Comparative study of two highly purified forms of liver microsomal cytochrome P-450: circular dichroism and other properties

CYTOCHROME P450: Nature's Most Versatile Biological Catalyst

The author describes studies that led to the resolution and reconstitution of the cytochrome P450 enzyme system in microsomal membranes. The review indicates how purification and characterization of the cytochromes led to rigorous evidence for multiple isoforms of the oxygenases with distinct chemical and physical properties and different but somewhat overlapping substrate specificities. Present knowledge of the individual steps in the P450 and reductase reaction cycles is summarized, including evidence for the generation of multiple functional oxidants that may contribute to the exceptional diversity of the reactions catalyzed.

Allosteric phenomena in cytochrome P450-catalyzed monooxygenations

Allosteric regulation of monooxygenase activity is shown to occur with diverse cytochrome P450 isoforms and is characterized by kinetic patterns deviating from the Michaelis-Menten model. Homotropic and heterotropic phenomena are encountered in both substrate activation and productive coupling of the electron donors NADPH-cytochrome P450 reductase and cytochrome b5, and the lipid environment of the system also appears to play a role as an effector. Circumstantial analysis reveals the components of the electron transfer chain to be mutually beneficial in interactions with each other depending on the substrate used and type of cytochrome P450 operative. It is noteworthy that association of diatomic gaseous ligands may be amenable to allosteric regulation as well. Thus, dioxygen binding to cytochrome P450 displays nonhyperbolic kinetic profiles in the presence of certain substrates; the latter, together with redox proteins such as cytochrome b5, can exert efficient control of the abortive breakdown of the oxyferrous intermediates formed. Similarly, substrates may modulate the structural features of the access channel for solutes such as carbon monoxide in specific cytochrome P450 isozymes to either facilitate or impair ligand diffusion to the heme iron. The in vivo importance of allosteric regulation of enzyme activity is discussed in detail.

Some properties of mitochondrial adrenodoxin associated with its nonconventional electron donor function toward rabbit liver microsomal cytochrome P450 2B4

Mitochondrial adrenodoxin (Adx) was found to cross-react with microsomal cytochrome P450 2B4 (CYP2B4) as the terminal electron acceptor. When compared with NADPH-cytochrome P450 reductase (P450R), the natural redox partner of CYP2B4, Adx was less efficient both in transferring the first electron and in coupling the system. The ferredoxin yielded an unusual reverse type I spectral change with low-spin CYP2B4, which underwent transformation to a typical type I optical perturbation upon deletion of the signal anchor sequence (Delta2-27) of the hemoprotein. Truncation of CYP2B4 slightly fostered electron transfer from Adx, but was deleterious to reduction of the engineered isozyme by P450R. Addition of manganese-substituted cytochrome b5, which failed to serve as an electron donor to CYP2B4, augmented the amount of hemoprotein existing in form of a low-spin complex with Adx and affected the ferredoxin-dependent reduction kinetics through causing a proportional rise in both Km and Vmax. Conservative replacement of Asp-76 with glutamate in the Adx molecule was associated with a drastic drop in reductive efficiency toward CYP2B4, while spectral binding of the mutant to the hemoprotein was marginally changed. The results support the concept of an evolutionary relationship between the various cytochrome P450 forms as regards the conservation of surface regions participating in contacts with heterologous donor proteins.

Conformational change of cytochrome P450 1A2 induced by phospholipids and detergents

Recently, it was reported that the activity of rabbit P450 1A2 is markedly increased at elevated salt concentration (Yun, C-H., Song, M., Ahn, T., and Kim, H. (1996) J. Biol. Chem. 271, 31312-31316). The activity increase of P450 1A2 coincides with the raised alpha-helix content and decreased beta-sheet content. The presence of phospholipid magnified this effect. Here, possible structural change of rabbit P450 1A2 accompanying the phospholipid-induced increase in its enzyme activity was investigated by circular dichroism, fluorescence spectroscopy, and absorption spectroscopy. Studies with the reconstituted system supported by cumene hydroperoxide or NADPH showed that the P450 1A2 activities were found to be dependent on the head group and hydrocarbon chain length of phospholipid. Phosphatidylcholines having short hydrocarbon chains with a carbon number of 8-12 were very efficient for reconstitution of the P450-catalyzed reactions supported by both cumene hydroperoxide and NADPH. It was found that the phospholipid increased the alpha-helix content and lowered the beta-sheet content of P450. Intrinsic fluorescence intensity is also increased in the presence of phospholipid. The low spin iron configuration of P450 1A2 shifted toward the high spin configuration by most of the phospholipids in the endoplasmic reticulum. Some synthetic phospholipids having short hydrocarbon chains with a carbon number of 10-12 caused a shift in the spin equilibrium of P450 1A2 toward low spin. The effect of detergents on the activity and conformation of P450 1A2 was also studied. It was found that the addition of detergents to P450 1A2 solution increased the enzyme activity of P450 1A2. Detergents also increased the alpha-helix content and lowered the beta-sheet content of P450 1A2. Intrinsic fluorescence emissions also increased with the presence of detergents. Octyl glucoside and deoxycholate caused a shift toward high spin. On the other hand, cholate caused a shift toward low spin. It was found that the activity increase of rabbit P450 1A2 coincides with the conformational change including raised alpha-helix content. It is proposed that the interaction with the phospholipid molecules surrounding P450 1A2 in the endoplasmic reticulum is important for a functional conformation of P450 1A2 in a monooxygenase system including NADPH-P450 reductase.

