Multiple forms of cytochrome P-450 from kidney cortex microsomes of rabbits treated with phenobarbital

Two distinct forms of cytochrome P-450 (P-450), referred to as P-450a and P-450b, were separated and purified from kidney cortex microsomes of rabbits treated with phenobarbital. P-450a had a monomeric molecular weight of 53,000, and its CO-reduced difference spectral peak was at 450 nm. It catalyzed the omega-hydroxylation of prostaglandin A1 (PGA1), and the omega- and (omega-1)-hydroxylation of myristate, but it was inactive toward exogenous compounds tested. On the other hand, P-450b had a monomeric molecular weight of 49,000, and its CO-reduced difference spectral peak was at 451 nm. This cytochrome was not able to hydroxylate PGA1 at all. It hydroxylated myristate much more slowly than P-450a, and preferentially at the (omega-1)-position. Unlike P-450a, P-450b efficiently metabolized exogenous compounds such as benzphetamine, aminopyrine, 7-ethoxycoumarin and p-nitroanisole. It is suggested that P-450a and P-450b are specialized for the metabolism of PGA1 and exogenous compounds, respectively, in kidney cortex microsomes.     

Characterization of human neutrophil leukotriene B4 omega-hydroxylase as a system involving a unique cytochrome P-450 and NADPH-cytochrome P-450 reductase

Leukotriene B4 (LTB4), a potent chemotactic agent, was catabolized to 20-hydroxyleukotriene B4 (20-OH-LTB4) by the 150,000 x g pellet (microsomal fraction) of human neutrophil sonicate. The reaction required molecular oxygen and NADPH, and was significantly inhibited by carbon monoxide, suggesting that a cytochrome P-450 is involved. The neutrophil microsomal fraction showed a carbon monoxide difference spectrum with a peak at 450 nm in the presence of NADPH or dithionite, indicating the presence of a cytochrome P-450. The addition of LTB4 to the microsomal fraction gave a type-I spectral change with a peak at around 390 nm and a trough at 422 nm, indicating a direct interaction of LTB4 with the cytochrome P-450. The dissociation constant of LTB4, determined from the difference spectra, is 0.40 microM, in agreement with the kinetically determined apparent Km value for LTB4 (0.30 microM). Such a spectral change was not observed with prostaglandins A1, E1 and F2 alpha or lauric acid, none of which inhibited the LTB4 omega-hydroxylation. The inhibition of the LTB4 omega-hydroxylation by carbon monoxide was effectively reversed by irradiation with monochromatic light of 450 nm wavelength. The photochemical action spectrum of the light reversal of the inhibition corresponded remarkably well with the carbon monoxide difference spectrum. These observations provide direct evidence that the oxygen-activating component of the LTB4 omega-hydroxylase system is a cytochrome P-450. Ferricytochrome c inhibited the hydroxylation of LTB4 and the inhibition was fortified by cytochrome oxidase. An antibody raised against rat liver NADPH-cytochrome-P-450 reductase inhibited both LTB4 omega-hydroxylase activity and the NADPH-cytochrome-c reductase activity of human neutrophil microsomal fraction. These observations indicate that NADPH-cytochrome-P-450 reductase acts as an electron carrier in LTB4 omega-hydroxylase. On the other hand, an antibody raised against rat liver microsomal cytochrome b5 inhibited the NADH-cytochrome-c reductase activity but not the LTB4 omega-hydroxylase activity of human neutrophil microsomal fraction, suggesting that cytochrome b5 does not participate in the LTB4-hydroxylating system. These characteristics indicate that the isoenzyme of cytochrome P-450 in human neutrophils, LTB4 omega-hydroxylase, is different from the ones reported to be involved in omega-hydroxylation reactions of prostaglandins and fatty acids.     

Regulation of arachidonic acid metabolism by cytochrome P-450 in rabbit kidney.

