Kinetics of CO and O 2 complexes of rabbit liver microsomal cytochrome P 450

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

Spectroscopic characterization of the oxyferrous complex of prostacyclin synthase in solution and in trapped sol-gel matrix

Prostacyclin synthase (PGIS) is a member of the cytochrome P450 family in which the oxyferrous complexes are generally labile in the absence of substrate. At 4 degrees C, the on-rate constants and off-rate constants of oxygen binding to PGIS in solution are 5.9 x 10(5) m(-1).s(-1) and 29 s(-1), respectively. The oxyferrous complex decays to a ferric form at a rate of 12 s(-1). We report, for the first time, a stable oxyferrous complex of PGIS in a transparent sol-gel monolith. The encapsulated ferric PGIS retained the same spectroscopic features as in solution. The binding capabilities of the encapsulated PGIS were demonstrated by spectral changes upon the addition of O-based, N-based and C-based ligands. The peroxidase activity of PGIS in sol-gel was three orders of magnitude slower than that in solution owing to the restricted diffusion of the substrate in sol-gel. The oxyferrous complex in sol-gel was observable for 24 h at room temperature and displayed a much red-shifted Soret peak. Stabilization of the ferrous-carbon monoxide complex in sol-gel was observed as an enrichment of the 450-nm species over the 420-nm species. This result suggests that the sol-gel method may be applied to other P450s to generate a stable intermediate in the di-oxygen activation.

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.

Kinetics of carbon monoxide binding to phenobarbital-induced cytochrome P-450 from rat liver microsomes: a simple bimolecular process

The kinetics of carbon monoxide binding to phenobarbital-induced cytochrome P-450 (P-450PB) and to its enzymatically inactive form P-420PB have been investigated by both stopped-flow and flash-photolysis spectrophotometry. When the simultaneous presence of both forms of the enzyme is taken into account, the binding of CO to these two proteins can be described in terms of two bimolecular processes with rate constants of 4.5 X 10(6) M-1.S-1 and 4.7 X 10(5) M-1.S-1 for P-450PB and 1.7 X 10(7) M-1.S-1 and 1.5 X 10(6) M-1.S-1 for P-420PB. From kinetic studies of the binding of CO to P-450PB under different experimental conditions, investigations of the homogeneity of our P-450PB preparations, and comparative kinetic investigations of P-450s from different sources, we conclude that CO binding to reduced P-450PB is a simple bimolecular process and that the observed biphasic traces are due to heterogeneity of the proteins. This conclusion is in contrast with previous reports of complex reaction mechanisms for the binding of CO to P-450PB. Optical spectroscopy studies indicate the existence of an equilibrium between P-450PB and P-420PB, at least for the reduced carbonyl derivatives of the enzymes. The interconversion is strongly influenced by the aggregation state of the protein. Large differences between the CO binding properties of P-450PB and those of P-420PB are found. These are discussed in terms of possible effects of the proximal ligation state of the iron on heme reactivity.

Oxygen binding to dithionite-reduced chloroperoxidase

Both the kinetics of ferric chloroperoxidase reduction by dithionite and the binding of molecular oxygen to ferrous chloroperoxidase have been studied. The oxyferrous chloroperoxidase decays spontaneously to the ferric enzyme. In addition the corresponding rapid-scan spectra have been recorded. The reduction reaction is caused by SO-.2 with a rate constant of (7.7 +/- 1.0) X 10(4) M-1 S-1. Oxygen binding occurs with a rate constant of (5.5 +/- 1.0) X 10(5) M-1 S-1 over the pH range 3.5-6. Oxyferrous chloroperoxidase has a Soret absorption peak at 428 nm and two partially resolved peaks at 555 nm and 588 nm. Isosbestic points occur at the following wavelengths: between ferrous and oxyferrous chloroperoxidase at 419, 545, 555 and 580 nm; between oxyferrous and ferric chloroperoxidase at 419, 487, 540, 609 and 682 nm.

