Regulative mechanisms in NADH- and NADPH-supported N-oxidation of 4-chloroaniline catalyzed by cytochrome b5-enriched rabbit liver microsomal fractions

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.

Models and mechanisms of O-O bond activation by cytochrome P450. A critical assessment of the potential role of multiple active intermediates in oxidative catalysis

Cytochrome P450 enzymes promote a number of oxidative biotransformations including the hydroxylation of unactivated hydrocarbons. Whereas the long-standing consensus view of the P450 mechanism implicates a high-valent iron-oxene species as the predominant oxidant in the radicalar hydrogen abstraction/oxygen rebound pathway, more recent studies on isotope partitioning, product rearrangements with 'radical clocks', and the impact of threonine mutagenesis in P450s on hydroxylation rates support the notion of the nucleophilic and/or electrophilic (hydro)peroxo-iron intermediate(s) to be operative in P450 catalysis in addition to the electrophilic oxenoid-iron entity; this may contribute to the remarkable versatility of P450s in substrate modification. Precedent to this mechanistic concept is given by studies with natural and synthetic P450 biomimics. While the concept of an alternative electrophilic oxidant necessitates C-H hydroxylation to be brought about by a cationic insertion process, recent calculations employing density functional theory favour a 'two-state reactivity' scenario, implicating the usual ferryl-dependent oxygen rebound pathway to proceed via two spin states (doublet and quartet); state crossing is thought to be associated with either an insertion or a radicalar mechanism. Hence, challenge to future strategies should be to fold the disparate and sometimes contradictory data into a harmonized overall picture.

N-oxidative transformation of free and N-substituted amine functions by cytochrome P450 as means of bioactivation and detoxication

Indirect evidence for the participation of cytochrome P450 (P450) in the microsomal N-oxygenation of primary and N-substituted amine functions is presented by studies employing diagnostic modifiers of the hemoprotein system as well as immunochemical approaches. Experiments with recombinant hemoproteins or isozymes purified from the tissues of various animal species support the results obtained by the inhibitor assays. Amine substrates and the redox proteins of the microsomal electron transfer chain reveal to be mutually beneficial in interactions with P450s. Numerous N-substituted amines undergo P450-catalyzed N-oxidative transformation despite the presence of accessible alpha-C hydrogens in these structures rather thought to favor N-dealkylation. In these instances, stabilization of the initially formed aminium radicals by the specific active site orientation of the particular P450s obviously permits oxygen rebound. Apart from common iron-oxenoid chemistry involving a (FeO)3+ species, iron-bound hydroperoxide, (FeO2H)3+, appears to act as an electrophilic oxidant with certain N-substituted amines and P450 subforms. Generally, P450-mediated N-oxygenation of amines can produce cytotoxic and mutagenic metabolites, but equally can well yield hydrophilic products, that are readily excreted and thus promote detoxication.

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.

Primary aromatic amines: their N-oxidative bioactivation

There exists a diversity of pathways in mammalian cells serving to activate primary aromatic amines. 1 N-Oxidative mixed-function turnover usually involves participation of the cytochrome P450 superfamily, while catalysis by the flavin-containing monooxygenases is restricted to a few amines capable of forming imine tautomers. Surprisingly, haemoglobin metabolizes cytotoxic and carcinogenic arylamines via a monooxygenase-like mechanism, but peroxygenase activity is also operative. 2 In extrahepatic tissues that exhibit only a low level of monooxygenases, peroxidative transformations, as are brought about by prostaglandin H synthase, myeloperoxidase or lactoperoxidase, predominate in amine activation. Non-mammalian peroxidases frequently used as model systems include horseradish peroxidase and chloroperoxidase. 3 Non-enzymatic, light-induced conversion of aromatic amines to free radical or N-oxy products proceeds either via direct photolysis of the nitrogenous compounds or through attack by lipid-derived reactive intermediates generated during irradiation. 4 The interplay of the various tissue-specific processes of arylamine activation serves to explain differences in susceptibility toward the biological actions of primary aromatic amines.

Regulatory mechanisms in the activation of nitrogenous compounds by mammalian cytochrome P-450 isozymes

Metabolic activation of nitrogenous compounds by the cytochrome P-450 system is a highly complex process. Inherent substrate factors, such as basicity, electronic state, lipophilicity, and conformation control binding of the diverse classes of amines to cytochrome P-450. Accommodation of these compounds in the enzyme cavity and proper orientation of the molecules are governed by intrinsic properties of the peptide structure of cytochrome P-450, which may be subject to modification by the action of effectors. On the membrane level, phospholipid might have some impact on substrate binding. On the other hand, bound amine substrate is beneficial to the productive interaction of the electron transport chains with the terminal acceptor, improving economy of the system. Certain amines appear to regulate O2 association with cytochrome P-450 and stabilize the various oxy species formed. Considering the selective prerequisites for oxidative attack by cytochrome P-450 at vulnerable nitrogen centers, many cytotoxic amines belonging to the category of relatively rigid, planar molecules undergo N-oxidative activation by the cytochrome P-450IA subfamily, while more bulky amines with flexible conformation are N-oxygenated preferentially by phenobarbital-inducible cytochromes P-450. Small differences in protein structure between the various cytochrome P-450 subforms might serve to stabilize aminium radicals to permit oxygen rebound. Collectively, the selective regulatory mechanisms operative in the bioactivation of nitrogen-containing compounds appear to be determined largely by the type of substrate used and the isozyme involved in catalysis. With respect to the latter, the interplay of the multiple cytochromes P-450 in the various organs of animal species thus serves to rationalize the differences in the particular selectivities for amine substrates. These are responsible for the mode and/or extent to which activation of nitrogenous compounds, including promutagens and procarcinogens, occurs, and this may explain the tissue-specific response to the tumorigenic action of these agents.

