Cytochrome catalyzed oxidations

X. Zu, Z. Lu, Z. Zhang, J.B. Schenkman, and J.F. Rusling, Electroenzyme-catalyzed oxidation of styrene and cis-(3-methylstyrene using thin films of cytochrome P450cam and myoglobin. Langmuir 15, 7372-7377 (1999). 

CL reaction can be catalyzed by enzymes other than HRP (e.g., microperoxidase and catalase) and by other substances [hemoglobin, cytochrome c, Fe(III), and other metal complexes]. The presence of suitable molecules such as phenols (p-iodophenol), naphthols (l-bromo-2-naphthol), or amines (p-anisidine) increases the light production deriving from the HRP-catalyzed oxidation of luminol and produces glow-type kinetics [6, 7], The use of other enzymes, such as glucose-6-phosphate dehydrogenase [38-41], P-galactosidase [42], and xanthine oxidase [43-46], as CL labels has been reported. 

Although the oxidation (hydroxylation) of hydrocarbons is usually believed to occur via hydrogen atom abstraction [51], the one-electron transfer mechanism of cytochrome P-450 catalyzed oxidation has also been proposed for the oxidation of Ar, Ar-dialkylaniIines [52]. This mechanism (Figure 24.4) is generally preferred for the substrates with low reduction... 

LOX-dependent superoxide production was also registered under ex vivo conditions [55]. It has been shown that the intravenous administration of lipopolysaccharide to rats stimulated superoxide production by alveolar and peritoneal macrophages. O Donnell and Azzi [56] proposed that a relatively high rate of superoxide production by cultured human fibroblasts in the presence of NADH was relevant to 15-LOX-catalyzed oxidation of unsaturated acids and was independent of NADPH oxidase, prostaglandin H synthase, xanthine oxidase, and cytochrome P-450 activation or mitochondrial respiration. LOX might also be involved in the superoxide production by epidermal growth factor-stimulated pheochromo-cytoma cells [57]. 

Andronik-Lion, V., Boucher, J. L., Delaforge, M., Henry, Y., Mansuy, D., Formation of nitric oxide by cytochrome P450-catalyzed oxidation of aromatic amidoximes, Biochem. 

A methodology that has turned out to be a very powerful tool in trying to unravel the intricacies of the mechanism of cytochrome P450-catalyzed oxidation reactions has been the use of deuterium isotope effects. The use of intramolecular deuterium isotope effects have been particularly important in this regard as will be described in chapter 4 where a number of such studies are presented. But, before describing the specific technique that intramolecular isotope effect studies entail, a quick mini review on the nature of deuterium isotope effects is probably in order. 

FIGURE 4.4 Cytochrome P450-catalyzed oxidation of fr-a/w-l,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane. 

FIGURE 4.5 Cytochrome P450-catalyzed oxidation of 1,1,2,2-tetramethylcyclopropane and hexamethylcyclopropane. 

FIGURE 4.50 Cytochrome P450-catalyzed oxidative N-dealkylation of Mdocaine. 

FIGURE 4.57 Examples (/V-rncthylcarbazole, N,N-dimethylaniIinc, N,/V-dimethylbenzamide, iV-methylbenzamidine, and /V,/V-dimcthylnitrosamine) of cytochrome P450-catalyzed oxidative N-dealkylation. 

Rietjens IM, denBestenC, Hanzlik RP, etal. Cytochrome P450-catalyzed oxidations of haloben-zene. Chem Res Toxicol 1997 10(6) 629-635. 

The pharmacokinetic implications of these findings are not straightforward. One important factor that must also be considered is hepatic extraction, which is higher for lovastatin than for its hydroxy acid metabolite [188], Some lactones are useful prodrugs of HMG-CoA reductase inhibitors due to this organ selectivity coupled with the efficiency of enzymatic hydrolysis. However, other factors may also influence the therapeutic response, in particular the extent and rate of metabolic reactions that compete with or follow hydrolysis, e.g., cytochrome P450 catalyzed oxidations, /3-oxidation, and tau-... 

The most important example to be discussed here is that of the drug chloramphenicol (11.39, R = 2-hydroxy-l-(hydroxymethyl)-2-(4-nitrophen-yl)ethyl, Fig. 11.7), the many metabolic pathways of which have yielded a wealth of information [75], The dichloroacetyl moiety is especially of interest in that dechlorination proceeds by three proven routes glutathione-dependent dechlorination, cytochrome P450 catalyzed oxidation, and hydrolysis. Of particular value is that the oxidative and hydrolytic routes can be unambiguously distinguished by at least one product, as shown in Fig. 11.7. Oxidation at the geminal H-atom produces an unstable (dichloro)hydroxyacet-amido intermediate that spontaneously eliminates HC1 to yield the oxamoyl... 

Fig. 11.7. The two pathways of dechlorination of chloramphenicol (11.39). Cytochrome P450 catalyzed oxidation to yield the oxamic acid derivative (11.40), and hydrolytic dechlorination to yield both oxamic acid (11.40) and primary alcohol (11.41) metabolites [75].

The correlation shown in Figure 7.4 would not be expected to hold for PAHs, when using linlish as a test species. This observation is based on the much higher levels of cytochrome P 50 dependent enzymes in linlish than in shellfish (Buhler and Williams, 1989). The P-450 system mediates Phase 1 metabolism of PAHs, which in this case consists of enzyme-catalyzed oxidation of the non-polar parent PAH molecule. 

Although in the majority of cases, cytochromes P-450 catalyze oxidation. 

The most important enzyme involved in bio transformation is cytochrome P-450, which catalyzes many phase 1 reactions. This enzyme is located primarily in the SER (microsomal fraction) of the cell and is especially abundant in liver cells. Cytochrome P-450 primarily catalyzes oxidation reactions and consists of many isoforms (isozymes). These isoenzymes have overlapping substrate specificities. The most important subfamily in humans is CYP3A4, although there is considerable variation in CYP3A4 expression between individuals. 

In different models for cytochrome P-450, Zn,152 Zn amalgam,153 and sodium ascorbate154 are used as the reductant. In contrast with all the abovementioned metal complex-catalyzed oxidations, the Mn(TPP)Cl-ascorbate system oxidizes alkanes with predominant formation of ketones.154 Certain complexes, however,... 

---