Inactivation of phenobarbital-inducible rabbit-liver microsomal cytochrome P-450 by allylisopropylacetamide: impact on electron transfer

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

Probability current in protein electron transfer reactions: A Green function pathway model

In a biological electron transfer reaction problem we have found distinct electron probability current representations of macromolecular currents, amino acid and interamino acid currents, atomic and interatomic currents. We present a general interatomic current approach to obtain the electron probability current from donor to acceptor sites, and vice versa. Symmetric (or antisymmetric) renormalizable interatomic currents provide exact (or approximate) effective tunneling matrix element between donor and acceptor states. The electron probability currents allow us to investigate stationary tunneling pathways through protein bridge medium, from molecular to subatomic resolution at any level of electronic description of the biological structure. Analytical comparison among interatomic pathway models is presented and we found equivalent interatomic current representations with distinct dependence on the tunneling energy from the weak to strong coupling between protein bridge and the donoracceptor sites.

Functional interaction of cytochrome P450 with its redox partners: a critical assessment and update of the topology of predicted contact regions

The problem of donor-acceptor recognition has been the most important and intriguing one in the area of P450 research. The present review outlines the topological background of electron-transfer complex formation, showing that the progress in collaborative investigations, combining physical techniques with chemical-modification and immunolocalization studies as well as site-directed mutagenesis experiments, has increasingly enabled the substantiation of hypothetical work resulting from homology modelling of P450s. Circumstantial analysis reveals the contact regions for redox proteins to cluster on the proximal face of P450s, constituting parts of the highly conserved, heme-binding core fold. However, more variable structural components located in the periphery of the hemoprotein molecules also participate in donor docking. The cross-reactivity of electron carriers, purified from pro- and eukaryotic sources, with a diversity of P450 species points at a possible evolutionary conservation of common anchoring domains. While electrostatic mechanisms appear to dominate orientation toward each other of the redox partners to generate pre-collisional encounter complexes, hydrophobic forces are likely to foster electron transfer events by through-bonding or pi-stacking interactions. Moreover, electron-tunneling pathways seem to be operative as well. The availability of new P450 crystal structures together with improved validation strategies will undoubtedly permit the production of increasingly satisfactory three-dimensional donor-acceptor models serving to better understand the molecular principles governing functional association of the redox proteins.

Antihypertensive effect of mechanism-based inhibition of renal arachidonic acid omega-hydroxylase activity

The cytochrome P-450 eicosanoid 20-hydroxyeicosatetraenoic acid (20-HETE) is a potent vasoconstrictor that is implicated in the regulation of blood pressure. The identification of selective inhibitors of renal 20-HETE formation for use in vivo would facilitate studies to determine the systemic effects of this eicosanoid. We characterized the acetylenic fatty acid sodium 10-undecynyl sulfate (10-SUYS) as a potent and selective mechanism-based inhibitor of renal 20-HETE formation. A single dose of 10-SUYS caused an acute reduction in mean arterial blood pressure in 8-wk-old spontaneously hypertensive rats. The decrease in mean arterial pressure was maximal 6 h after 10-SUYS treatment (17.9 +/- 3.2 mmHg; P < 0.05), and blood pressure returned to baseline levels within 24 h after treatment. Treatment with 10-SUYS was associated with a decrease in urinary 20-HETE formation in vivo and attenuation of the vasoconstrictor response of renal interlobar arteries to ANG II in vitro. These results provide further evidence that 20-HETE plays an important role in the regulation of blood pressure in the spontaneously hypertensive rat.

Interactions between redox partners in various cytochrome P450 systems: functional and structural aspects

The various types of redox partner interactions employed in cytochrome P450 systems are described. The similarities and differences between the redox components in the major categories of P450 systems present in bacteria, mitochondria and microsomes are discussed in the light of the accumulated evidence from X-ray crystallographic and NMR spectroscopic determinations. Molecular modeling of the interactions between the redox components in various P450 mono-oxygenase systems is proposed on the basis of structural and mutagenesis information, together with experimental findings based on chemical modification of key residues likely to be associated with complementary binding sites on certain typical P450 isoforms and their respective redox partners.

Residue 285 in cytochrome P450 2B4 lacking the NH(2)-terminal hydrophobic sequence has a role in the functional association of NADPH-cytochrome P450 reductase

Cytochrome P450 2B4 (CYP2B4) lacking the NH(2)-terminal signal anchor sequence (2-27) was used to study the impact of replacement of histidine with alanine at position 285 on electron transfer from NADPH-cytochrome P450 reductase (P450R). Absorption and circular dichroism spectra of the recombinant hemoproteins indicated that amino acid substitution neither grossly perturbed the geometry of the immediate heme vicinity nor the global polypeptide backbone folding. Fitting of the initial-velocity patterns of P450R-directed reduction of the ferric CYP2B4 (2-27) forms to the Michaelis-Menten kinetics revealed an approximately 3.5-fold increase in the apparent K(m) value for the electron donor of the H285A mutant, while its reductive capacity (V(max)) remained unchanged; this caused a strong drop in reductive efficiency of the engineered enzyme. Circumstantial analysis suggested that impaired association of the redox partners accounted for this phenomenon. Thus, deletion of the positive charge at position 285 of CYP2B4 (2-27) might have disrupted contacts with oppositely charged entities on the P450R surface. Measurements of the stoichiometry of aerobic NADPH consumption and H(2)O(2) production disclosed the oxyferrous H285A species to autoxidize more readily compared with the shortened wild type. This was assumed to arise from less efficient coupling of the system due to defective donation of the second electron by P450R. These results are consistent with the view that His-285 in the truncated CYP2B4 is of importance in the functional interaction with the flavoprotein reductase.

