Crystalline cytochrome P-450cam

Recollection of the early years of the research on cytochrome P450

Since the publication of the first paper on "cytochrome P450" in 1962, the biochemical research on this novel hemoprotein expanded rapidly in the 1960s and the 1970s as its principal roles in various important metabolic processes including steroid hormone biosynthesis in the steroidogenic organs and drug metabolism in the liver were elucidated. Establishment of the purification procedures of microsomal and mitochondrial P450s in the middle of the 1970s together with the introduction of molecular biological techniques accelerated the remarkable expansion of the research on P450 in the following years. This review paper summarizes the important developments in the research on P450 in the early years, for about two decades from the beginning, together with my personal recollections.

Purification and characterization of 2-ethoxyphenol-induced cytochrome P450 fromCorynebacteriumsp. strain EP1

A soluble cytochrome P450 (P450EP1A) induced by 2-ethoxyphenol was purified to apparent homogeneity from Corynebacterium sp. strain EP1. The P450EP1Ashowed a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a molecular weight of about 45 kDa. The CO-reduced difference spectra of P450EP1Ahad a Soret maximum at 447.6 nm. The substrate difference spectra with 2-ethoxyphenol showed an absorption maximum at 394.0 nm. The purified P450EP1Adegraded 2-ethoxyphenol in an assay system composed of spinach ferredoxin-NADP+oxidoreductase and NADPH. The reaction activity decreased to 1.4% of its original activity by addition of CO. The existence of catechol in the reaction mixture was confirmed after the metabolic reaction, indicating that P450EP1Acatalyzes O-dealkylation of 2-ethoxyphenol. In addition to 2-ethoxyphenol, the P450EP1Ametabolized 2-methoxyphenol, 1,1,1-trichloroethane, carbon tetrachloride, benzene, and toluene.Key words: cytochrome P450, Corynebacterium sp., 2-ethoxyphenol, enzyme purification, biodegradation.

Crystallization and preliminary x-ray diffraction analysis of P450terp and the hemoprotein domain of P450BM-3, enzymes belonging to two distinct classes of the cytochrome P450 superfamily

Cytochromes P450 are members of a superfamily of hemoproteins that are involved in the metabolism of various physiologic and xenobiotic organic compounds. This superfamily of proteins can be divided into two classes based on the electron donor proximal to the P450: an iron-sulfur protein for class I P450s or a flavoprotein for class II. The only known tertiary structure of any of the cytochromes P450 is that of P450cam, a class I soluble enzyme isolated from Pseudomonas putida (product of the CYP101 gene). To understand the details of the structure-function relationships within and between the two classes, structural studies on additional cytochromes P450 are crucial. We report here characterization of the crystal forms of two soluble, bacterial enzymes: cytochrome P450terp [class I enzyme from a Pseudomonas species (product of CYP108 gene)] and the hemoprotein domain of cytochrome P450BM-3 [class II enzyme from Bacillus megaterium (product of the CYP102 gene)]. The crystals of cytochrome P450terp are hexagonal and belong to the space group P6(1)22 (or its enantiomorph, P6(5)22) with unit cell dimensions a = b = 68.9 A and c = 458.7 A. The crystals of the hemoprotein domain of cytochrome P450BM-3 are monoclinic and belong to the space group P2(1) with unit cell dimensions a = 59.4 A, b = 154.0 A, c = 62.2 A, and beta = 94.7 degrees. Diffraction data for the crystals of these two proteins were obtained to a resolution better than 2.2 A. Assuming the presence of two molecules in the asymmetric unit for the hemoprotein domain of P450BM-3 and one molecule for P450terp, the calculated values of Vm are 2.6 and 3.3 A3/Da, respectively.

Crystallization of mitochondrial ubiquinol-cytochrome c reductase

Ubiquinol-cytochrome c reductase of beef heart mitochondria was crystallized in the presence of decanoyl-N-methylglucamide, heptanetriol, and sodium chloride with poly(ethylene glycol) as precipitant. The largest crystal has dimensions of 4 x 2 x 1 mm. The crystalline enzyme is composed of 10 subunits. It contains 2.5 nmol of ubiquinone, 8.4 nmol of cytochrome b, 4.2 nmol of cytochrome c1, 4.2 nmol of iron-sulfur cluster, and 140 nmol of phospholipid per milligram of protein. Of the last, 36% is with diphosphatidylglycerol. The crystals are very stable in the cold and show full enzymatic activity when redissolved in aqueous solution. Absorption spectra of the redissolved crystals show a Soret to UV ratio of 0.88 and 1.01 in the oxidized and the reduced forms, respectively.