Comparative studies on the cumene hydroperoxide- and NADPH-supported N-oxidation of 4-chloroaniline by cytochrome P-450

The present study confirms that cytochrome P-450 can act as a catalyst in the cumene hydroperoxide-supported N-oxidation of 4-chloroaniline. Analogous to the NADPH/O2-driven N-oxidation process, product dissociation is likely to limit the overall rate of cytochrome P-450 cycling also in the peroxidatic pathway. The oxy complexes involved in either metabolic route differ with respect to stability, spectral properties and need for thiolate-mediated resonance stabilization. With the organic hydroperoxide, the metabolic profile is shifted from the preponderant production of N-(4-chlorophenyl)hydroxylamine to the formation of 1-chloro-4-nitrobenzene. This finding suggests that the peroxide-sustained N-oxidation mechanism differs in several ways from that functional in the NADPH/O2-dependent oxenoid reaction. Thus one-electron oxidation, triggered by homolytic cleavage of the oxygen donor, is proposed as the mechanism of peroxidatic transformation of 4-chloroaniline.

Interaction between NADPH-cytochrome P-450 reductase and cytochrome P-450 in the membrane of phosphatidylcholine vesicles

Cytochrome P-450 and NADPH-cytochrome P-450 REDUctase, both purified from liver microsomes of phenobarbital-pretreated rabbits, have been incorporated into the membrane of phosphoaditylcholine vesicles by the cholate dialysis method. The reduction of cytochrome P-450 by NADPH in this system is biphasic, consisting of two first-order reactions. The rate constant of the fast phase, in which 80--90% of the total cytochrome is reduced, increases as the molar ratio of the reductase to the cytochrome is increased at a fixed ratio of the cytochrome to phosphatidylcholine, suggesting that the rate-limiting step of the fast phase is the interaction between the reductase and the cytochrome. The rate constant of the fast phase also increases when the amount of phosphatidylcholine, relative to those of the two proteins, is decreased. This latter observation suggests that the interaction between the two proteins is effected by their random collision caused by their lateral mobilities on the plane of the membrane of phosphatidylcholine vesicles. The rate constant of the slow phase as well as the fraction of cytochrome P-450 reducible in the slow phase, on the other hand, remains essentially constant even upon alteration in the ratio of the reductase to the cytochrome or in that of the two proteins to phosphatidylcholine. No satisfactory explanation is as yet available for the cause of the slow-phase reduction of cytochrome P-450. The overall activity of benzphetamine N-demethylation catalyzed by the reconstituted vesicles responds to changes in the composition of the sysTEM IN A SIMILAR WAY TO THE FAST-PHASE REDUCTION OF CYTOCHROME P-450, though the latter is not the rate-limiting step of the overall reaction.

The effects of agents that bind to cytochrome P-450 on hypoxic pulmonary vasoconstriction

The relationship between pulmonary arterial pressure (Ppa) and blood (Q) was determined during normoxia and hypoxia in ventilated pig lungs perfused in situ with the animal's own blood. Hypoxia shifted the Ppa-Q relationship to the right and decreased its slope, indicating pulmonary vasoconstriction. Carbon monoxide (11.5% in the inspired gas) and metyrapone ditartrate (10 mg/min into the perfusate) caused vasodilation when oxygenation was normal and reduced the vasoconstriction caused by hypoxia. Since the only pharmacological property CO and metyrapone are thought to have in common at the concentrations employed is the ability to bind to the heme iron of cytochrome P-450, these results are consistent with the hypothesis that desaturation of this cytochrome leads to pulmonary vasoconstriction. Prostaglandin F2alpha, infused into the pulmonary artery at 0.01 mg/min, when oxygenation was normal, had effects on the Ppa-Q relationship similar to those of hypoxia. The F2alpha response was also reduced by CO and metyrapone, suggesting either that P-450 was involved in the F2alpha response or that CO and metyrapone were toxic to pulmonary vascular smooth muscle. Proadifen hydrochloride (1 mg/min), which is thought to bind to the protein moiety of P-450 also reduced the hypoxic response, but was a vasoconstrictor during normoxia and did not affect the F2alpha response.