Cytochrome demethylation activity

The formation of the methylenedioxy bridge in Berberis has been found to be caused by the demethylating activity of a peroxidase (POD) found within the vesicle. It was also found that the cytochrome P450-requiring enzyme (canadine synthase) from microsomes of Berberis, Thalictrum and Coptis species formed the methylene bridge in (S)-tetrahydrocolumbamine (Ikezawa et al, 2003), but not in the quaternary alkaloid columbamine (Galneder et al, 1988 Zenk, 1995). Because of the substrate specificity of canadine s)mthase, the berberine pathway is considered to be that presented in Fig. 2.5 (Rueffer and Zenk, 1994). Columbamine, once proposed as an alternative route to berberine, is however converted to palmatine by a specific methyltransferase first isolated from Berberis wilsoniae cell cultures (Rueffer and Zenk, 1985 Ikezawa et al, 2003). 

Kim D, Guengerich FP. Enhancement of 7-methoxyresorufin O-demethylation activity of human cytochrome P450 1A2 by molecular breeding. Arch Biochem Biophys 2004 432 102-8. 

This may be a reason for the unique role of sulfur in the hemoprotein cytochrome P450. In order to explain the N-demethylation activity of ferric microsomal cytochrome P450 in the presence of hydrogen peroxide, Ullrich et al. have formulated the following reaction ... 

Ascorbic acid deficiency in guinea pigs has been shown to decrease microsomal cytochrome P-450 activity to about 50% of its normal value (96, 97,192, 203, 268) cytochrome bs (96,97, 174,192, 302, 358), NADPH-linked cytochrome c reductase (174, 358), and O- and A/-demethylase activities (174, 185, 192, 358). Conflicting data by Kato et al. (170) in which the above-mentioned enzyme activities were not affected by ascorbate deficiency seem to be the result of a shorter depletion period (12 days). Zannoni and Sato (Ref. 174, p. 119) have shown that microsomal enzyme activities are not decreased significantly in a 10-day depletion experiment but are significantly decreased after 21 days. Repletion experiments with scorbutic guinea pigs have shown that supplementation with ascorbic acid returned cytochrome P-450 and demethylation activities to normal within 48 hr (97,192, 203). 

In this study, we have utilized 2,2, 4,4 -tetrachlorobiphenyl and 3,3, 4,4 -tetrachlorobiphenyl as representative non-coplanar and coplanar isomers respectively. The 3,3, 4,4 -tetrachlordbi-phenyl isomer (0.3mmole/kg) induced ethoxycoumarin-and ethoxy-resorufin-O-deethylations in the rainbow trout to a similar extent as did Aroclor 1242 (Table III). However, the non-coplanar 2,2 -4,4 -tetrachlorobiphenyl was without effect upon any of the monooxygenase activities examined. Cytochrome P -450-like activity as determined by ethoxyresorufin-O-deethylation was increased by the planar 3,3, 4,4 -tetrachlorobiphenyl while cytochrome P-450-like activity (benzphetamine-N-demethylation) was unaffected. 

As with adults, the primary organ responsible for drug metabolism in children is the liver. Although the cytochrome P450 system is fully developed at birth, it functions more slowly than in adults. Phase I oxidation reactions and demethylation enzyme systems are significantly reduced at birth. However, the reductive enzyme systems approach adult levels and the methylation pathways are enhanced at birth. This often contributes to the production of different metabolites in newborns from those in adults. For example, newborns metabolize approximately 30% of theophylline to caffeine rather than to uric acid derivatives, as occurs in adults. While most phase I enzymes have reached adult levels by 6 months of age, alcohol dehydrogenase activity appears around 2 months of age and approaches adult levels only by age 5 years. 

Human colon microsomal cytochrome P450 enzymes can demethylate 1,2-dimethylhydrazine, yielding formaldehyde, and activities in the descending colon are always higher than in the ascending or transverse colon (Newaz et al., 1983). A-Demethylation was also detected in microsomes from a human colon tumour line, adenocarcinoma LS 174T cells. The activity was inducible by phenobarbital and/or hydrocortisone. 