Renal microsomal cytochrome P-450-dependent arachidonic acid metabolism was correlated with the level of cytochrome P-450 in the rabbit kidney. Cobalt, an inducer of haem oxygenase, reduced cytochrome P-450 in both the cortex and medulla in association with a 2-fold decrease in aryl-hydrocarbon hydroxylase, an index of cytochrome P-450 activity, and a similar decrease in the formation of cytochrome P-450-dependent arachidonic acid metabolites by renal microsomes (microsomal fractions). Formation of the latter was absolutely dependent on NADPH addition and was prevented by SKF-525A, an inhibitor of cytochrome P-450-dependent enzymes. Arachidonate metabolites of cortical microsomes were identified by g.c.-m.s. as 20- and 19-hydroxyeicosatetraenoic acid, 11,12-epoxyeicosatrienoic acid and 11,12-dihydroxyeicosatrienoic acid. The profile of arachidonic acid metabolites was the same for the medullary microsomes. Induction of cytochrome P-450 by 3-methylcholanthrene and beta-naphthoflavone increased cytochrome P-450 content and aryl-hydrocarbon hydroxylase activity by 2-fold in the cortex and medulla, and this correlated with a 2-fold increase in arachidonic acid metabolites via the cytochrome P-450 pathway. These changes can also be demonstrated in cells isolated from the medullary segment of the thick ascending limb of the loop of Henle, which previously have been shown to metabolize arachidonic acid specifically via the cytochrome P-450-dependent pathway. The specific activity for the formation of arachidonic acid metabolites by this pathway is higher in the kidney than in the liver, the highest activity being in the outer medulla, namely 7.9 microgram as against 2.5 micrograms of arachidonic acid transformed/30 min per nmol of cytochrome P-450 for microsomes obtained from outer medulla and liver respectively. These findings are consistent with high levels of cytochrome P-450 isoenzyme(s), specific for arachidonic acid metabolism, primarily localized in the outer medulla.     

Spectral and catalytic properties of cytochrome P-450 from a diazinon-resistant housefly strain

Microsomes from the diazinon-resistant Rutgers strain of housefly contain amounts of cytochrome P-450 that are larger than those reported for rat liver, but the specific activity expressed as nmole of cytochrome P-450 per mg protein is much lower. The hemoprotein shows that spectral changes type I, II and IV are essentially in the low-spin form as judged by the n-octylamine and ethyl isocyanide difference spectra, and is unstable at pH below 6.5 and above 8.0. Cytochrome P-420 is also produced with time when CO-difference spectra are recorded. This is accelerated at pH above 8.0. The presence of contaminating amounts of cytochrome P-420, due to denaturation during spectral analysis or to the method used to isolate the microsomes, makes questionable the practice of characterizing the hemoprotein on the basis of the 455 nm peak in the ethyl isocyanide spectra, since a 434 nm peak is produced with concomitant decrease of the 455 nm peak. Microsomes hydroxylate naphthalene, aminopyrine and aniline, but the activity when expressed as nmole of product per nmole of cytochrome P-450 is the same or lower than that reported for other resistant housefly strains.     

Separation of two constitutive forms of cytochrome P-450 active in aminopyrine N-demethylation from rabbit intestinal mucosa microsomes

Two constitutive forms of cytochrome P-450, designated P-450ib and P-450ic, were purified from intestinal mucosa microsomes of untreated rabbits. P-450ib and P-450ic have minimal molecular weights of 56 000 and 49 000, respectively, as determined by calibrated sodium dodecyl sulphate polyacrylamide gel electrophoresis. The CO-reduced difference spectral maximum of cytochrome P-450ib is at 450 nm and P-450ic is at 451 nm. Both the cytochromes preferentially demethylate aminopyrine, benzphetamine and N,N-dimethylaniline in the presence of NADPH-cytochrome P-450 reductase. Cytochrome P-450ib has absorption maxima at 417, 535 and 573 nm in the oxidized form, indicating that this cytochrome is in a low-spin state. Ouchterlony double-diffusion studies show that cytochrome P-450ib does not cross-react with antisera against liver cytochrome P-450LM2 purified from phenobarbital-treated rabbits, but P-450ic cross-reacts with spur formation. Unlike cytochrome P-450ib, P-450ic is very similar, if not identical, to liver cytochrome P-450LM2 on the basis of its molecular weight, spectral properties, catalytic activities and immunochemical properties.     