Conformational change of cytochrome P450 1A2 induced by sodium chloride

Recently, it was reported that the activity of rabbit P450 1A2 is markedly increased at elevated sodium phosphate concentration. Here, the possible structural change of rabbit P450 1A2 accompanying the NaCl-induced increase in its enzyme activity is investigated by fluorescence spectroscopy, circular dichroism, and absorption spectroscopy. It was found that NaCl increased alpha-helix content and lowered beta-sheet content of P450 1A2 in the presence as well as in the absence of a phospholipid. Intrinsic fluorescence emissions also increased with increasing NaCl concentration. The low spin iron configuration of P450 1A2 shifted toward the high spin configuration in response to the increased salt concentration. The effect of increased potassium phosphate and NaCl on the P450 1A2 activity was also studied. It was found that the activity increase of rabbit P450 1A2 occurs concomitantly with the conformational change including raised alpha-helix content.

Identification of nitric oxide synthase as a thiolate-ligated heme protein using magnetic circular dichroism spectroscopy. Comparison with cytochrome P-450-CAM and chloroperoxidase

Nitric oxide (NO) has recently been recognized as an important biomolecule playing diverse physiological roles. It is synthesized in several different tissues from L-Arg and O2, using NADPH as an electron donor, by a family of heme-containing catalytically self-sufficient monooxygenases known as nitric oxide synthases (NOS). Recently, the CO complex of reduced NOS has been shown to exhibit an absorption maximum near 450 nm, a characteristic spectral feature of cytochrome P-450 (P-450). Yet, the amino acid sequences of NOS and P-450 have no homology. To further probe the active site heme coordination structure and the heme environment of NOS, we have employed magnetic circular dichroism (MCD) and CD spectroscopy in the present study. MCD spectra of several derivatives of rat brain neuronal NOS strikingly resemble those of analogous derivatives of bacterial P-450-CAM and fungal chloroperoxidase, two known thiolate-ligated heme proteins. Given the proven fingerprinting capability of MCD spectroscopy, this provides convincing evidence for endogenous thiolate (cysteinate) ligation to the heme iron of NOS. Furthermore, the heme-related Soret CD bands of NOS (positive) and P-450s (negative), as represented by P-450-CAM, are almost mirror images, whereas chloroperoxidase exhibits totally different CD band shapes. This suggests that the active sites of NOS and P-450 may share some common structural features, but significant distinctions exist between their heme environments in certain aspects such as hydrophobicity or size.

Kinetics of cytochromes P-450 IA1 and IIB1 in reconstituted systems with dilauroyl- and distearoyl-glycerophosphocholine

In the present study the effect of changing the fatty acyl moiety of phosphatidylcholine from dilauroyl to distearoyl on the kinetic parameters of O-dealkylation of alkoxyresorufins and ethoxycoumarin dependent on reconstituted cytochromes P-450 IA1 and IIB1 has been investigated. The results demonstrate that (a) the maximum rate of O-dealkylation (V) for both P-450 enzymes was about two times higher in the L-alpha-dilauroyl-sn-glycero-3-phosphocholine (Lau2GroPCho) system and (b) changes in the fatty acyl moiety of phosphatidylcholine (acyl2GroPCho) from dilauroyl to distearoyl affected the apparent Km for the substrate (Kms) of P-450 IA1 and IIB1 in a different way. In addition, (c) the kinetic parameters appeared to be dependent on the acyl2GroPCho/P-450 ratio and a change in this ratio affected the kinetic parameters of P-450 IA1 and IIB1 in a different manner. From these last two observations it was concluded that the mechanism by which phospholipids influence P-450-IIB1-dependent O-dealkylation of ethoxycoumarin is different from that by which they influence P-450-IIB1-dependent O-dealkylation of this substrate. Furthermore, the results of the present study demonstrate that the increase in the rate of O-dealkylation of ethoxycoumarin, reported in the literature for reconstituted systems in the presence of Lau2GroPCho, results from an effect of Lau2GroPCho on both the Kms and the V. In a number of additional experiments possible mechanisms underlying the observed differential effect of Lau2GroPCho and Ste2GroPCho on the Kms and V of P-450 IA1 and IIB1 were investigated. This was done by studying the effect of the two acyl2GroPCho species on the kinetic parameters of some of the different steps of the P-450 cycle, namely substrate binding, oxygen binding and the rate of electron transfer. The results demonstrate an influence of Lau2GroPCho and Ste2GroPCho on (a) substrate binding to cytochrome P-450, (b) the affinity of cytochromes P-450 for NADPH-cytochrome reductase and thus on (c) the electron flow through the reconstituted system. Based on the results from these experiments it was concluded that the increased V of P-450 IA1 and IIB1 in the presence of Lau2GroPCho compared to the systems with Ste2GroPCho was at least in part due to an increased affinity of both P-450 enzymes for NADPH-cytochrome reductase in the presence of Lau2GroPCho compared to Ste2GroPCho.(ABSTRACT TRUNCATED AT 400 WORDS)