Epoxidation of aflatoxin B1 by Aspergillus flavus microsomes in vitro: interaction with DNA and formation of aflatoxin B1-glutathione conjugate

Metabolism of aflatoxin B1 (AFB1) by subcellular preparations of Aspergillus flavus is least understood. The results reported here have demonstrated for the first time the epoxidation of AFB1 and subsequent conjugation with glutathione (GSH). Microsomes prepared from toxigenic mycelia catalysed [3H]AFB1 to calf thymus DNA to a greater extent (approximately 2-fold) as compared to that of non-toxigenic. The binding of [3H]AFB1 to exogenous and A. flavus nuclear DNA catalyzed by A. flavus microsomes was found to be comparable with that of mammalian extrahepatic tissue such as lung. Addition of phenobarbitone to the growing cultures resulted in 1.5-fold increase in [3H]AFB1-DNA binding mediated by microsomes prepared from either of the two strains. Tolnaftate, an inhibitor of aflatoxin synthesis enhanced the epoxidation rate in a dose-related manner. The binding of [3H]AFB1 to DNA catalyzed by A. flavus microsomes was significantly reduced (50% of control) upon addition of hamster liver cytosol, thereby substantiating the formation of the carcinogen adduct with DNA as reported in mammalian tissues. The metabolite formed by subcellular preparation of A. flavus was found to be AFB1-GSH having Rf value (6.5) similar to that obtained for mammalian liver preparations.

Enhanced decomposition of oxyferrous cytochrome P450CIA1 (P450cam) by the chemopreventive agent 3-t-butyl-4-hydroxyanisole

The efficacy of 2(3)-t-butyl-4-hydroxyanisole (BHA) and other chemicals as chemopreventive agents against chemically induced cancer or toxicity may involve direct modulation of cytochrome P450 activity. Direct interaction of BHA with cytochrome P450 was investigated using substrate-bound, oxyferrous cytochrome P450CIA1 either in a reconstituted system containing cytochrome P450CIA1, putidaredoxin, and putidaredoxin reductase with NADH as electron donor or in the absence of physiological electron donors. In the reconstituted system, BHA caused a concentration-dependent decrease in the production of 5-exo-hydroxycamphor and a substoichiometric increase in hydrogen peroxide production. However, BHA did not appreciably inhibit either NADH oxidation or oxygen utilization under conditions optimal for accumulation of oxyferrous cytochrome P450CIA1 during steady-state metabolism of camphor. In the absence of electron donor, BHA enhanced decomposition of the ternary oxyferrous substrate complex of cytochrome P450CIA1 without the formation of any apparent spectral intermediate(s). The rate of decomposition of the oxyferrous complex was pseudo-first order and was dependent upon the concentration of BHA present. Enhanced decomposition of the complex was not attributable to catalytic turnover of cytochrome P450CIA1 (i.e., acquisition of a second electron from an indeterminate source) since no appreciable metabolism of either camphor or BHA was observed. The enhanced decomposition was accompanied by a substoichiometric increase in hydrogen peroxide production, suggesting that BHA may facilitate four-electron reduction of molecular oxygen to water. These results indicate that BHA inhibits cytochrome P450 function, presumably by enhancing autoxidation of the substrate-bound oxyferrous complex.

Magnetic circular dichroism of Pseudomonas putida cytochrome P-450 in near infrared region

Magnetic circular dichroism spectra of oxidized, reduced and carbonmonoxy reduced forms of cytochrome P-450 from D-camphor grown Pseudomonas putida (P-450cam) were studied in the near infrared region (650 to 1200 nm) at various temperatures in the presence of D-camphor. Oxidized P-450cam with camphor exhibited positive (+) and negative (-) magnetic CD bands at 825 and 970 nm, respectively, and both of them were assigned to Faraday B terms. The magnetic CD spectrum of reduced P-450cam in the presence of D-camphor exhibited at least five components in the region between 650 to 1175 nm and one of them at 760 nm showed considerably smaller magnitude than that of the corresponding band of deoxymyoglobin. These results were interpreted to mean that the heme-iron in both oxidized and reduced P-450cam has a ligand field symmetry lower than C4v, i.e. a strong rhombic character of the heme in cytochrome P-450. Carbonmonoxide complex of reduced P-450cam exhibited no detectable magnitude of magnetic CD in the near infrared region but showed CD bands at 710 (-) and 850 (+) nm. The results were compared and discussed with those of carbonmonoxy hemoglobin and myoglobin. In addition, temperature dependent changes in the spin state of oxidized P-450cam from high to low by decrease of temperature were observed by measuring both magnetic CD and absorption spectra in the near ultraviolet and visible regions (300 to 650 nm), provided that the temperature of the sample was varied slowly (approximately 3 degrees C/min) between room and liquid nitrogen temperature in a 0.03 M phosphate buffer (pH 7.2) containing a saturated amount of D-camphor and 70% (v/v) glycerol. The significance of this phenomenon is also discussed.

