Hydroxylation by CYP

A second example of sulfate bioactivation derives from the observed carcinogenicity of aromatic amineS/ such as those derived from coal tar (44). The polycyclic aromatic amines are N-hydroxylated by CYPs and then sulfated to form unstable N-O-sulfates that decompose and produce reactive nitrenium ion intermediates/ which form DNA and protein adducts. One environmental/genetic hypothesis of colon cancer etiology involves the interaction between dietary aromatic amines and the polymorphic expression of the appropriate STs for their activation to procarcinogenic reactive intermediates (44/ 45). 

The metabolism of nitrobenzenes to quinone-imines arises from a six-electron reduction of the nitro group to the corresponding aniline metabolite via the intermediate nitroso and hydroxylamine analogs. Aromatic ring hydroxylation by CYP ortho or para to the aniline nitrogen then generates the aminophenol derivative. The conversion of a nitrobenzene derivative to a quinone-imine is illustrated with the catechol-O-methyltransferase inhibitor tolcapone, an... 

MgCl2 (10 mM) increased the apparent Km (83 to 173 /am) and reduced the Vjnax (3.4 to 2.4 min-1) of triazolam 4-hydroxylation by expressed CYP3A4 [21]. However, both MgCl2 (30 mM) and CaCl2 (30 mM) significantly increased reaction rates of testosterone 6/3-hydroxylation (approximately threefold) and nifedipine oxidation (three- to six-fold) by human liver microsomes (HLMs) or recombinant CYP3A4 (reconstituted with b5 and GSH) [15]. It was suggested that divalent cation stimulation on the activity was related to involvement of b5 in CYP 3A4 reaction. 

Perphenazine 20% 1 to3h Sulfoxidat hydroxylal dealkylatior glucuronid by CYP ... 

Eventually, flavonoids can be hydroxylated or demethylated by CYPs. For instance, hesperetin (4 -methoxy-3, 5,7-trihydroxyflavanone) is specifically demethylated by CYPs lAl and IBl, but not by CYPs 1A2 and 3A4. In addition, 3,5,7-trihydroxyflavone undergoes sequential CYP lAl-catalyzed hydroxylation at C4 and C3 to finally yield querce-tin. These reactions may be relevant to flavonoid metabolism and cytotoxicity since the corresponding products are more reducing and thus more prone to autoxidation with simultaneous ROS production. 

Cyclosporine is slowly absorbed, and peak concentrations are reached in 4 to 6 hours. Cyclosporine is 90% protein bound and concentrated in erythrocytes. " The degree of concentration in erythrocytes is temperature dependent in vitro for this reason, measurement of plasma concentration is not recommended. " The optimal specimen for analysis is whole blood. The ehmination profile of cyclosporine is biphasic. An early elimination phase with an apparent half-life that typically ranges from 3 to 7 hours is followed by a slower elimination phase with an apparent half-life ranging from 18 to 25 hours. The volume of distribution is 17L/kg. Cyclosporine undergoes extensive metabolism by CyP 3A4. Many of the 31 known metabolites of cyclosporine are inactive. One of the major metabolites, hydroxylated at the... 

Figure 43-1 I Schematic view of the role of NAT enzymes in the metabolism of aromatic amines. N-acetylation might be a detoxification reaction in a number of cases however, after N-hydroxylation of aromatic amines (e.g., by CYP enzymes), NAT enzymes can bioactivate these intermediates by either 0-acetylation or intramolecular N,0-acety transfer, leading to the formation of nitrenium ions, which might react with DNA or alternatively be detoxified by, for example, GST enzymes. Importantly, it is shown that a number of other biotransformation enzymes are also involved in the metabolism of aromatic amines as well. (Redrawn from Wormhoudt LW, Commandeur jNM, Vermeuien NPE. Genetic polymorphisms of human N-acetyitransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes relevance to xenobiotic metabolism and toxicity. Crit Rev Toxicol 1999 29 59-124. Reproduced by permission from Taylor and Francis, Inc.)...

