Cytochrome p 450

Cytochrome P-450 is frequently the oxygenase which detoxifies xenobiotics, including herbicides. Blocking the metaboHsm of a herbicide increases the activity or delays the inactivation, thus increasing the effectiveness of such herbicides as chlortoluron [15545-48-9] and bentazon [25057-89-0]... 

Other compounds of this general class which have been found to have antiestrogenic properties include the cytochrome P-450 inhibitor, SKF 525A P02-33-0](Sl) (24) JV, JV-diethyl-2-[(4-phenylmethyl)phenoxy]ethanamine [98774-23-3] (DPPE)(58) (42) /-Butylphenoxyethyl diethylamine [57586-10-4] (BPEA)(59) (43) and cyclofenil [110042-18-7] (60, R = C H ) (24) analogues. 

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

The principal route of macroHde excretion is by way of the Hver. Effects of macrohdes on hepatic metaboHc enzymes, particularly cytochrome P-450, have been studied in order to identify and reduce potential interference with metaboHsm of other dmgs (21—23,444—447). Several macrohdes are initially... 

Clotrimazole and other azole derivatives have a different mode of action than the polyenes, eg, amphotericin B. The latter biad to the ergosterol present ia the membranes of yeasts and fungi, but azole derivatives inhibit the cytochrome P-450 dependent biosynthesis of ergosterol (8—11). This inhibition not only results in a reduction of ergosterol, but also in an accumulation of C-14 methyl sterols. They disturb membrane permeabiUty, inhibit cell rephcation, and are basically responsible, in combination with the reduction of ergosterol levels, for the antifungal action. 

Miconazole. Miconazole nitrate [22832-87-7] (Fig. 2), the 1-phenethyl-imidazole derivative first described in 1969, interferes at low doses with the cytochrome P-450 dependent ergosterol biosynthesis in yeasts and fungi. The result is accumulation of C-14 methylated sterols on the one hand and reduction of the ergosterol levels in the membranes on the other hand (12). Analogous to clotrimazole, this leads to a disturbance in the membranes it results in inhibition of ceU repHcation, mycelium development (in C. albicans) and finally, ceU death. High concentrations of miconazole, which may be achieved with topical use, disturb the orientation of phosphoHpids in the membranes, which produces leaks (13). 

Like the a2ole derivatives, it inhibits the biosynthesis of ergosterol. However, naftifine [65472-88-0] does not inhibit the cytochrome P-450 dependent C-14-demethylase, but the epoxidation of squalene. Squalene epoxidase cataly2es the first step in the conversion of squalene via lanosterol to ergosterol in yeasts and fungi or to cholesterol in mammalian cells. The squalene epoxidase in C. albicans is 150 times more sensitive to naftifine, C2 H2 N, than the en2yme in rat fiver (15). Naftifine is available as a 1% cream. 

Plasma levels of 3—5 p.g/mL are obtained two hours after adraiinistration of 200 mg ketoconazole. No accumulation in the bloodstream was noted after a 30-wk treatment with this dose. The half-life is approximately eight hours. When ketoconazole is taken with meals, higher plasma levels are obtained. Distribution studies using radioactive ketoconazole in rats show radioactivity mainly in the Hver and the connective tissue. Radioactivity is also present in the subcutaneous tissue and the sebaceous glands. After one dose of 200 mg in humans, ketoconazole is found in urine, saUva, sebum, and cenimen. Like miconazole, the mode of action is based on inhibition of the cytochrome P-450 dependent biosynthesis of ergosterol. This results in disturbed membrane permeabiUty and membrane-bound enzymes (8,10,23,25). 

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

FIGURE 24.27 Dicarboxylic acids can be formed by oxidation of the methyl group of fatty acids in a cytochrome P-450-dependent reaction. 

Another important group of cytochromes, found in plants, bacteria and animals is cytochrome P-450, so-called because of the absorption at 450 nm characteristic of their complexes with CO. Their function is to activate... 

Chemical synthesis of cytochrome P-450-dependent metabolites (epoxyeico-satriene acids and other metabolites possessing heterocyclic fragments) 98MI9. 

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 P-450 and other heme-containing oxygenases. J. T. Groves, Adv. Inorg. Biochem., 1979,1,119-145 (109). 

Cytochrome P-450 — an effective catalyst of the oxidation of organic compounds by peroxides. D. I. Metelitsa, Russ. Chem. Rev. (Engl. Transl), 1981, 50,1058-1073 (147). 

Chemical mechanisms of catalysis by cytochromes P-450 towards a unified view. F. P. Guengerich andT. L. Macdonald, Acc. Chem. Res., 1984,17, 9-16 (68). 

Catalysis. Cytochrome P-450 model compounds catalyze the epoxidation of alkenes by hypochlorite ions.16 A typical catalyst is OMn(TMP)L+. 

Oae and coworkers oxidized several diaryl, dialkyl and alkyl aryl sulfides to their corresponding sulfoxides using purified cytochrome P-450 obtained from rabbit liver microsomes138. In agreement with expectations, this enzyme did not exhibit much stereospecificity. Some examples including the observed e.e. values are shown by 121-125. A model was proposed to account for the absolute configurations of the sulfoxides produced (126). The sulfur atom is preferentially oxidized from the direction indicated. 

Asymmetric oxidation of this sulphide was also catalyzed by two isocytochromes P 450 purified from phenobarbital induced rat liver309. Both P 450 isocytochromes, termed PB-1 and PB-4, when reconstituted with purified rat liver NADPH-cytochrome P 450 reductase and cytochrome b5 afforded ethyl p-tolyl sulphoxide with S-configuration at the sulphur atom. In the case of PB-1 optical purity of this sulphoxide was 58% whereas with PB-4 it was 78%. 

A more detailed study of the biological oxidation of sulphoxides to sulphones has been reported165. In this study cytochrome P-450 was obtained in a purified form from rabbit cells and was found to promote the oxidation of a series of sulphoxides to sulphones by NADPH and oxygen (equation 56). Kinetic measurements showed that the process proceeds by a one-electron transfer to the activated enzymatic intermediate [an oxenoid represented by (FeO)3+] according to equation (57). 

Enzyme-mediated chiral sulfoxidation has been reviewed comprehensively in historical context [188-191]. The biotransformation can be mediated by cytochrome P-450 and flavin-dependent MOs, peroxidases, and haloperoxidases. Owing to limited stability and troublesome protein isolation, a majority of biotransformations were reported using whole-cells or crude preparations. In particular, fungi have been identified as valuable sources of such biocatalysts and the catalytic entities have not been fully identified in all cases. 

Henry M, Jolivet JP, Livage J (1991) Aqueous Chemistry of Metal Cations Hydrolysis, Condensation and Complexation. 77 153-206 Hider RC (1984) Siderophores Mediated Absorption of Iron. 57 25-88 Hill HAO, Rdder A, Williams RJP (1970) The Chemical Nature and Reactivity of Cytochrome P-450. 8 123-151... 

Komori M, Nishio K, Kitada M, et al. 1990. Fetus-specific expression of a form of cytochrome P-450 in human liver. Biochemistry 29 4430-4433. 

---