The role of cytochrome P-450 in the synthesis of polar metabolites of aldosterone by microsomes of male rat liver

Among the tissues of the male rat studied, the largest quantities of the neutral polar metabolites of aldosterone were synthesized by the hepatic microsomal fraction. The polar metabolites of aldosterone were separated by HPLC into six peaks. Three peaks of non-polar (reduced) metabolites were also synthesized. Synthesis of at least four of the neutral polar metabolites was induced by phenobarbital and inhibited by both CO and SKF-525A. The rates of synthesis of these metabolites, which were linear up to 5 minutes, correlated well with the concentration of cytochrome P-450 in the liver microsomes. Addition of aldosterone to the microsomal fraction caused a pronounced type 1 change in the cytochrome P-450 spectrum. The half maximal spectral change (K_s) for aldosterone was calculated to be 8 μM. These experiments indicate that the neutral polar metabolites of aldosterone are produced by cytochrome P-450 dependent hydroxy lations.     

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.     

Restoration of hydroperoxide-dependent lipid peroxidation by 3-methylcholanthrene induction of cytochrome P-448 in hepatoma microsomes

Microsomal membranes from the slow-growing Morris hepatoma 9618A catalyze, in the presence of t-butyl hydroperoxide, lower rates of lipid peroxidation than rat liver microsomes. The cytochrome P-450 content of hepatoma microsomes is about 40% that of the liver. SKF 525-A, an inhibitor of mixed-function oxidase, produces in hepatoma microsomes a P-450 type I binding spectrum similar to that of hepatic microsomes. The concentration of the inhibitor required for half-maximal spectral change is about 2 microM in both microsome types. SKF 525-A or ethylmorphine inhibit lipid peroxidation of normal and tumor microsomes to the same extent (about 60%). Treatment of the tumor-bearing rats with 3-methylcholanthrene increases the hepatoma cytochrome P-450 to values comparable to those of control membranes, although the hemoprotein has a peak in the CO-reduced difference absorption spectrum at 448 nm. The cytochrome P-448 induction is accompanied by an almost complete restoration of the hydroperoxide-dependent lipid peroxidation.     

The involvement of cytochrome P-450 in hepatic microsomal steroid hydroxylation reactions supported by sodium periodate, sodium chlorite, and organic hydroperoxides.

The mechanism of steroid hydroxylation in rat liver microsomes has been investigated by employing NaIO4, NaClO2, and various organic hydroperoxides as hydroxylating agents and comparing the reaction rates and steroid products formed with those of the NADPH-dependent reaction. Androstenedione, testosterone, progesterone, and 17beta-estradiol were found to act as good substrates. NaIO4 was by far the most effective hydroxylating agent followed by cumene hydroperoxide, NADPH, NaClO2, pregnenolone 17alpha-hydroperoxide, tert-butyl hydroperoxide, and linoleic acid hydroperoxide. Androstenedione was chosen as the model substrate for inducer and inhibitor studies. The steroid was converted to its respective 6beta-, 7alpha, 15-, and 16alpha-hydroxy derivatives when incubated with microsomal fractions fortified with hydroxylating agent. Evidence for cytochrome P-450 involvement in androstenedione hydroxylation included a marked inhibition by substrates and modifiers of cytochrome P-450 and by reagents which convert cytochrome P-450 to cytochrome P-420. The ratios of the steroid products varied according to the type of hydroxylating agent used and were also modified by in vivo phenobarbital pretreatment. It was suggested that multiple forms of cytochrome P-450 exhibiting different affinities for hydroxylating agent are responsible for these different ratios. Horse-radish peroxidase, catalase, and metmyoglobin could not catalyze androstenedione hydroxylation. Addition of NaIO4, NaClO2, cumene hydroperoxide and other organic hydroperoxides to microsomal suspensions resulted in the appearance of a transient spectral change in the difference spectrum characterized by a peak at about 440 nm and a trough at 420 nm. The efficiency of these oxidizing agents in promoting steroid hydroxylation in microsomes appeared to be related to their effectiveness in eliciting the spectral complex. Electron donors, substrates, and modifiers of cytochrome P-450 greatly diminished the magnitude of the spectral change. It is proposed that NaIO4, NaClO2, and organic hydroperoxides promote steroid hydroxylation by forming a transient ferryl ion (compound I) of cytochrome P-450 which may be the common intermediate hydroxylating species involved in hydroxylations catalyzed by cytochrome P-450.     

