were oxidized to the corresponding sulfoxides or thiosulfinates, while thiosulfinate was oxidized to thiolsulfonate. The reconstituted systems required oxygen and NADPH and were not affected by the catalase which decomposes H2O2, or by 1,4-diazabicyclo-[2,2,2]octane (DABCO), which is a good quencher of singlet oxygen. The differences in the binding of substrates such as sulfides and disulfides with the enzyme system are discussed in connection with differences in the spectra of the substrates in the reconstituted system with pure cytochrome P-450. A correlation was found between the rates of oxidation of the substrates and the rates of oxidation of NADPH.     

Oxidation of pyrazole by reconstituted systems containing cytochrome P-450 IIE1

Rat liver microsomes oxidize pyrazole to 4-hydroxypyrazole and this oxidation is increased in microsomes isolated from rats treated with inducers of cytochrome P-450 IIE1, such as pyrazole or ethanol. A reconstituted system containing the P-450 IIE1, purified from pyrazole-treated rats, oxidized pyrazole to 4-hydroxypyrazole in a time- and P-450-dependent manner. Oxidation of pyrazole was dependent on the concentration of pyrazole over the range of 0.15 mM to 1.0 mM. In isolated microsomes, glycerol inhibited pyrazole oxidation by about 50% under concentration conditions which occur in the reconstituted system; hence, the values for pyrazole oxidation by the reconstituted systems are underestimated because of the presence of glycerol. Oxidation of pyrazole was inhibited by competitive substrates for P-450 IIE1, such as 4-methylpyrazole, aniline and ethanol, as well as by an antibody raised against the pyrazole-induced P-450 IIE1. Thus, pyrazole is an effective substrate for oxidation by purified P-450 IIE1, extending the substrate specificity of this isozyme to potent inhibitors of alcohol dehydrogenase.     

Dimethylnitrosamine demethylation by reconstituted liver microsomal cytochrome P-450 enzyme system.

Oxidative demethylation of dimethylnitosamine was studied with both reconstituted and unresolved liver microsomal cytochrome P-450 enzyme systems from rats and hamsters. Proteinase treatment of liver microsomal preparations yielded cytochrome P-450 particulate fractions. Both cytochrome P-450 and NADPH- cytochrome c reductase fractions were required for optimum demethylation activity. Particulate cytochrome P-450 fractions were more effecient than either Triton X-100- or cholatesolubilized preparations of these particles in demethylation activity with rat and hamster liver preparations appear to be due to differences in specificity in their cytochrome P-450 fractions.     

Ring-andN-hydroxylation of 2-acetamidofluorene by rat liver reconstituted cytochrome P-450 enzyme system.

The N- and ring-hydroxylation of 2-acetamidofluorene were studied with a reconstituted cytochrome P-450 enzyme from microsomal fractions of liver from both control and 3-methylcholanthrene-pretreated rats. Proteinase treatment and Triton X-100 solubilization were two important steps for partial purification of the cytochrome P-450 fraction. Both cytochrome P-450 and NADPH-cytochrome c reductase fractions were required for optimum N- and ring-hydroxylation activity. Hydroxylation activity was determined by the source of cytochrome P-450 fraction; cytochrome P-450 fraction from pretreated animals was severalfold more active than the fraction from controls. Formation of N-hydroxylated metabolites with reconstituted systems from both control and pretreated animals was greater than that with their respective whole microsomal fractions.     

Reduction and catalytic properties of cytochrome P-450 LM2 in reconstituted system containing monomeric carriers

The membrane microsomal monooxygenase system can be reconstituted in solution from NADPH-specific flavoprotein and cytochrome P-450 which exist in the monomeric state in the presence of Emulgen 913 at molar ratio of the proteins and detergent of 1:1:300. Oxidized and dithionite-reduced monomers of cytochrome P-450 were much less thermostable than its initial aggregates, while thermal stability of NADPH-specific flavoprotein did not depend on its aggregation state. Binding spectra of cytochrome P-450 monomers with benzphetamine were atypical and had an absorbance minimum at 422 nm only. The addition of benzphetamine and/or flavoprotein to cytochrome P-450 monomers did not cause the spin equilibrium shift and the low-spin form content was higher than 85% in all cases. Investigation of the dependence of the initial rates of NADPH-dependent cytochrome P-450 reduction and benzphetamine oxidation on the stoichiometry of the flavoprotein and cytochrome P-450 at their constant total concentration showed that the molar ratio of 1:1 was required for maximal activity. Thus this system works in full accordance with the mass action law.     

