Hydroxylation reaction, P450 monooxygenase

Hydroxylation and epoxidation reactions performed by monooxygenases are of particular interest, the cytochrome P450 monooxygenases representing a superfamily that catalyzes such reactions. 

A feature of some pterocarpan phytoalexins (e.g., pisatin and glyceollin of pea and soybean, respectively) is their hydroxylation at position 6a, a reaction catalyzed by a microsomal cytochrome P450 monooxygenase.49 50 A cDNA encoding this enzyme was recently characterized from elicited soybean cell cultures.51 The microsomal protein, expressed in yeast cells, catalyzed the stereoselective hydroxylation of (6a/ , lla/ )-3,9-dihydroxypterocarpan to its 6a-hydroxy derivative. It was also demonstrated that the enzyme expression is regulated at the transcriptional level.51... 

Among the reactions performed by the cytochrome P450 monooxygenase system, epox-idation, hydroxylation, N-dealkylation, O-dealkylation, desulfuration, and sulfoxidation are the most important with respect to pesticide metabolism ... 

Some of the important reactions catalyzed by the P450 monooxygenase system include aliphatic hydroxylation, aromatic hydroxylation, epoxidation, heteroatom (N-, 0-, and S-)dealkylation, nitrogen oxidation, oxidative deamination, oxidative dehalo-genation, oxidative denitrification, and oxidative desulfuration. Most of these reactions result from the initial oxidation of a carbon atom, another reason that P450 is so important in the oxidative biotrans-formation of lipophilic chemicals. Some P450-cataly-zed oxidation reactions are illustrated in Table 2. 

Carbon monoxide inhibited the 6/3-. la-, and 16a-hydroxylation of testosterone by rat liver microsomes to different extents. A C0/02 ratio of 0.5 inhibited the la-, 6/i-, and 16a-hydroxylation reactions by 14%, 25%, and 36%, respectively, and the ratio of C0/02 needed for 50% inhibition of testosterone hydroxylation in the 16a-, 6/3-, and 7a-positions was 0.93, 1.54, and 2.36, respectively (36,48). Studies on the photochemical action spectrum revealed that CO inhibition of the three hydroxylation reactions was maximally reversed by monochromatic light at 450 nm, but there were differences in the shape of the photochemical reactivation spectra for the 6/3-, la-, and 16a-hydroxylation reactions (36,48). The data from our laboratory summarized above and at the First International Symposium on Microsomes and Drug Oxidation in 1968 pointed to multiple cytochromes P450 with different catalytic activities that were under separate regulatory control (36,45,46), and we indicated that the actual number of cytochromes that participate in the multiple hydroxylation reactions must await the solubilization and purification of the microsomal system (36). The use of different inducers of liver microsomal monooxygenases caused selective increases in the concentration of specific cytochromes P450 in fiver microsomes that greatly facilitated the isolation and purification of these hemoproteins. 

The catalytic cycle of cytochrome P-450, typical of monooxygenases, is presented in Figure 13.12 (Johnston, Ouellet, Podust, Ortiz de Montellano, 2011). This substrate hydroxylation reaction is mediated by the Compound I -like ferryl species formed during the catalytic turnover of P450 enzymes. The Fe(lV) haem iron... 

P.F. Hollenberg, D.A. K.opp et al. (2002). Evaluation of norcarane as a probe for radicals in cyto-chome P450-and soluble methane monooxygenase-catalyzed hydroxylation reactions. J. Am. Chem. Soc. 124, 6879-6886. 

Cytochrome P4S0 and Biological Hydroxylation Reactions Table 1. Cytochrome P450-dependent monooxygenases... 

Describe the hydroxylation reactions involving cytochrome P450. Indicate the role of these monooxygenase reactions in steroid biosynthesis, the detoxification of xenobiotic compounds, and the generation of carcinogens. 

Steroid hydroxylation reactions mediated by a cell extract containing P450 monooxygenase CYP106A2-catalyzed were 18-fold faster than in whole cells. Cell-free protein synthesis has been widely used to synthesize and purify numerous proteins within several hours, more rapidly than microbial fermentations, which usually last several days. ... 

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