Mixed-function oxidases reactions

The oxidation of carotenes results in the formation of a diverse array of xanthophylls (Fig. 13.7). Zeaxanthin is synthesised from P-carotene by the hydroxylation of C-3 and C-3 of the P-rings via the mono-hydroxylated intermediate P-cryptoxanthin, a process requiring molecular oxygen in a mixed-function oxidase reaction. The gene encoding P-carotene hydroxylase (crtZ) has been cloned from a number of non-photosynthetic prokaryotes (reviewed by Armstrong, 1994) and from Arabidopsis (Sun et al, 1996). Zeaxanthin is converted to violaxanthin by zeaxanthin epoxidase which epoxidises both P-rings of zeaxanthin at the 5,6 positions (Fig. 13.7). The... 

Approximately 10-20% of -hexane absorbed by inhalation is excreted unchanged in exhaled air the remainder is metabolized. Metabolism takes place via mixed-function oxidase reactions in the liver. In a study in which metabolites were measured in workers exposed to 77-hexane (Perbellini et al. 1981), mean concentrations of 77-hexane metabolites in urine were 2,5-hexanedione, 5.4 mg/L 2,5-dimethylfuran,... 

Hildebrandt A., Estabrook RW. 1971. Evidence for the participation of cytochrome b5 in hepatic microsomal mixed-function oxidase reactions. Arch Biochem Biophys 143 66-79. 

Renal tissue metabolism and transport mechanisms play important roles in the excretion and detoxification of xenobiotics (and/or their metabolites). Although the kidneys play an important part in the detoxification of xenobiotics, renal tissue may produce or increase the amounts of toxic metabolites received via the renal blood supply by metabolism (e.g., mixed-function oxidase reactions or concentrating effects within the nephron) (Piperno 1981 Commandeur and Vermeulen 1990 Goldstein 1994 Diamond and Zalups 1998 Endou 1998 Tarloff and Lash 2004). 

Recent studies have revealed insight into the dehydrogenation reaction of palmitoyl-CoA (Bloomfield et al. 1958). The reaction appears to be a mixed function oxidase reaction. The only cofactors required are molecular oxygen and reduced NADP+. This enzyme introduces the isolated double bond with high stereo-, geometrical and positional specificity as shown with 9-D and 9-L- H-stearic acid. Only the 9-D hydrogen is lost (Schroepfer et al. 1964). 

Only mixed-function oxidase reactions will be seen in quantity. 

Lipophilic xenobiotic species in the body tend to undergo Phase I reactions that make them more water-soluble and reactive by the attachment of polar functional groups, such as -OH (Figure 23.6). Most Phase I processes are microsomal mixed-function oxidase reactions catalyzed by the cytochrome P-450 enz3mie system that... 

Many examples of microbial hydroxylation of sterols/steroids have been reported. These hydroxylations usually involve mixed function oxidases which utilise molecular oxygen and cytochrome P-450. The reaction can be represented by ... 

Cytochrome P450 (CYP) mono-oxygenases, also called mixed function oxidases, are versatile hemoprotein enzymes that catalyze the cleavage of molecular oxygen to incoiporate one oxygen atom into a substrate molecule and one atom into water [1]. The general stoichiometry of the reaction is as follows (S-H, substrate) ... 

Peptidyl hydroxyprohne and hydroxylysine are formed by hydroxylation of peptidyl proline or lysine in reactions catalyzed by mixed-function oxidases that require vitamin C as cofactor. The nutritional disease scurvy reflects impaired hydroxylation due to a deficiency of vitamin C. 

Cytochrome P450 is considered the most versatile biocatalyst known. The actual reaction mechanism is complex and has been briefly described previously (Figure 11-6). It has been shown by the use of that one atom of oxygen enters R—OH and one atom enters water. This dual fate of the oxygen accounts for the former naming of monooxygenases as mixed-function oxidases. The reaction catalyzed by cytochrome P450 can also be represented as follows ... 

The answers are 34-g, 35-a, 36-d. (Katzung, pp 53—56J There are four major components to the mixed-function oxidase system (1) cytochrome P450, (2) NAD PH, or reduced nicotinamide adenine dinucleotide phosphate, (3) NAD PH—cytochrome P450 reductase, and (4) molecular oxygen. The figure that follows shows the catalytic cycle for the reactions dependent upon cytochrome P450. 