These oxidations, which are usually catalyzed by CYP monooxygenases, produce hydroxylated derivatives. When the C-H bond is located in the a position to a heteroatom (such as O, S, N, halogen), the a hydroxylated derivative obtained is usually unstable and may be further oxidized or cleaved (Figure 33.5). 

Thiabendazole s fungicidal properties had already been reported in 1964, but prior to that it had been used as an anthelmintic in human and veterinary medicine. In aqueous solution it is stable, but in mammals it is hydroxylated in the benzene ring by CYP enzymes. 

The reactions show some typical oxidations of insecticides catalyzed by CYP enzymes. Carbofuran is hydroxylated to another active compound carbaryl is demethylated or hydroxylated and aldrin is epoxidated to diel-drin, which also has insecticidal properties. Phosphorothionates must be oxidized to the phosphates by CYP enzymes in order to become inhibitors of acetyl cholinesterase. Parathion-methyl is transformed to the oxon analogue, paraoxon-methyl, which is the toxic compound. It also can be demethylated to the inactive desmethyl-parathion-methyl. 

Amines, ethers, and thioethers can sometimes seem like magnets for oxidative metabolism, being subject to a variety of reactions catalyzed by CYPs or FMOs. An attempt to simplify, but hopefully not oversimplify, the possible metabolic fates of amines in the presence of various CYPs is shown in Figure 9.19. A radical cation is initially formed at nitrogen. This sometimes goes on to yield an amine A-oxide for tertiary amines or a hydroxylamine for primary or secondary ones. More often, however, an o-hydrogen is extracted and a carbi-nolamine, the net product of o-hydroxylation, is formed. 

Mace and colleagues have examined the ability of camosol and camosic acid from rosemary as well as the synthetic dithiolethione, oltipraz, to block the formation of DNA adducts, and their effects on the expression of phase I and phase II enzymes. It was found that both rosemary extracts and oltipraz inhibited BaP- or aflatoxin Bi-induced DNA adduct formation by efficiently inhibiting CYP activities and inducing the expression of GST. Treatment of female CD-I mice with a 2% methanol extract of rosemary in AIN-76A diet for 3 weeks increased the liver microsomal 2-hydroxylation of estradiol and estrone by approximately 150%, increased their 6-hydroxylation by approximately 30%, and inhibited the 16a-hydroxylation of estradiol by approximately 50%. The same treatment of rosemary also stimulated the liver microsomal glucuronidation of estradiol and estrone by 54 to 67% and 37 to 56%, respectively. In additional studies, feeding 2% rosemary diet to ovariectomized CD-I mice for 3 weeks inhibited the uterotropic action of estradiol and estrone by 35 to 50% compared with animals fed a control diet. 

In addition to LOX-mediated reactions, AA-derived HETEs and LA-derived HODEs can also be biosynthesized by CYP pathways (Bylund et al. 1998). HODEs which have been determined as CYP products include 9-, 13-, and 18-HODE (Bylund et al. 1998), while some work on rodents and primates found the additional products 8-HODE, 11-HODE (acid-labile converts into 9/13-HODE), and 14-HODE (Oliw et al. 1993). 8-HODE, 11-HODE, and 14-HODE were all formed via an allylic hydroxylation, whereas the 9/J-HODE and 131 -HODE were stereoselectively biosynthesized via hydroxylation with double bond migration (Oliw et al. 1993). AA-derived HETEs that can be generated by CYP include 7-, 10-, and 13-HETE (acid-labile) and 5,6-diHETrE. CYP enzymes can also catalyze the formation of terminal and subterminal hydroxylated species such as 16-, 18-, 19-, 20-HETE, all of which have been shown to exhibit vasodilatory properties (Carroll et al. 1996). 19-HETE has been reported to affect vascular tone and ion transport. 20-HETE is a CYP4 family AA oxidation product, which has been found to be a potent vasoconstrictor but it also has been identified in some studies as having a pro-carcinogenic role contributing to cell proliferation and tumor growth (Guo et al. 2008 Alexanian et al. 2009 Liu et al. 2010). 

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