Reduction of hexavalent chromium in a reconstituted system of cytochrome P-450 and cytochrome b5

The reduction of hexavalent chromium (Cr(VI] by the monooxygenase components was studied. Both a reconstituted system of cytochrome P-450 (P-450) and cytochrome b5 (b5) with NADPH was capable of reducing Na2CrO4 (30 microM) provided anaerobic atmosphere. The rates were 1.29 nmol Cr.min-1 nmol P-450(-1) and 0.73 nmol Cr.min-1 nmol b5(-1). Using NADH instead of NADPH gave very low reducing activities, confirming the enzymic nature of the P-450 dependent Cr(VI) reductase reaction. Oxygen, 22% (air) and 0.1% gave 89% and 69% inhibition of Cr(VI) reducing activity, respectively. Carbon monoxide (100%) caused an inhibition of about 37% and 44% for P-450 and b5, respectively. Externally added flavin mononucleotide (FMN) (3 microM) or Fe-ADP (10 microM) to the complete system stimulated the enzymatic reaction about 2-fold and 3-fold, respectively.     

Induction of cytochrome P-450 and ethanol oxidation in Yarrowia lipolytica

The study of the effect of different ethanol concentrations in the medium on the growth and the activity of enzymatic systems involved in ethanol oxidation in Yarrowia lipolytica showed that the cultivation of yeast cells on 1 and 2% ethanol caused their rapid growth and a drastic increase in cell respiration and sensitivity to cyanide already in the first hours of cultivation. At the same time, during cultivation on 3, 4, and 5% ethanol, the growth and respiration of yeast cells were considerably suppressed. All of the ethanol concentrations studied induced the synthesis of cytochrome P-450, its dynamics in cells being dependent on the initial concentration of ethanol in the medium. When the initial concentration of ethanol was 1 and 2%, the content of cytochrome P-450 in cells steeply decreased after a short period of induction. But when the initial concentration of ethanol in the medium was 3 to 5%, the content of cytochrome P-450 in cells was high throughout the cultivation period. The induction of cytochrome P-450 in cells preceded the induction of the NAD-dependent enzymes alcohol dehydrogenase and catalase, which, like cytochrome P-450, are also involved in ethanol oxidation by yeasts. The activity of catalase was higher in the yeast cells grown in the presence of 3 to 5% ethanol than in the cells grown in the presence of 1 and 2% ethanol. The roles played by cytochrome P-450, alcohol dehydrogenase, and catalase in ethanol oxidation by yeast cells are discussed.     

Anaerobic dehalogenation of halothane by reconstituted liver microsomal cytochrome P-450 enzyme system

Cytochrome P-450 from liver microsomes of phenobarbital-treated rabbits catalyzed anaerobic dehalogenation of halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) when combined with NADPH and NADPH-cytochrome P-450 reductase. Cytochromes P-450B₁ and P-448 from liver microsomes of untreated rabbits were less active. Triton X-100 accelerated the reaction. Unlike anaerobic dehalogenation of halothane in microsomes, the major product was 2-chloro-1,1,1-trifluoroethane and 2-chloro-1,1-difluoroethylene was negligible. These products were not detected under aerobic conditions, and dehalogenation activity was inhibited by carbon monoxide, phenyl isocyanide and metyrapone.     

Synthesis of aldosterone by a reconstituted system of cytochrome P-45011 beta from bovine adrenocortical mitochondria

When corticosterone was incubated with cytochrome P-45011 beta purified from bovine adrenocortical mitochondria in the presence of adrenodoxin, NADPH-adrenodoxin reductase and an NADPH generating system, aldosterone as well as 18-hydroxycorticosterone were formed with turnover numbers of 0.23 and 1.1 nmol/min/nmol P-450, respectively. Phospholipids extracted from adrenocortical mitochondria remarkably enhanced the activity of aldosterone formation by the cytochrome P-45011 beta-reconstituted system. The apparent Km and turnover number were estimated to be 6.9 microM and 2.0 nmol/min/nmol P-450 for aldosterone formation in the presence of the lipidic extract. When 18-hydroxycorticosterone was tested as a substrate, cytochrome P-45011 beta showed catalytic activity for aldosterone synthesis with an apparent Km and turnover number of 325 microM and 5.3 nmol/min/nmol P-450, respectively. Carbon monoxide and metyrapone inhibited the production of aldosterone from corticosterone and that from 18-hydroxycorticosterone. These results suggest that conversion of corticosterone and of 18-hydroxycorticosterone to aldosterone occurs through P-45011 beta-catalyzed reaction.     

Inhibition mechanism of reconstituted cytochrome P-450scc-linked monooxygenase system by antimycotic reagents and other inhibitors.