Insect cytochrome P-450

Two approaches may be used to study the function of cytochrome P-450 in insects: (a) an evaluation of the spectral and catalytic properties of the hemoprotein while associated with microsomal membranes; (b) the solubilization, resolution and purification of the microsomal mixed-function oxidase system. The first approach has provided some understanding of the biochemical factors involved in the metabolism of a variety of compounds, including pesticides, drugs, hormones and many other xenobiotics. However, solubilization of the monooxygenase system allows the study of each of its components individually, providing a better insight on the sequence of events leading to the hydroxylation of a substrate, the type of intermediates formed, and the rate-limiting step(s). This report discusses studies carried out with the monooxygenase system associated with microsomal membranes, as well as procedures to solubilize and partially purify its components from housefly microsomes. The latter involves solubilization with either Triton X-100 or sodium cholate, followed by either ammonium sulfate fractionation, Sephadex G-200, DEAE-Sephadex A-50 column chromatography or by omega-amino-n-octyl-Sepharose 4B affinity chromatography. These procedures have shown that two cytochrome P-450 species (P-450 and P-450I) are present in microsomes isolated from a resistant housefly strain. Induction with either naphthalene or phenobarbital appears to increase cytochrome P-450I preferentially.

A thermodynamic model of regulation: modulation of redox equilibria in camphor monoxygenase

Regulation of biological phenomena occurs in all types of systems, being manifested in many different reaction types, from allosteric behavior in proteins, through modulation in energy and information transfer, to the control of growth and differentiation in cells, organelles, and organisms. In this communication, a modulation in oxidation/reduction potential via ligation of substrate and protein components in the camphor 5-exo-monoxygenase system is described in terms of a four-state system using as fundamental parameters the transition free energies between equilibrium states. This approach provides a concise description of the data and is useful for describing many aspects of regulatory phenomena.

A role of the putidaredoxin COOH-terminus in P-450cam (cytochrome m) hydroxylations

Methylene hydroxylation by cytochrome P-450(cam) (cytochrome m) can be resolved into four distinct steps: substrate addition, m(o) --> m(os); reduction, m(os) --> m(rs); dioxygen addition, m(rs) --> m(O2) (rs); followed by a second putidaredoxin (Pseudomonas putida ferredoxin)-mediated reduction and product formation. The isolated ferrous oxy-substrate complex exhibits first-order decay kinetics with the relatively slow rate constant of k [unk] 0.01 sec(-1), at 25 degrees , without product release. Putidaredoxin addition accelerates the decomposition with second-order kinetics, k [unk] 51,000 M(-1) sec(-1), and initiation of product formation. Cytochrome m forms a complex with putidaredoxin with dissociation constant of K(D) = 3 muM. In the complete three-protein hydroxylase system, consisting of cytochrome m, putidaredoxin, and the reductase (a DPNH-specific flavo-protein), camphor hydroxylation occurs with a stoichiometry of 1 mole each of DPNH and O(2) used per mole of product formed; the K(M) for putidaredoxin is about 4.2 muM.Putidaredoxin, on treatment with carboxypeptidase A, loses one molecule each of tryptophan and glutamine sequentially from the carboxy terminus to expose a terminal arginine. The tryptophan-free product has been separated from native putidaredoxin and other impurities, and retains the visible and electron paramagnetic resonance spectra and the redox potential of the active center of native putidaredoxin. This modified redoxin binds less tightly to cytochrome m, K(D) [unk] 150 muM, and is 50 times less effective in stimulation of the m(O2) (rs) decay rate. A similar decrease in specific activity is observed in the complete hydroxylase system.

Dynamics of carbon monoxide binding by heme proteins

Rebinding of carbon monoxide to myoglobin and to cytochrome P-450 after removal by a light flash occurs down to 50 degrees K for myoglobin and 25 degrees K for cytochrome P-450 in glycerol-water solution. Above 240 degrees K the reaction is second order; between 240 degrees and 200 degrees K the rebinding becomes exponential and independent of the carbon monoxide concentration. Below 150 degrees K the reaction follows a power law and is approximately 10(3) times faster for cytochrome P-450 than for myoglobin.

Photoreduction of NADP + sensitized by synthetic pigment systems

Two synthetic pigment systems capable of enzymatically photoreducing NADP(+) are described. One system contains proflavine; the other, acridine. The complete system consists of ethylene diamine tetraacetic acid (EDTA), proflavine (or acridine), ferredoxin-NADP reductase (EC 1.6.99.4), and NADP(+). The two pigments initiate the photoreduction of NADP(+) in different portions of the electromagnetic spectrum. Proflavine photosensitizes in the visible portion; acridine, in the ultraviolet. Neither proflavine nor acridine is structurally related to chlorophyll. The acridine system has the attractive property that the enzyme, ferredoxin-NADP reductase, is the only component of the system that absorbs appreciably in the visible region of the spectrum.