The cholesterol-side-chain-cleaving cytochrome P450 spin-state equilibrium. 2. Conformational analysis

We have confirmed and characterised structurally the enzyme conformational changes deduced from the preceding thermodynamic analysis of the adrenal mitochondrial cholesterol-side-chain-cleaving cytochrome P450 spin-state equilibrium. The spin-transition kinetics following rapid pH jumps were multiphasic in aqueous buffer and biphasic in the presence of 35% ethylene glycol. The activation energy between -2 degrees C and 30 degrees C of both phases was exceptionally high (Ea = 147 kJ.mol-1), suggesting the involvement of large-scale conformational changes. The pH and temperature effects on the CD spectrum show that the enzyme is in equilibrium between at least two conformations which are predicted by the thermodynamic model, but which are not directly correlated to the spin state. The CD changes between 260 nm and 280 nm indicate that the conformation prevailing at high temperatures is characterised by a decreased polarity of the tyrosine environments; the changes between 200 nm and 250 nm suggest furthermore a 4% decreased protein helical content.

Multichannel circular dichroism investigations of the structural stability of bacterial cytochrome P-450

The thermal unfolding of cytochrome P-450 LIN and P-450 CAM measured in presence and absence of their specific substrates was analyzed by circular dichroism (CD) and the alpha-helix content was estimated. Both proteins show, independent of the presence or absence of the substrates, nearly the same amount of loss of the CD in the peptide region. The comparison of the half transition temperatures determined from different chromophores and different methods indicates a non-two-state transition of the thermal unfolding. For such analysis we developed a new spectrometer, which is capable of measuring the CD simultaneously at all wavelengths in a limited wavelength region.

The structure of the dihaem cytochrome b of fumarate reductase in Wolinella succinogenes: circular dichroism and sequence analysis studies

The fumarate reductase from Wolinella succinogenes contains two haem groups with markedly different midpoint potentials (-20 mV and -200 mV). The enzyme is made up of three subunits, the lipophilic one of which (cytochrome b) ligates the haems. Circular dichroism (CD) spectroscopy has been applied to the reductase in order to obtain information on the structure of the haems and of their environment. This approach is integrated with amino acid sequence comparison of the cytochrome b with other quinone-reacting membrane haemoproteins for predicting the axial ligands of the haems as well as their location relative to the membrane. The following results have been obtained: (1) the CD spectra in the Soret region show exciton coupling indicating haem-haem interaction, which is particularly evident in the reduced state and disappears upon denaturation of the enzyme; (2) The apoprotein of cytochrome b is predicted to consist of five hydrophobic helices (helices A-D and cd), four of which should span the membrane. Helices A, B, C and cd contain a histidine residue each which possibly forms one of the ligands of the haems. It is proposed that haem b (-20 mV) is ligated by H44 and H93, and haem b (-200 mV) by H143 and H182.

Heme maintains catalytically active structure of cytochrome P-450

Treatment of purified cytochrome P-450 LM2 and its liposome-bound form with hydrogen peroxide led to complete destruction of the P-450 heme. The apoenzyme thus produced could be reconstituted to catalytically active cytochrome P-450 by incubation with hemin, the reconstitution efficiency being 50% for the soluble enzyme and 80% for the liposome-bound enzyme. The removal of heme from the soluble hemoprotein resulted in a 3-fold decrease in the efficiency of its incorporation into sonicated liposomes. The contents of 5 secondary structure forms in the native, apo- and reconstituted holoenzymes were estimated from their circular dichroism spectra. It was thus found that the helix content increased from 34% to 60% upon removal of the heme from the native enzyme. We suggest that the increase in the helix content leads to a reduction of the incorporation efficiency into liposomal membranes.