Cocaine may be TV-demethyl ate d by the cytochrome P450 system to produce an active metabolite, norcocaine. Further breakdown produces A-hydroxynorcocaine and norcocaine nitroxide. Further metabolism produces a highly reactive free radical that is thought to be responsible for the hepatotoxicity observed in cocaine users.1... 

Another consequence of these findings is that the same adduct can be formed by a free radical mediated pathway from MAB following a one electron oxidation by peroxides as that formed from methylol or me thimine by a two electron oxidation catalyzed by cytochrome P450. Clearly identification of the GSH adducts of carcinogens in vivo may not distinguish both metabolic activation systems. It is also still not clear whether cytochrome P450 and peroxidases form common intermediates during N-demethylation reactions (22-24). 

Ichikawa, Y., Yamano, T., Fujishima, H. (1969) Relationships between the interconversion of cytochrome P540 and its activities in hydroxylations and demethylations by P450 oxidase systems. Biochem. Biophys. Acta 171, 32-46. 

Dayer P, Leemann T, Stribemi R. Dextromethorphan O-demethylation in liver microsomes as a prototype reaction to monitor cytochrome P-450 dbl activity. Clin Pharmacol Ther 1989 45 34-40. 

Figure 19 Time course of heat inactivation (50°C) of benzydamine N-oxygenation (FMO) and N-demethylation (CYP) by human liver microsomes. Human liver microsomes were preincubated at 50°C for 0, 1, 2, 3, 5, 7, and 10 minutes prior to measuring FMO and CYP activity toward benzydamine as outlined in Figure 18. Abbreviations FMO, flavin monooxygenase CYP, cytochrome P450.

Jones D, Gorski J, Haehner B, et al. Determination of cytochrome P450 3A4/5 activity in vivo with dextromethorphan iV-demethylation. Clin Pharmacol Ther 1996 60 374-384. 

Mode of action Ketoconazole interacts with C-14 a-demethylase (a cytochrome P-450 enzyme) to block demethylation of lanosterol to ergosterol, the principal sterol of fungal membranes Figure 34.4). This inhibition disrupts membrane function and increases permeability. Ketoconazole acts in an additive manner with flucytosine against Candida, but antagonizes amphotericin B s antifungal activity. 

Similarly, for tertiary amines a distinction can be made between oxometal and peroxometal pathways. Cytochrome P450 monooxygenases catalyze the oxidative N-demethylation of amines in which the active oxidant is a high-valent oxoiron species. This reaction can be mimicked with some oxometal complexes (Ruv=0), while oxidation via peroxometal complexes results in oxidation of the N atom (Fig. 4.93 a and b) [261]. A combination of MTO/hydrogen peroxide can... 

Four subjects had markedly reduced O-demethylation of dextromethorphan after they had taken moclobemide 300 mg bd for 9 days (60). A-demethylation was not affected. This result supports the hypothesis that moclobemide or a metabolite reduces the activity of the cytochrome enzyme CYP2D6. The clinical implications of this particular interaction remain to be clarified. 

A very important conclusion was reached based on the effect of p-mer-curibenzoate on the NADPH oxidase and the NADPH-cjrtochrome c reductase activities of microsomes, namely, that the natural acceptor might be a component reactive with oxygen and involved in hydroxyla-tions or demethylations (11). It was found that in the absence of cytochrome c, the oxidase activity was largely inhibited by p-mercuri-benzoate. In the presence of cytochrome c, NADPH oxidation exceeded cytochrome c reduction in the absence of p-mercuribenzoate and the two rates equaled each other in the presence of p-mercuribenzoate. Thus, a mercurial sensitive oxidase distinct from the reductase was indicated, and this component was hypothesized to be connected with hydroxylation and/or demethyktion (11). 

Anhydrous Fe Cl3 catalyzes the stereospecific epoxidation of norbomene, the demethylation of A, A-dimethylaniline, and the oxidative cleavage of PhCMe(OH)CMe(OH)Ph (and other a-diols) by hydrogen peroxide (Table 11 and Scheme 4). For each class of substrate, the products parallel those that result from their enzymatic oxidation by cytochrome P-450. The close congruence of the prodncts indicates that the reactive oxygen in the Fe Cl3/HOOH model system and in the active form of cytochrome P-450 is essentially the same, with strong electrophilic oxene character (stabilized singlet atomic oxygen). 

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