The effects of various antimycotic reagents and some other reagents on a cytochrome P-450-linked monooxygenase system were investigated with respect to the activities of NADPH-ferricyanide reductase. NADPH-cytochrome c reductase of NADPH-adreno-ferredoxin reductase from NADPH to cytochrome c via adreno-ferredoxin, NADPH-cytochrome P-450-phenylisocyanide complex reductase, and the cholesterol side chain cleavage of the cytochrome P-450scc-linked monooxygenase system. No reagents inhibited the NADPH-ferricyanide reductase activity. Only cloconazole inhibited about 50% of NADPH-cytochrome c reductase activity. Cloconazole, econazole, clotrimazole, etomidate and ketoconazole inhibited both NADPH-cytochrome P-450-phenylisocyanide complex reductase and the side chain cleavage activity of cholesterol of the cytochrome P-450scc-linked monooxygenase system. Cloconazole, econazole, etomidate and ketoconazole behaved like non-competitive inhibitors for NADPH-cytochrome P-450-phenylisocyanide reductase activities and their Ki values were 10(-4)-10(-6) M. Cloconazole was a non-competitive inhibitor of NADPH-cytochrome c reductase and its Ki value was 8.3 x 10(-4) M. Cloconazole, clotrimazole, econazole, etomidate, ketoconazole and mitotane completely inhibited the side chain cleavage activity of cholesterol.     

Immunochemical evidence for a role of cytochrome P-450 in liver microsomal ethanol oxidation

Antibodies to cytochrome P-450 isozyme 3a, the ethanol-inducible isozyme in rabbit liver, were used to determine the role of this enzyme in the microsomal oxidation of alcohols and the p-hydroxylation of aniline. P-450 isozymes, 2, 3b, 3c, 4, and 6 did not crossreact with anti-3a IgG as judged by Ouchterlony double diffusion, and radioimmunoassays indicated a crossreactivity of less than 1%. Greater than 90% of the activity of purified form 3a toward aniline, ethanol, n-butanol, and n-pentanol was inhibited by the antibody in the reconstituted system. The catalytic activity of liver microsomes from control or ethanol-treated rabbits was unaffected by the addition of either desferrioxamine (up to 1.0 mM) or EDTA (0.1 mM), suggesting that reactions involving the production of hydroxyl radicals from H2O2 and any contaminating iron in the system did not make a significant contribution to the microsomal activity. The addition of anti-3a IgG to hepatic microsomes from ethanol-treated rabbits inhibited the metabolism of ethanol, n-butanol, n-pentanol, and aniline by about 75, 70, 80, and 60%, respectively, while the inhibition of the activity of microsomes from control animals was only about one-half as great. The rate of microsomal H2O2 formation was inhibited to a lesser extent than the formation of acetaldehyde, thus suggesting that the antibody was acting to prevent the direct oxidation of ethanol by form 3a. Under conditions where purified NADPH-cytochrome P-450 reductase-catalyzed substrate oxidations was minimal, the P-450 isozymes other than 3a had low but significant activity toward the four substrates examined. The residual activity at maximal concentrations of the antibody most likely represents the sum of the activities of P-450 isozymes other than 3a present in the microsomal preparations. The results thus indicate that the enhanced monooxygenase activity of liver microsomes from ethanol-treated animals represents catalysis by P-450 isozyme 3a.     

The effects of quinone analogues on cytochrome b6 reduction and oxidation in a reconstituted system

The reconstituted system containing Photosystem I, plastocyanin and the cytochrome b6-f complex is used to study the effects of various quinone analogues on the redox behavior of cytochrome b6. The effects of DBMIB, DNP-INT and HQNO are compared in an attempt to discern the modes of action of these quinone analogues. Both DBMIB and DNP-INT are potent inhibitors of the plastocyanin reductase activity of the isolated cytochrome complex. However, while DBMIB abolished the oxidant-induced reduction of cytochrome b6, DNP-INT only inhibited about 25% of the net reduction. On the other hand, HQNO does not show any significant inhibition of plastocyanin reductase activity of the isolated cytochrome complex at concentrations up to 20 microM. An enhancement of the net amount of cytochrome b6 reduced is observed in the presence of HQNO. Both DNP-INT and HQNO inhibited the dark oxidation rate of cytochrome b6. The possible identity of the oxidant for cytochrome b6 is discussed. Plastoquinone is concluded to be the most likely candidate. DNP-INT is concluded to have at least two sites of inhibition in the cytochrome complex. The implications of these findings on quinone functions in the cytochrome b6-f complex are discussed.