On the role of phospholipids in the reconstituted cytochrome P-450 system. A model study using dilauroyl and distearoyl glycerophosphocholine

The difference in pentoxyresorufin O-dealkylating activity observed in a reconstituted system containing dilauroylglycerophosphocholine (Lau2GroPCho) or distearoylglycerophosphocholine (Ste2GroPCho) was used as a model to study the role of phospholipids in the reconstituted cytochrome P-450b (IIB1) system. The hypotheses proposed in the literature for the role of phospholipids in the reconstituted cytochrome P-450 system, mainly based on the comparison of systems without phospholipid and with Lau2GroPCho, were either validated or shown to be unlikely when tested by comparing reconstituted systems with different phosphatidylcholines. The higher activity in the Lau2GroPCho system as compared to the Ste2GroPCho system cannot be ascribed to (a) an increased affinity of cytochrome P-450 for the NADPH-cytochrome reductase in the Lau2GroPCho system, also not to (b) a Lau2GroPCho-dependent dissociation of protein multimers, nor to (c) a change in the spin state of the heme. We found a different apparent Km for pentoxyresorufin in the Lau2GroPCho system compared with the Ste2GroPCho system. Furthermore, we found a difference between the cytochrome P-450b tryptophan fluorescence polarization of the Lau2GroPCho system and the Ste2GroPCho system as well as with a system without phosphatidylcholine. From these observations it is concluded that the higher activity of the Lau2GroPCho system compared with the Ste2GroPCho system or with a system without additional phosphatidylcholine may at least in part be caused by a difference in the conformation of the cytochrome P-450 molecules in these systems. Furthermore, the different effects of both phosphatidylcholines on the Km and V for pentoxyresorufin not only suggest a role of phospholipids in the binding of the substrate to the active site of the cytochrome P-450 molecule, but also on the efficiency of electron transfer from NADPH-cytochrome reductase to cytochrome P-450.

Point mutations at threonine-301 modify substrate specificity of rabbit liver microsomal cytochromes P-450 (laurate (omega-1)-hydroxylase and testosterone 16 alpha-hydroxylase)

Thr-301 of cytochrome P-450 (laurate (omega-1)-hydroxylase) was replaced by Ser, Val, Ile, or Asn via site-directed mutagenesis. The Ser-, Val-, and Asn-mutants had lower laurate (omega-1)-hydroxylase activities than the wild-type P-450. The mutation to Ser did not affect caprate (omega-1)-hydroxylase activity and rather increased caprate omega-hydroxylase activity, but the Val- and Asn-mutants could not hydroxylate caprate. The Ile-mutant was devoid of the hydroxylase activities. The mutation also led to changes in the affinities for the fatty acids and exogenous ligands. Replacement of Thr-301 of cytochrome P-450 (testosterone 16 alpha-hydroxylase) by Ser or Val also affected the activities toward testosterone and progesterone in different ways. These findings indicate that residue 301 of the P-450s plays an important role in determining their substrate specificities.

Secondary structure prediction of liver microsomal cytochrome P-450; proposed model of spatial arrangement in a membrane

The secondary structure of rabbit liver microsomal cytochrome P-450 LM2, rat liver microsomal cytochromes P-450b and P-450e (phenobarbital-inducible), and rat liver microsomal cytochromes P-450c, P-450d (3-methylcholanthrene-inducible) was predicted by a combination of methods (i) identifying the transmembrane parts of integral membrane proteins, and (ii) statistically predicting the secondary structure of globular proteins. The results are similar for all phenobarbital-inducible enzymes and make it possible to construct two structural models with seven or four transmembrane alpha-helices. The cytochromes of the second group obviously form a second structural family with four membrane-spanning alpha-helices. In both cases, a large ectodomain with several consecutive alpha-helices, which may provide the heme-binding pocket, is exposed out of the membrane.

Chemical modification of cytochrome P-450 LM4. Identification of functionally linked tyrosine residues

Cytochrome P-450 LM4 (RH, reduced flavoprotein:oxygen oxidoreductase (RH-hydroxylating), EC 1.14.14.1) from rabbit liver microsomes was chemically modified with tetranitromethane. Nitration of two tyrosine residues inhibits the p-nitrophenetole O-deethylase activity of the enzyme by about 80%. Sequencing the 3-nitrotyrosine-containing peptides after HPLC tryptic peptide mapping reveals that mainly Tyr-243 and Tyr-271 are nitrated, whereas Tyr-71, Tyr-188 and Tyr-365 are modified to a lower extent. Nitration of tyrosine residues affects the complex formation with p-nitrophenetole, alpha-naphthoflavone and metyrapone as indicated by an increased affinity towards p-nitrophenetole and by a decreased affinity for the latter compounds. Furthermore, nitration interferes with the electron transfer from NADPH-cytochrome P-450-reductase to cytochrome P-450 LM4 resulting in a slowed down reduction reaction. The results suggest that Tyr-243 and Tyr-271 of cytochrome P-450 LM4 are functionally involved in the interaction with NADPH-cytochrome P-450 reductase.