Hexachloroethane is metabolized by the mixed function oxidase system by way of a two-step reduction reaction involving cytochrome P-450 and either reduced nicotinamide adenine dinucleotide phosphate (NADPH) or cytochrome b5 as an electron donor. The first step of the reduction reaction results in the formation of the pentachloroethyl free radical. In the second step, tetrachloroethene is formed as the primary metabolite. Two chloride ions are released. Pentachloroethane is a minor metabolic product that is generated from the pentachloroethyl free radical. 

Liver necrosis is another concern following hexachloroethane exposure. Hexachloroethane is metabolized in the centrilobular area of the liver by way of the microsomal mixed function oxidase system. The relatively nonpolar pentachloroethyl free radical is an intermediate in this pathway. The reaction of the free radical with unsaturated lipids in the cellular or organelle membranes could contribute to hepatocyte damage and necrosis. 

Environmental agents that influence microsomal reactions will influence hexachloroethane toxicity. The production of tetrachloroethene as a metabolite is increased by agents like phenobarbital that induce certain cytochrome P-450 isozymes (Nastainczyk et al. 1982a Thompson et al. 1984). Exposure to food material or other xenobiotics that influence the availability of mixed function oxidase enzymes and/or cofactors will change the reaction rate and end products of hexachloroethane metabolism and thus influence its toxicity. 

The CYP enzymes active in phase I reactions are often oxidases or hydroxylases, sometimes called mixed function oxidase (MFO). An oxidase enzyme introduces into the substrate (i.e. the unwanted compound) both atoms of an oxygen molecule whilst... 

Table II summarizes the results together with the detailed experimental conditions. As is evident, metabolic activities were detectable in these 3 aquatic species, but the rate was far lower as compared with mammalian hepatic enzume preparations, and the oxidative activities in snail were particularly low although the possibility was not ruled out of the presence of inhibitors of mixed-function oxidases in the fractions. The O-demethylation reaction proceeds extremely slowly in the enzyme preparation of aquatic animals, at less than one hundredth that of mammals.

The marine environment acts as a sink for a large proportion of polyaromatic hydrocarbons (PAH) and these compounds have become a major area of interest in aquatic toxicology. Mixed function oxidases (MFO) are a class of microsomal enzymes involved in oxidative transformation, the primary biochemical process in hydrocarbon detoxification as well as mutagen-carcinogen activation (1,2). The reactions carried out by these enzymes are mediated by multiple forms of cytochrome P-450 which controls the substrate specificity of the system (3). One class of MFO, the aromatic hydrocarbon hydroxylases (AHH), has received considerable attention in relation to their role in hydrocarbon hydroxylation. AHH are found in various species of fish (4) and although limited data is available it appears that these enzymes may be present in a variety of aquatic animals (5,6,7,8). 

Considerable interest has developed concerning the nature of the mixed function oxidase system in fish and the role that this system may play in the development of toxic responses in these animals. Studies have shown that components of the mixed function oxidase system are present in relatively high concentrations in fish liver (4, 5, 6) and that the enzyme systems in this organ are capable of many of the biotransformation reactions already described for the mammalian liver (7, 8, 9). The presence of this complement of enzymes in the livers of many fishes suggests that this organ too may be particularly sensitive to insult from sub lethal concentrations of many waterborne toxicants. For this reason, methods to evaluate liver function in fish may be particularly useful in identifying the sublethal effects of certain classes of toxicants. 

N-dealkylation results from an alkyl substitution on an aromatic molecule, which is one of the first places where microorganisms initiate catabolic transformation of atrazine, a xenobiotic molecule (Fig. 15.2). It is a typical example of a reaction leading to transformation of pesticides like phenyl ureas, acylanihdes, carbamates, s-tri-azines, and dinitranilines. The enzyme mediating the reaction is a mixed-function oxidase, requiring a reduced nicotinamide nucleotide as an H donor. 

Most phase one reactions are catalyzed by the drug-metabolizing enzymes (mixed function oxidases, oxygenases) located in the endoplasmic reticulum of liver and, to a lesser extent, in intestine, kidney, and lung. These enzymes have been the subject of intensive research (G7, G8, LI). 

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