Synthetic peptide mimics of a predicted topographical interaction surface: the cytochrome P450 2B1 recognition domain for NADPH-cytochrome P450 reductase

In order to identify the cytochrome P450-binding domain for NADPH-cytochrome P450 reductase, synthetic peptide mimics of predicted surface regions of rat cytochrome P450 2B 1 were constructed and evaluated for inhibition of the P450-reductase interaction. A peptide corresponding to residues 116-134, which includes the C helix, completely inhibited reductase-mediated benzphetamine demethylation by purified P450 2B1. Replacement of Arg-125 by Glu yielded a noninhibitory peptide, suggesting that this residue significantly contributes to the reductase-P450 interaction. Additional P450 peptides were prepared which correspond to combinations of regions distant in primary sequence, but predicted to be spatially proximate. A peptide derived from segments of the C and L helices was a more potent inhibitor than peptides derived from either segment alone. This topographically designed peptide not only inhibited P450 2B1 in its purified form, but also when membrane-bound in rat liver microsomes. The peptide also inhibited microsomal aryl hydrocarbon hydroxylase, aniline hydroxylase, and erythromycin demethylase activities derived from other P450s. These results indicate that the C and L helices contribute to a reductase-binding site common to multiple P450s, and present a peptide mimic for this region that is useful for inhibition of P450-mediated microsomal activities.

Histidine residues in rabbit liver microsomal cytochrome P-450 2B4 control electron transfer from NADPH-cytochrome P-450 reductase and cytochrome b5

Treatment of cytochrome P-450 2B4 (P-450 2B4) with diethylpyrocarbonate to introduce 10-11 equivalents of acylating agent per polypeptide chain resulted in the selective derivatization of histidine residues characterized by differential susceptibility toward the modifier. Second-derivative spectral analysis as well as fluorescence measurements disproved gross alterations in P-450 2B4 structure as a consequence of labelling. The modified haemoprotein retained its ability to bind hexobarbital and catalyse cumene hydroperoxide-sustained N-demethylation of the barbiturate. However, there was a steady attenuation of NAD(P)H-driven electron flux with increasing extent of P-450 2B4 carbethoxylation in reconstituted systems fortified with either NADPH-cytochrome P-450 reductase or NADH-cytochrome b5 reductase/cytochrome b5 as the redox partners, with 50% inhibition occurring when 6-7 histidines were blocked. Hampered P-450 2B4 reductase activities recovered to differing degrees upon treatment of the acylated mono-oxygenase with neutral hydroxylamine. Spectral data indicated that docking of the redox components to derivatized P-450 2B4 was not perturbed, so that disruption of the electron flows most likely resulted from some injury of the electron-transfer mechanisms.

Hydrogen peroxide-mediated inactivation of microsomal cytochrome P450 during monooxygenase reactions

Cytochrome P450 can undergo inactivation following monooxygenase reactions in liver microsomes of untreated, phenobarbital and 3-methylcholanthrene-treated rats and rabbits. The acceleration of cytochrome P450 loss in the presence of catalase inhibitors (sodium azide, hydroxylamine) indicates that hydrogen peroxide is involved in hemoprotein degradation. It was revealed that cytochrome P450 is inactivated mainly by H2O2 formed through peroxy complex breakdown, whereas H2O2 formed via the dismutation of superoxide anions produces a slight inactivating effect. The hydrogen peroxide added outside or formed by a glucose-glucose oxidase system has less of an inactivating effect than H2O2 produced within the cytochrome P450 active center. Self-inactivation of cytochrome P450 during oxygenase reactions is highly specific. Other components of the monooxygenase system, such as cytochrome b5, NADH- and NADPH-specific flavoproteins, undergo no inactivation. The alterations in phospholipid content and in the rate of lipid peroxidation were not observed as well. The inactivation of cytochrome P450 by H2O2 is the result of heme loss or destruction without cytochrome P420 formation. Such a mechanism operates with different substrates and cytochrome P450 species catalyzing the partially coupled monooxygenase reactions.

Chemical modification of Tyr34 and Tyr129 in rabbit liver microsomal cytochrome b5 affects interaction with cytochrome P-450 2B4

Rabbit liver microsomal cytochrome b5 was allowed to react with tetranitromethane. Up to three tyrosine residues in each cytochrome b5 molecule were found to be accessible to the nitrating agent. Co-modification of tryptophan and histidine residues could be disregarded. CD-spectral measurements disproved gross changes in cytochrome b5 structure as a consequence of derivatization. Introduction of 1.6 nitro groups/polypeptide chain resulted in a fivefold increase in binding affinity for cytochrome P-450 2B4 (P-450 2B4), whereas spectral interaction with cytochrome c remained unaffected. Furthermore, the capacity of nitrated cytochrome b5 to shift the spin equilibrium to the high-spin conformer of P-4502B4 was diminished by 44% compared with the control. This corresponded with the partial disruption of NADH-dependent electron flow to ferric P-450 2B4. Changes in the redox potential of cytochrome b5 could be discounted as being responsible for this effect. The overall oxidative turnover of 4-nitroanisole did not respond to cytochrome b5 modification. MS analysis and sequencing of peptide fragments produced by tryptic digestion of modified cytochrome b5 permitted the detection of three nitrated tyrosine residues located at positions 11, 34 and 129. Derivatization of cytochrome b5 in the presence of a protective amount of P-450 2B4 provided evidence of the involvement of Tyr34 and Tyr129 in complexation of the two hemoproteins. It is proposed that Tyr129 might control docking of cytochrome b5 to P-450 2B4, whereas Tyr34 could be of functional importance in electron transfer.