Cytochrome redox cycle

The electron transfer properties of the cytochromes involve cycling of the iron between the +2 and +3 oxidation states (Cytochrome)Fe + e" (Cytochrome)Fe ° = -0.3Vto+ 0.4V Different cytochromes have different side groups attached to the porphyrin ring. These side groups modify the electron density in the delocalized iz system of the porphyrin, which in turn changes the redox potential of the iron cation in the heme. 

Gardner et al. [165] have shown that the redox-cycling agent phenazine methosulfate (PMS), mitochondrial ubiquinol-cytochrome c oxidoreductase, or hypoxia inactivated aco-nitase in mammalian cells. It has been proposed that the inactivation of aconitase is mediated by superoxide produced by prooxidants because the overproduction of mitochondrial MnSOD protected aconitase from inactivation by the prooxidants mentioned above except hyperoxia. Later on, the reaction of superoxide with aconitases began to be considered as one of the most important ways to NTBI generation in vivo. 

Possible errors due to the competition of cytochrome c reduction with the reversible reduction of quinones by superoxide are frequently neglected. For example, it has been found that quinones (Q), benzoquinone (BQ), and menadione (MD) enhanced the SOD-inhibitable cytochrome c reduction by xanthine oxidase [6]. This seems to be a mystery because only menadione may enhance superoxide production by redox cycling ( °p)"]/ [MD] =-0.20 V against ,0[02 ]/[02] 0.16 V) via Reactions (3) and (4), whereas for... 

Shen T, Hollenberg PF. The mechanism of stimulation of NADPH oxidation during the mechanism-based inactivation of cytochrome P450 2B1 by N-methylcarbazole redox cycling and DNA scission. Chem Res Toxicol 1994 7(2) 231-238. 

Tertiary amine oxides and hydroxy la mines are also reduced by cytochromes P-450. Hydroxylamines, as well as being reduced by cytochromes P-450, are also reduced by a flavoprotein, which is part of a system, which requires NADH and includes NADH cytochrome b5 reductase and cytochrome b5. Quinones, such as the anticancer drug adriamycin (doxorubicin) and menadione, can undergo one-electron reduction catalyzed by NADPH cytochrome P-450 reductase. The semiquinone product may be oxidized back to the quinone with the concomitant production of superoxide anion radical, giving rise to redox cycling and potential cytotoxicity. This underlies the cardiac toxicity of adriamycin (see chap. 6). 

This is the terminal complex in the electron transport chain, which transfers electrons to oxygen, reducing it to water (Fig. 7.68). Cyanide binds to the Fe3+ form of iron (Fig. 7.69), which is found in cytochromes such as cytochrome a3, which undergo redox cycling. Thus, oxidized hemoglobin, cytochrome P-450, and cytochrome c are all targets. However, the effects on the mitochondria are the most significant because of the rapid effects on cell metabolism. 

Figure 28.4. Mechanism of quinone drug redox cycling by liver microsomal NADPH cytochrome P450 reductase. A one-electron pathway competes with a two-electron process (catalyzed by DT-diaphorase) and is coupled with the monoxygenation and glucuronidation and sulfation systems.

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

Dutton, D.R., Reed, G.A., Parkinson, A. (1989). Redox cycling of resorufin catalyzed by rat liver microsomal NADPH-cytochrome P450 reductase. Hrc/i. Biochem. Biophys. 268 605-16. 

FIGURE 16.13 Partial scheme for the metabolism of diethylstilbestrol (DES). DES is administered as the trans isomer (E-DES), which, in solution, is in equilibrium with the cis isomer (Z-DES). Cytochrome P450 enzymes oxidize E-DES and Z-DES to a postulated chemically reactive semiquinone (1), which is further oxidized to a quinone (2), thereby generating reactive oxygen species (ROS) that oxidize cellular macromolecules. Redox cycling is perpetuated and ROS formation is amplified by two enzymes, cytochrome P450 or cytochrome bs reductase, which reduce the quinone back to the semiquinone. The unstable semiquinone and diol epoxide (3) metabolites are presumably those that bind to DNA to form adducts and initiate carcinogenesis. 

Ruthenium complexes are excellent reagents for protein modification and electron-transfer studies. Ru +-aquo complexes readily react with surface His residues on proteins to form stable derivatives [20, 21]. Low-spin pseudo-octahedral Ru-complexes exhibit small structural changes upon redox cycling between the Ru + and Ru + formal oxidation states [3, 22]. Hence, the inner-sphere barriers to electron transfer (Ai) are small. With the appropriate choice of ligand, the Ru + + reduction potential can be varied from <0.0 to >1.5 V versus NHE [23]. Ru-bpy complexes bound to Lys and Cys residues have been employed to great advantage in studies of protein-protein ET reactions. The kinetics of electron transfer in cytochrome 65/cytochrome c [24], cytochrome c/cytochrome c peroxidase [12], and cytochrome c/cytochrome c oxidase [25] complexes have been measured with the aid of laser-initiated ET from a Ru-bpy label. 

Moreover, intracellular accumulation and cytochrome P450 catalyzed bioactivation of p-lactams such as cephaloridine overwhelms of the GSH redox cycle by inhibiting glutathione reductase activity [35, 56], depletion of GSH and accumulation of GSSG [35, 42, 49,56]. Most of GSSG formed is subsequently reduced by glutathione reductase and GSH is regenerated with concomitant oxidation (consumption) of NADPH to NADP+ [104]. 

Superoxide generated by xanthine oxidase or in the redox cycling of paraquat can cause the reductive release of F3 from ferritin, a process that is dependent on the activity of microsomal NADPH-cytochrome P-450 reductase [119]. Iron appears to be an essential component in the formation of reactive species such as superoxide and hydroxyl radical via redox cycling of cephaloridine. Addition of EDTA or of the specific iron chelator desferrioxamine to an incubation system containing renal cortex microsomes and cephaloridine depressed cephaloridine-induced peroxidation of microsomal lipids significantly EDTA showed a weaker effect than desferrioxamine at equimolar concentrations. By chelating F3 preferentially [120], desferrioxamine reduced the availability of F2 produced by the iron redox cycle and decreased cephaloridine-stimu-lated peroxidation of membrane lipids [36, 37]. 

The mechanism of 4-aminophenol nephrotoxicity remains to be determined with certainty. The current hypothesis suggests that 4-aminophenol is oxidized by cytochrome P450 isozymes or peroxidases to p-benzoquinoneimine which can arylate renal mac-romolecules and/or redox cycle between 4-aminophenol and p-benzoquinoneimine to form reactive oxygen species. Recent studies have suggested that... 

Aerobic organisms produce minor fluxes of superoxide ion during respiration and oxidative metabolism. Thus, up to 15% of the O2 reduced by cytochrome-c oxidase and by xanthine oxidase passes through the HOO /O2 - state.70 The reductase of the latter system is a flavoprotein i that probably reduces O2 to HOOH via a redox cycle similar to that outlined by Eqs. (7-19) - (7-22). Thus, the observed flux of O2 -, which is the carrier of the auto-oxidation cycle, is due to leakage during turnover of xanthine/xanthine oxidase (see Scheme 7-14 for a reasonable mechanistic pathway). 

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