Properties of the tryptophan residue in rabbit liver microsomal cytochrome P-450 isozyme 2 as determined by fluorescence

Cytochrome P-450 isozyme 2 from rabbit liver microsomes fluoresces upon excitation at 295 nm due to the single tryptophyl residue (Trp121) in the protein. The fluorescence spectrum, which is not altered by the presence of phospholipid or substrates, has a maximum at 335 nm, which suggests that the environment of the residue is hydrophobic. The fluorescence intensity decreases linearly with increase of specific content of the cytochrome preparations, and the holoenzyme was estimated to exhibit, at most, 6% as much fluorescence as the apoenzyme. This indicates that the fluorescence of the tryptophan is quenched by energy transfer to the heme. The distance between the tryptophyl residue and the heme was estimated to be less than 40 A. From enhancement of the fluorescence by methanol and ethanol, 30 to 50% of the Trp residue was found to be accessible to these solvents. On the other hand, the accessibility to iodide and cesium ions, as estimated by quenching effects, is less than 14%. From such evidence, the tryptophyl residue is believed to be partly buried. Since Trp121 is conserved at or near the same position in all mammalian P-450's so far sequenced, the results obtained may be applicable to these related cytochromes as well.

Spin state studies on cytochrome P-450 in liver microsomes from obese and diabetic animals

The spin state of liver microsomal cytochrome P-450 from obese mice and streptozotocin-diabetic mice and rats has been studied both by the temperature and the type I substrates-induced spectral changes. The high spin cytochrome P-450 is significantly decreased in these animals. Moreover absolute spectra indicate that low spin cytochrome P-450 is stabilized in streptozotocin induced-diabetic animals. Thus the physiopathological state may modify the in vivo spin state of cytochrome P-450 and modifications of the microsomal fatty acid composition might contribute to these changes.

Chemical modification of adrenocortical cytochrome P-450scc with tetranitromethane

Selective chemical modification of adrenocortical cytochrome P-450scc, responsible for key stages of steroid biogenesis, with tetranitromethane has been carried out. Nitration of the cytochrome P-450scc tyrosine residues results in heme protein inactivation with syncatalytic loss of enzyme activity. Analysis of the cytochrome P-450scc inactivation kinetics indicates that there are several pools of tyrosine residues, differing in their accessibility to tetranitromethane. The modification of cytochrome P-450scc results in changes in the hemeprotein spectral properties and its conformation which indicates to the involvement of essential tyrosine residue(s) in the heme-protein interaction. Cholesterol and adrenodoxin (high-spin effectors) prevent the inactivation of cytochrome P-450scc with tetranitromethane, i.e., protect the essential tyrosine residue(s) from modification. Possible functions of the tyrosine residues in the cytochrome P-450scc molecule are discussed.

Extensive studies of the heme coordination structure of indoleamine 2,3-dioxygenase and of tryptophan binding with magnetic and natural circular dichroism and electron paramagnetic resonance spectroscopy

In order to probe the active site of the heme protein indoleamine 2,3-dioxygenase, magnetic and natural circular dichroism (MCD and CD) and electron paramagnetic resonance (EPR) studies of the substrate (L-tryptophan)-free and substrate-bound enzyme with and without various exogenous ligands have been carried out. The MCD spectra of the ferric and ferrous derivatives are similar to those of the analogous myoglobin and horseradish peroxidase species. This provides strong support for histidine imidazole as the fifth ligand to the heme iron of indoleamine 2,3-dioxygenase. The substrate-free native ferric enzyme exhibits predominantly high-spin EPR signals (g perpendicular = 6, g parallel = 2) along with weak low-spin signals (g perpendicular = 2.86, 2.28, 1.60); similar EPR, spin-state and MCD features are found for the benzimidazole adduct of ferric myoglobin. This suggests that the substrate-free ferric enzyme has a sterically hindered histidine imidazole nitrogen donor sixth ligand. Upon substrate binding, noticeable MCD and EPR spectral changes are detected that are indicative of an increased low spin content (from 30 to over 70% at ambient temperature). Concomitantly, new low spin EPR signals (g = 2.53, 2.18, 1.86) and MCD features characteristic of hydroxide complexes of histidine-ligated heme proteins appear. For almost all of the other ferric and ferrous derivatives, only small substrate effects are observed with MCD spectroscopy, while substantial substrate effects are seen with CD spectroscopy. Thus, changes in the heme coordination structure of the ferric enzyme and in the protein conformation at the active site of the ferric and ferrous enzyme are induced by substrate binding. The observed substrate effects on the ferric enzyme may correlate with the previously observed kinetic substrate inhibition of indoleamine 2,3-dioxygenase activity, while such effects on the ferrous enzyme suggest the possibility that the substrate is activated during turnover.