Spectral studies on drug-cytochrome P-450 interaction in isolated rat liver cells

Various drugs including hexobarbital, lidocaine and nortriptyline were added to suspensions of liver cells isolated from untreated and phenobarbital-treated male rats. Upon drug addition, there was a fast binding to cytochrome P-450, as revealed by the appearance of a rapidly growing type I spectral change in the difference spectrum. When this had reached optimal magnitude, an absorption peak at 437 nm could often be seen to appear and quickly disappear, followed by yet another increase in absorption at about 446 nm; the latter and the type I spectral change then rapidly disappeared. These spectral changes were most pronounced with liver cells from phenobarbital-treated rats which contained markedly increased levels of cytochrome P-450. Also the rate of hexobarbital binding to cytochrome P-450 seemed to be increased after phenobarbital pretreatment. Finally, evidence was obtained that the major part of cytochrome P-450 in the isolated liver cells is present in an oxidized, non-substrate-bound form.     

Purification and characterization of two constitutive cytochromes P-450 (F-1 and F-2) from adult female rats: identification of P-450F-1 as the phenobarbital-inducible cytochrome P-450 in male rat liver

Two hepatic microsomal cytochromes P-450, P-450F-1 and P-450F-2 were purified to electrophoretic homogeneity from untreated adult female rats by high-performance liquid chromatography (HPLC) with anion-exchange, cation-exchange, and hydroxyapatite columns. Cytochromes P-450F-1 and P-450F-2 were not adsorbed with the anion-exchange column, but were retained on a cation-exchange column and were separated poorly. These forms separated on hydroxyapatite HPLC. The molecular weights of cytochromes P-450F-1 and P-450F-2 were 50,000 and 49,000, respectively. The absolute spectrum of the oxidized forms indicated that they had the low-spin state of heme, and the CO-reduced spectral maxima of cytochromes P-450F-1 and P-450F-2 were at 450 and 448 nm, respectively. Both forms catalyzed the N-demethylation of benzphetamine and had low catalytic activity for 7-ethoxycoumarin. Cytochrome P-450F-1 had low 2 alpha-hydroxylation activity toward testosterone. Cytochrome P-450F-2 had low 15 alpha-hydroxylation activity. On the basis of these results and those of NH2-terminal sequence analysis, cytochrome P-450F-2 seemed to be the typical female-specific cytochrome P-450. The NH2-terminal sequence of cytochrome P-450F-1 was identical to that of cytochrome P-450PB-2 purified from hepatic microsomes of male rats treated with phenobarbital. Cytochromes P-450F-1 and P-450PB-2 had identical chromatographic properties, minimum molecular weight, spectral properties, and peptide maps. Furthermore, the antibody to phenobarbital-inducible cytochrome P-450PB-2 gave a single immunoprecipitin band with cytochrome P-450F-1 by Ouchterlony double-diffusion analysis.     

Studies on the microsomal electron-transport system of anaerobically grown yeast. III. Spectral characterization of cytochrome P-450.

A carbon monoxide-binding pigment which shows an absorption peak at about 450 nm in the reduced carbon monoxide difference spectrum was purified from the microsomal fraction of yeast grown anaerobically. The spectral characteristics of the pigment were practically identical with those of cytochrome P-450 of hepatic microsomes, especially from polycyclic hydrocarbon-induced animals. The pigment was denatured to P-420, and bound with ethyl isocyanide in the reduced state. Although Type I spectral change was not evident, the pigment showed Type II and modified Type II spectral changes upon binding with some organic compounds, as in the case of hepatic cytochrome P-450. These observations clearly indicate that the carbon monoxide-binding pigment of yeast microsomes may be designated as cytochrome P-450 of yeast.     

Interaction of ligands with cytochrome P-450. On the 442 nm spectral species generated during the oxidative metabolism of pyridine.

When added to aerobic rabbit liver microsomal fractions fortified with an NADPH-generating system, pyridine initially produces a type II difference spectrum such as is observed with other aromatic amines. There is a time-dependent conversion of this perturbation into a new spectral species characterized by an absorbance maximum at 442 nm and a minor peak at 389 nm. Experiments with inhibitors of the cytochrome P-450-dependent electron-transport chain suggest that these species originate from binding to the haemoprotein of metabolic intermediate(s) derived from the amine substrate. Analysis of the incubation mixtures by t.l.c., high-pressure liquid chromatography, u.v.- and mass-spectrometry reveals the presence of a single metabolite arising from cytochrome P-450-catalysed oxidation of the heteroaromatic tertiary amine, which was identified as pyridine N-oxide, obviously accounting for adduct formation. This view is supported by comparative studies on the spectral changes generated by exogenous amine oxide with NADPH-reduced cytochrome P-450. Moreover, dithiothreitol, a potent N-oxidase inhibitor, strongly suppresses development of the 442 nm and 389 nm complexes. The ability of forming low-spin adducts with ferrous cytochrome P-450 absorbing around 440 nm appears to be an inherent property of different types of N-oxides. Considering the dipole character of the N+-O- function, a co-ordinate iron-oxygen bond is proposed to be formed in these complexes.     

