Cytochrome P450s transformations

Over the last three decades it has become known that certain synthetic metalloporphyrins can also act as oxidative catalysts thus mimicking the action of cytochrome P450. Transformations concerning substrates epoxidation and hydroxylation have been widely studied [ 19,55,56]. 

Cytochrome P450 enzymes are the most widespread, active, and most versatile in their xenobiotic Phase I transformation activity. These enzymes are composed of heme-containing enzymes in the ferric ion state. In transformations the ferric ion is reduced to the ferrous ion that can bind Oj and CO. These enzymes basically add oxygen or remove hydrogen in a step-wise process to generate Phase I biotransformation products. Most cytochrome P450 transformations require an additional enzyme (co-enzyme) to assist in the transfer of electrons. Cytochrome P-450 enzymes carry out many kinds of oxidations - hydroxylations, epoxidations, heteroatom oxidations, N-hydroxylations, dealkylations, ester hydrolysis, and dehydrogenation. 

Imidazole antimycotics, ketoconazole, clotrimazole, and miconazole are potent inhibitors of various cytochrome P450-isoenzymes that also affect the metabolism of retinoids. They were fust shown to inhibit the metabolism of RA in F9 embryonal carcinoma cells. When tested in vitm liarazole, a potent CYP-inhibitor, suppressed neoplastic transformation and upregulated gap junctional communication in murine and human fibroblasts, which appeared to be due to the presence of retinoids in the serum component of the cell culture medium. Furthermore, liarazole magnified the cancer chemopreventive activity of RA and (3-carotene in these experiments by inhibiting RA-catabolism as demonstrated by absence of a decrease in RA-levels in the culture medium in the presence of liarazole over 48 h, whereas without liarazole 99% of RA was catabolized. In vivo, treatment with liarazole and ketoconazole reduced the accelerated catabolism of retinoids and increased the mean plasma all-irans-RA-concentration in patients with acute promyelocytic leukemia and other cancels. 

Some of the major enzyme groups that facilitate this transformation are heme-containing MOs of the cytochrome P450 type [111], alkane hydroxylases, xylene monooxygenases, styrene monooxygenases [105], and haloperoxidases [112],... 

Remarkably, alkanes are oxidatively transformed by biological organisms at benign temperatures and pressures. Clearly, enzymatic transformations of alkanes and their well studied mechanisms (e.g., for cytochrome P450) are beyond the... 

P450 systems (Sariaslani 1991), their widespread role in the transformation of xenobiotics (Smith and Davis 1968), and their occurrence and activities in yeasts (Kappeli 1986). The essential features of prokaryotic and eukaryotic cytochrome P450 systems are compared in Figure 3.17. 

Sulfonylureas are the basis of a large group of herbicides. Cytochrome P450 enzymes in Streptomyces griseolus transform the sulfonylureas by hydroxylation (Omer et al. 1990) leaving the -SO2NHCONH- part of the structure unaltered (Harder et al. 1991). 

Attention has been directed to the reactions catalyzed by cytochrome P450s that bring about important reactions leading to the loss of angular methyl groups at C-10 (with concomitant aromatiza-tion of ring A) and C-14, and the C-17 -COCHj side chain. These reactions are discussed further in Chapter 3, Part 1. The transformations of steroids and their precursor lanosterol have been extensively... 

It is appropriate here to note that styrene is transformed by the black yeast Exophilia jeanselmei to phenylacetate by a pathway similar to that of the Xanthobacter sp. already noted. The initial monooxygenation was carried out by a cytochrome P450, and phenylacetate was further metabolized to 2-hydroxy- and 2,5-dihydroxyphenylacetate (Cox et al. 1996). 

A cytochrome P450 has been purified from Saccharomyces cerevisiae that has benzo[a]pyrene hydroxylase activity (King et al. 1984), and metabolizes benzo[fl]pyrene to 3- and 9-hydroxybenzo[fl]pyrene and benzo[fl]pyrene-7,8-dihydrodiol (Wiseman and Woods 1979). The transformation of PAHs by Candida Upolytica produced predominantly monohydroxyl-ated products naphth-l-ol from naphthalene, 4-hydroxybiphenyl from biphenyl and 3- and 9-hydroxybenzo[fl]pyrene from benzo[fl]pyrene (Cerniglia and Crow 1981). The transformation of phenanthrene was demonstrated in a number of yeasts isolated from littoral sediments and of these, Trichosporumpenicillatum was the most active. In contrast, biotransformation of benz[fl]anthracene by Candida krusei and Rhodotorula minuta was much slower (MacGillivray and Shiaris 1993). 

Hydrocarbon formation involves the removal of one carbon from an acyl-CoA to produce a one carbon shorter hydrocarbon. The mechanism behind this transformation is controversial. It has been suggested that it is either a decarbonylation or a decarboxylation reaction. The decarbonylation reaction involves reduction to an aldehyde intermediate and then decarbonylation to the hydrocarbon and releasing carbon monoxide without the requirement of oxygen or other cofactors [88,89]. In contrast, other work has shown that acyl-CoA is reduced to an aldehyde intermediate and then decarboxylated to the hydrocarbon, releasing carbon dioxide [90]. This reaction requires oxygen and NADPH and is apparently catalyzed by a cytochrome P450 [91]. Whether or not a decarbonylation reaction or a decarboxylation reaction produces hydrocarbons in insects awaits further research on the specific enzymes involved. 

Cytochrome P450 2E1, rabbit Hairy root Atropa belladonna A. rhizogenes transformation of leaf explant CaMV 35S Not reported Not reported 22... 

The answer is b. (Hardmanr p 906.) Cimetidine reversibly inhibits cytochrome P450. This is important in phase I bio transformation reactions and inhibits the metabolism of such drugs as warfarin, phenytoin, propranolol, metoprolol, quinidine, and theophylline. None of the other enzymes are significantly affected. 

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