Phosphorylation of rabbit liver cytochrome P-450 LM2 and its effect on monooxygenase activity

The phosphorylation of rabbit liver microsomal cytochrome P-450 LM2 by catalytic subunit of cyclic AMP-dependent protein kinase (W. Pyerin et al. (1983) Carcinogenesis 4, 573) has now been studied in detail with purified soluble form of cytochrome P-450 as well as with the purified protein incorporated into model membranes. The apparent Km values for P-450 of the phosphorylation reaction in all experimental systems were in a range of 2-8 microM, while the Vmax values were dependent on the state of P-450. Upon phosphorylation, the reconstituted enzyme activities with benzphetamine (N-demethylation) and 7-ethoxycoumarin (O-deethylation) as substrates were reduced to 30-40% of control.

The influence of oxygen donor ligation on the spectroscopic properties of ferric cytochrome P-450: ester, ether and ketone co-ordination to the haem iron

Homogeneous low-spin complexes of cytochrome P-450-CAM with esters, ethers and ketones have been prepared and characterized by u.v.-visible absorption, circular dichroism (CD), magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopy. Direct haem iron ligation has been verified by competition for binding with well-known haem ligands. The u.v.-visible spectra of the oxygen-donor complexes exhibit near-u.v. (delta) transitions near 356 nm, Soret maxima at 417 +/- 1 nm, beta bands near 536 nm and alpha peaks near 569 nm, with alpha greater than beta in intensity. Negative delta and Soret CD troughs are seen. The MCD spectra have minima at approximately 356 nm, intense derivative-shaped Soret features centred at approximately 416 nm and four characteristic features beyond 450 nm. The EPR spectra of these complexes, while similar to that of the native enzyme, exhibit slight variances. Anomalous spectral and substrate binding properties have been reported in the study of cytochrome P-450 under conditions employing solvents and non-phosphate buffers containing oxygen functionalities, and have been attributed to 'solvent effects'. The present work, in combination with our previous report of alcohol, amide and carboxylate oxygen donor complexes of cytochrome P-450, is evidence that a wide variety of oxygen-donor species are capable of direct ligation to the haem iron of cytochrome P-450. This leads us to suggest oxygen-donor ligation to cytochrome P-450 as the origin of spectral and substrate binding anomalies previously attributed to solvent effects.

Circular dichroism studies of low-spin ferric cytochrome P-450CAM ligand complexes

Circular dichroism (CD) spectroscopy has been used to probe the active site of bacterial ferric cytochrome P-450CAM. The endogenous sixth ligand to the heme iron has been displaced by an extensive series of exogenous oxygen, nitrogen, sulfur and other neutral and anionic donor ligands in an attempt to examine systematically the steric and electronic factors that influence the coupling of the heme chromophore to its protein environment. General trends for each ligand class are reported and discussed. Both the wavelengths and the intensities of the CD bands vary with ligand type and structure. All but one of the complexes exhibit negative CD maxima in their delta and Soret bands. Comparison to ferric myoglobin-thiolate complexes indicates that the negative sign observed for the cytochrome P-450 spectra is not a property of the thiolate fifth ligand, but rather arises from a different interaction of the cytochrome P-450 heme with its protein environment. Complexes with neutral oxygen donors display CD spectra that most closely resemble the spectrum of the native low-spin enzyme. Hyperporphyrin (split Soret) cytochrome P-450 complexes with thiolates, phosphines and cyanide trans to cysteinate have complex CD spectra, reflecting the intrinsic non-degeneracy of the Soret pi pi transitions. The extensive work presented herein provides an empirical foundation for use in analyzing the interaction of heme chromophores with their protein surroundings, not only for the cytochrome P-450 monooxygenases but also for heme proteins in general.