Cytochrome P-450 enzyme-specific control of the regio- and enantiofacial selectivity of the microsomal arachidonic acid epoxygenase

Chiral analysis of the rat liver microsomal arachidonic acid epoxygenase metabolites shows enantioselective formation of 8,9-, 11,12-, and 14,15-cis-epoxyeicosatrienoic acids in an approximately 2:1, 4:1, and 2:1 ratio of antipodes, respectively. Animal treatment with the cytochrome P-450 inducer phenobarbital increased the overall enantiofacial selectivity of the microsomal epoxygenase and caused a concomitant inversion in the absolute configurations of its metabolites. These effects of phenobarbital were time-dependent and temporally linked to increases in the concentration of microsomal cytochrome P-450 enzymes. Reconstitution of the epoxygenase reaction utilizing several purified cytochrome P-450 demonstrated that the asymmetry of epoxidation is under cytochrome P-450 enzyme control. These results established that the chirality of the hepatic arachidonic acid epoxygenase is under regulatory control and confirm cytochromes P-450 IIB1 and IIB2 as two of the endogenous epoxygenases induced in vivo by phenobarbital.     

Purification and characterization of a previously unreported form of cytochrome P-448 from the liver of 3-methylcholanthrene-pretreated rats.

At least four hepatic isoenzymes of cytochrome P-450 were purified and characterized from rats treated with 3-methylcholanthrene. A monoclonal antibody developed against one of the forms (designated cytochrome P-450 MC-B) and polyclonal antibodies against others were used to demonstrate that form MC-B is immunologically distinct from other methylcholanthrene-inducible forms. Limited N-terminal amino acid sequencing showed that cytochrome P-450 MC-B has a primary structure that differs from the N-terminal sequences of other established rat isoenzymes. Cytochrome P-450 MC-B has a minimum Mr of 53,000, a CO-reduced spectral maximum at 448 nm, a Soret maximum of 417 nm in the absolute oxidized spectrum and a pattern of substrate preferences that differs from those of the other methylcholanthrene-induced forms. The other forms (MC-A, MC-C and MC-D) share characteristics with isoenzymes previously reported by other investigators.     

Properties of electrophoretically homogeneous phenobarbital-inducible and beta-naphthoflavone-inducible forms of liver microsomal cytochrome P-450.

Procedures are described for the isolation of two forms of rabbit liver microsomal liver microsomal cytochrome P-450 (P-450LM) in homogeneous state. They are designated by their relative electrophoretic mobilities on polyacrylamide gel in the presence of sodium dodecyl sulfate as P-450LM2 and P-450LM4. P-450LM2, which was isolated from phenobarbital-induced animals, has a subunit molecular weight of 48,700. The best preparations contain 20 nmol of the cytochrome per mg of protein and 1 molecule of heme per polypeptide chain. P-450LM4, which is induced by beta-naphthoflavone but is also present in phenobarbital-induced and untreated animals, was isolated from all three sources and found to have a subunit molecular weight of 55,300. The best preparations contain 17nmol of the cytochrome per mg of protein and 1 molecule of heme per polypeptide chain. Some of the purified preparations of the cytochromes, although electrophoretically homogeneous, contain apoenzyme due to heme loss during purification. The purified proteins contain no detectable NADPH-cytochrome P-450 reductase, cytochrome b5, or NADH-cytochrome b5 reductase, and only low levels of phospholipid (about 1 molecule per subunit). Amino acid analysis indicated that P-450LM2 and P-450LM4 are similar in composition, but the latter protein has about 60 additional residues. The COOH-terminal amino acid of P-450LM2 is arginine, as shown by carboxypeptidase treatment, whereas that of P-450LM4 is lysine. NH2-terminal amino acid residues could not be detected. Carbohydrate analysis indicated that both cytochromes contain 1 residue of glucosamine and 2 of mannose per polypeptide subunit. The optical spectra of the oxidized and reduced cytochromes and carbon monoxide complexes were determined. Oxidized P-450LM2 has maxima at 568, 535, and 418 nm characteristic of a low spin hemeprotein, and P450LM4 from beta-naphthoflavone-induced, phenobarbital-induced, or control microsomes has maxima at 645 and 394 nm, characteristic of the high spin state. The spectrum of -450lm4 becomes similar to that of P-450LM2 at high protein concentrations or upon the addition of detergent (Renex), whereas the spectrum of P-450LM2 is unaffected by the protein concentration or the presence of detergent. Electron paramagnetic resonance spectrometry of the purified cytochromes indicated that oxidized -450lm2 is in the low spin state, whereas P-450LM4 is largely, but not entirely, in the high spin state.     