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

Application of a single dose of allylisopropylacetamide (AIA) to phenobarbital-pretreated rabbits resulted in partial destruction of the heme moiety of liver microsomal cytochrome P-450. A minor fraction of chromophore loss was accounted for by heme-derived product(s) covalently attached to microsomal proteins. Interestingly, cytochrome P-450 appeared to have undergone significant drug-mediated alkylation of the apohemoprotein. The modified species was purified to apparent homogeneity and shown to arise from AIA-induced blockage of about 2 histidines in the cytochrome P-450LM2 molecule located close to the heme edge. AIA administration to the animals caused inhibition of hexobarbital-promoted electron flow from NADPH-cytochrome P-450 reductase to phenobarbital-inducible ferricytochrome P-450 both in microsomal particles and reconstituted systems. The impaired interaction between the proteins was shown not to originate from decreased capacity to bind each other but more likely to be due to some defect in a step subsequent to complex formation. In contrast, treatment with the porphyrogenic agent did not affect microsomal electron transmission from cytochrome b5 to the ferric monooxygenase. However, when the intermediate carrier was to donate reducing equivalents to the ferrous oxycytochrome in the presence of benzphetamine, there was a pronounced deceleration of the electron flux observable. These findings were interpreted to mean that there exist multiple reductase- and cytochrome-b5-binding domains in phenobarbital-inducible cytochrome P-450, some of which seem to be common to the two redox proteins. This sheds interesting light on the molecular organization of the catalytic electron transfer complexes.

Chemical modification of lysine residues in cytochrome P450LM2 (P450IIB4): influence on heme liganding of arylamines

Treatment of cytochrome P450LM2 with fluorescein isothiocyanate to introduce up to two equivalents of fluorophore per polypeptide chain resulted in the selective derivatization of lysine residues. CD-spectral measurements revealed the overall conformation as well as the immediate heme environment of the hemoprotein to remain unaffected by attachment of the label. Modification caused decreased affinity of p-phenylenediamine and other 4-substituted anilines for the heme site, whereas there was a rise in the extent of substrate interaction. Experiments with pigment containing acetylated lysines gave analogous results, suggesting that the observed phenomenon was due to charge neutralization. There was linear correlation between the Hammett sigma P values and both the optical dissociation constants for arylamine binding to intact enzyme and the dipole moments of the anilines, indicating that basicity along with electronic factors controlled heme liganding; lipophilicity appeared to be of minor importance. Introduction of fluorescein isothiocyanate into the oxygenase was found to influence the bond-making process through modulating basicity of the nitrogenous compounds, but perturbation of optimal spacial orientation of the amine nitrogen toward the heme iron also might have been operative. The lysines studied seem to represent metabolically inactive elements of the substrate channel located on the cytosolic surface of the aggregates, as evidenced by steady-state fluorescence measurements. A hydrophilic segment in the cytochrome P450LM2 molecule that would accommodate the critical residues is discussed.

Hepatic microsomal N-hydroxylation of aniline and 4-chloroaniline by rainbow trout (Onchorhyncus mykiss)

1. N-Hydroxylation of aniline and 4-chloroaniline was quantified in rainbow trout microsomal preparations using h.p.l.c.-liquid scintillation methods. Radioactive phenylhydroxylamine and 4-chlorophenylhydroxylamine metabolites were identified by co-elution with non-labelled standards. The method provided resolution of metabolite standards, and quantification of both N-hydroxylated metabolites was achieved without derivatization. 2. The maximum velocities at 25 degrees C were 33.8 +/- 1.40 and 22.0 +/- 0.98 pmol/min per mg for aniline and 4-chloroaniline N-hydroxylation, respectively. The Km values were 1.0 +/- 0.11 and 0.8 +/- 0.11 mM for aniline and 4-chloroaniline N-hydroxylation, respectively. These activities were not induced by treatment of the trout with Aroclor 1254 under the conditions of this study. 3. When incubations were performed at 11 degrees C, the physiological temperature of rainbow trout in this study, the Vmax for 4-chloroaniline N-hydroxylation decreased from 22.0 to 6.4 pmol/min per mg and the Km decreased from 0.8 to 0.5 mM. 4. The pH optimum for 4-chloroaniline N-hydroxylation was 8.0 while the pH optimum for aniline N-hydroxylation ranged from 7.4 to 8.0, suggesting the possible contribution of different isoenzymes. 5. The demonstration of aniline and 4-chloroaniline N-hydroxylation by rainbow trout microsomes provides further insight into the high acute:subchronic toxicity ratios observed in fish exposed to these compounds.