Modification of cytochrome P-450 with fluorescein isothiocyanate

Fluorescein isothiocyanate (FITC) has been shown to be selectively attached to the N-terminus of cytochrome P-450 LM2. The N-demethylase activity of cytochrome P-450 LM2 reconstituted systems modified in this way was inhibited by 25%. As revealed by CD measurements the overall conformation as well as the immediate heme environment of cytochrome P-450 LM2 remained unchanged after attachment of the FITC molecule. The binding affinity of modified cytochrome P-450 LM2 toward benzphetamine and aniline and the cumene hydroperoxide- or H2O2-supported N-demethylation of benzphetamine are maintained. However, the introduction of the electron via NADPH-cytochrome P-450 reductase (EC 1.6.2.4) is impaired after modification of the alpha-amino group. The extent of reduced modified cytochrome P-450 LM2 in the cytochrome P-450 reductase-supported reduction reaction is diminished and the half-time of the reduction is increased. The diminished reducibility is ascribed to steric hindrance of groups directly involved in the interaction between cytochrome P-450 LM2 and NADPH-cytochrome P-450 reductase or to blocking of the charge-pair interactions between the alpha-amino group of P-450 LM2 and the respective negatively charged group of NADPH-cytochrome P-450 reductase. By energy-transfer measurements distances between the heme and the alpha-amino group of 2.65 and 3.97 nm for the oligomeric and the monomeric forms of P-450 LM2, respectively, have been determined.

Fluorescent energy transfer measurements on fluorescein isothiocyanate modified cytochrome P-450 LM2

The distance between the heme iron and the N-terminus of cytochrome P-450 LM2 was determined by fluorescence energy transfer measurements. Fluorescein isothiocyanate which was covalently bound to the N-terminal methionine was used as donor chromophor. The Ro value between fluorescein isothiocyanate and the heme was calculated to be 3.98 nm. The distance between the nitrogen of the N-terminal methionine and the heme was estimated with 2.84 +/- 0.23 nm excluding most likely the N-terminal amino acid of cytochrome P-450 LM2 to participate in the electron transfer to the heme iron. A cytochrome P-450 LM2 membrane model is proposed.

Thermotropic behaviour of phospholipid vesicles reconstituted with rat liver microsomal cytochrome P-450

The interaction of rat liver microsomal cytochrome P-450 with phospholipids has been investigated employing differential scanning microcalorimetry. It is shown that the thermotropic behaviour of phospholipid vesicles reconstituted with cytochrome P-450 depends on liposome composition, protein concentration and the mode of reconstituted system preparation. From the delta H dependence on protein concentration in proteoliposomes it was calculated that one cytochrome P-450 molecule influence 350 +/- 50 dimyristoylphosphatidylcholine (DMPC) molecules. The electrostatic interaction of cytochrome P-450 and negatively charged phospholipid, phosphatidylinositol (PI), mixed with DMPC involves the temperature stabilization of proteoliposomes at a phase transition of phospholipid bilayers. The thermal denaturation temperature is increased due to negatively charged PI added.

Hydroxylations in biosynthesis of bile acids. Cytochrome P-450 LM4 and 12alpha-hydroxylation of 5beta-cholestane-3alpha, 7alpha-diol

12alpha-Hydroxylation of 5beta-cholestane-3alpha, 7alpha-diol was studied in reconstituted systems consisting of electrophoretically homogeneous cytochrome P-450 LM4 fractions and NADPH-cytochrome P-450 reductase from rabbit liver microsomes. Cytochrome P-450 LM4 fractions were prepared from untreated, phenobarbital-treated, beta-naphthoflavone-treated and starved rabbits. The purified cytochromes catalyzed 12alpha-hydroxylation more efficiently than the corresponding microsomes. In the reconstituted systems, carbon monoxide inhibited 12alpha-hydroxylation by 50-80%. The rate of 12alpha-hydroxylation was three to four times higher with cytochrome P-450 LM4 fractions from starved rabbits than with cytochrome P-450 LM4 fractions from untreated, phenobarbital-treated and beta-naphthoflavone-treated animals. Amino acid analyses, peptide mapping experiments as well as absorption spectral and circular dichroism spectral analyses revealed physical differences between cytochrome P-450 LM4 fractions from starved animals and preparations from phenobarbital-treated animals. The results indicate the presence of a cytochrome P-450 species in the cytochrome P-450 LM4 fraction specific for 12alpha-hydroxylation.