The occurrence of multiple forms of cytochrome P-450 in hepatic microsomes from untreated rats and mice

The hepatic microsomes of rat and mice were subfractionated by the procedure of Dallner. When a 1.3 M sucrose lower layer was used for the two-step discontinuous gradient, no differences in spectral characteristics were noted between subfractions, though the smooth fractions (SER) had higher oxidative activity towards the substrates tested. When lower layers of 1.05, 1.1 or 1.15 M sucrose were used, the SER isolated contained cytochrome P-450 with significantly different spectral characteristics from that of the rough fraction (RER). The SER cytochrome P-450 had a wavelength maximum in the carbon-monoxide reduced difference spectrum that was significantly lower (ca. 1.0 nm) than that in the RER. In addition, the type I:CO-reduced spectral ratio of these fractions is significantly elevated. These data indicate that liver microsomes from untreated rats and mice contain more than one cytochrome P-450 and that these cytochromes may be located in different parts of the endoplasmic reticulum.     

Participation of cytochrome P-450 in nicotine oxidation

Addition of nicotine to phenobarbital-inducible cytochrome P-450 caused a shift of maximum of Soret peak toward the red approximately 3 nm. The difference spectrum produced by nicotine showed a type 2 spectral change with a peak at 427 nm and a trough at 393 nm. A spectral dissociation constant of phenobarbital-inducible cytochrome P-450 was found to be 0.16 mM for nicotine. Nicotine oxidation in the reconstituted system depended on cytochrome P-450, NADPH-cytochrome P-450 reductase and NADPH. These results indicate that phenobarbital-inducible cytochrome P-450 participates in nicotine oxidation.     

Purification and characterization of a soybean flour-induced cytochrome P-450 from Streptomyces griseus.

A soybean flour-induced, soluble cytochrome P-450 (P-450soy) was purified 130-fold to homogeneity from Streptomyces griseus. Native cytochrome P-450soy is a single polypeptide, with a molecular weight of 47,500, in association with one ferriprotoporphyrin IX prosthetic group. Oxidized P-450soy exhibited visible absorption maxima at 394, 514, and 646 nm, characteristic of a high-spin cytochrome P-450. The CO-reduced difference spectrum of P-450soy had a Soret maximum at 448 nm. When reconstituted with spinach ferredoxin and spinach ferredoxin:NADP+ oxidoreductase, purified cytochrome P-450soy catalyzed the NADPH-dependent oxidation of the xenobiotic substrates precocene II and 7-ethoxycoumarin. In vitro proteolysis of cytochrome P-450soy generated a stable and catalytically active cytochrome P-450, designated P-450soy delta.     

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces changes in the heme spin state of microsomal cytochrome P-450

In vitro studies on the nature of interaction of the neurotoxin MPTP with hepatic microsomal cytochrome P-450 were carried out. Spectral perturbation studies showed nitrogenous ligand type binding between MPTP and cytochrome P-450 with a peak at 423 nm and a broad trough at 400 nm. Scatchard analysis of MPTP-cytochrome P-450 binding suggested that MPTP binds to at least 2 species of cytochrome P-450--a high affinity binding species with an apparent spectral dissociation constant (Ks) of 372 microM and a low affinity species with Ks of 37.6 mM. EPR studies confirmed that MPTP is a type II substrate for the forms of cytochrome P-450 with which it interacts and causes a shift from the high spin state of cytochrome P-450 to the low spin state. MPTP is, thus, likely to be an effective inhibitor of cytochrome P-450.