Conformational study of purified epoxide hydrolase from rat liver

Hepatic epoxide hydrolase (EC 3.3.2.3) was purified from phenobarbital-treated rats by ion-exchange chromatography followed by hydrophobic chromatography. The enzyme had a specific activity of 300--400 nmol min-1 mg-1 protein with benzo[a]pyrene-4,5-oxide as the substrate. Circular dichroism (CD) spectra of the purified enzyme gave two negative bands, centered at 210 nm and 222 nm, respectively. The mean residue ellipticity at 222 nm was 12,9000 deg X cm(2) X dmol(-1), which indicated the presence of about 35% alpha-helical structures. Sodium dodecyl sulfate (SDS) greatly affected the shape of the CD spectra, which were gradually shifted to the blue. This suggested a decrease in the aggregation state of the protein. Electrostatic interactions were important in the organization of the enzyme structure since the conformation was stable between pH 7.4 and pH 10. At pH-values 5.0, 6.0 and 12.0, the CD bands underwent considerable changes in both amplitude and shape. Moreover there was a good correlation between the optimal pH range of the epoxide hydrolase activity and the organization state of the protein. After membrane reconstitution with liposomes, the conformation of the enzyme was not significantly modified by the presence of dimyristoyl L-alpha-phosphatidylcholine or other phospholipids. This constancy was obtained over a wide range of molar ratios of phospholipids to protein (0--500). However, phospholipids did increase the thermal stability of the enzyme. Fluorescence measurements of diphenylhexatriene (DPH) bound to dimyristoyl L-alpha-phosphatidylcholine indicated that addition of epoxide hydrolase modified the thermal transition of the lipid phase. On the other hand, electron paramagnetic resonance (EPR) signals of the nitroxide-labelled fatty acid, 2-(14-carboxy-tetradecyl)-2-ethyl-4,4-dimethyl-3,3,-oxazolidiny-oxyl, bound to the phospholipid, indicated that the presence of the protein decreased by about 53% the correlation time of the label, suggesting that its motion had increased. In conclusion, phospholipid-epoxide hydrolase interactions enhanced the fluidity of dimyristoyl L-alpha-phosphatidylcholine liposomes without changing the secondary structure of the enzyme. Electrostatic interactions also played an important role in the conformational stability of the protein.

Magnetic and natural circular dichroism spectra of cytochgromes P-450(11) beta and P-450scc purified from bovine adrenal cortex

Magnetic (MCD) and natural circular dichroism (CD) spectra various complexes of cytochrome P-450(11) beta (P-450(11) beta) and cytochrome P-450scc (P-450scc) from bovine adrenal cortex were measured from 250 nm to 700 nm. MCD and CD spectral contours of cytochromes P-450(11) beta and P-450scc in the Soret and visible regions were, as a whole, analogous to those of cytochromes P-450 from rabbit liver microsomes and also from Pseudomonas putida in their high-spin ferric, high-spin ferrous and ferrous-CO complexes. MCD spectrum of the low-spin ferric P-450scc free from the substrate, cholesterol, was very similar to that caused by addition of 20 alpha-hydroxycholesterol, a reaction intermediate. However, it was distinct from those of the low-spin ferric P-450(11) beta and P-450scc complexes caused by addition of external nitrogenous ligands. The electronic states of the heme in the low-spin ferric P-450 free from substrates seemed to be subtly different from those of low-spin complexes coordinated with external nitrogenous ligands. Soret CD spectra of ferric low-spin complexes were not so different from each other. Upon reduction of high-spin ferric P-450(11) beta or P-450scc, the Soret CD magnitudes increased significantly in contrast with those of other P-450s, the Soret CD magnitudes of which decrease upon reduction. This may reflect an increased proximity of the neighbouring aromatic groups upon reduction of high-spin P-450(11) beta or P-450scc. High substrate specificity of adrenal P-450s compared with liver P-450s can be explained in view of the above findings. THe CD spectra in the near ultraviolet region (250-350 nm) were found to be quite sensitive to the spin change for ferric P-450scc, while the MCD spectra in this region did not reflect substantially the spin state of the enzyme. MCD parameters of cytochrome P-450s were compared to those of other hemoproteins in diagrams describing selected MCD spectral values of hemoproteins so far available and were discussed in connection with the structures of the heme environment of P-450.

Analysis of interactions among purified components of the liver microsomal cytochrome P-450-containing monooxygenase system by second derivative spectroscopy

Second derivative spectroscopy together with the respective difference spectroscopy offers an effective methodical tool to resolve overlapping bands and shoulders into distinct bands at eliminated background absorption. The improved resolution allows attribution of the distinct bands to individual amino acid residues. Both methods have been utilized to analyze interactions between the three essential components of the liver microsomal cytochrome P-450-containing monooxygenase system. The improved resolution of the aromatic amino acid residues in the derivative spectra of cytochrome P-450LM2 and reductase allows one to determine that in the interactions of the essential components tyrosine residue(s) are involved. The participation of phenylalanine is likely and the participation of tryptophan residues is excluded. The pH-dependent decrease of the tyrosine absorption bands in the medium ultraviolet region with increasing pH is accompanied by a concurrent decrease of the heme absorption in the Soret region. Based on this concurrence, the existence of a heme-linked tyrosine as one of the axial heme iron ligands in cytochrome P-450 is postulated.