Metabolite in vivo

Human exposure to environmental contaminants has been investigated through the analysis of adipose tissue, breast milk, blood and the monitoring of faecal and urinary excretion levels. However, while levels of persistent contaminants in human milk, for example, are extensively monitored, very little is known about foetal exposure to xenobiotics because the concentrations of persistent compounds in blood and trans-placental transmission are less well studied. Also, more information is needed in general about the behaviour of endocrine disruptive compounds (and their metabolites) in vivo, for example the way they bind to blood plasma proteins. 

Nonaqueous Systems In nonaqueous (nonpolar) solvent systems, nitrosatlon also proceeds. In these solvents, alpha-tocopherol acts as a lipid soluble blocking agent in much the same fashion as ascorbic acid functions in the aqueous phase. Alpha-tocopherol reacts with a nitrosating agent and reduces it to nitric oxide. At the same time, alpha-tocopherol is oxidized to tocoquinone, which is the first oxidation product of vitamin E and also a normal metabolite in vivo. 

In addition to the circumstantial iji vivo evidence for fluorocitrate as the ultimate biochemical lesion we desired to demonstrate unambiguously that it was produced as a metabolite of 29-fluorostigmasterol. The toxicity of fluorocitrate and the resulting lethal accumulation of citrate in mouse, fly and cockroach tissues have been shown in early experiments with fluoroacetamide and fluoroacetate(24). However, to our knowledge, complete characterization of (2R,3R)-2-fluorocitrate as the lethal metabolite in vivo has not previously been reported. We thus prepared( ) [29- ]-29-fluorostigmasterol, [29- H]-29-Huorositosterol and [16- H]-16-fluorohexadec-9-enoic acid to enable isolation of [2-3H]-2-fluorocitrate from vivo incubations using Manduca sexta. 

Schroder, L., Schmitz, C., and Bachert, P. (2004). Molecular dynamics and information on possible sites of interaction of intramyocellular metabolites in vivo from resolved dipolar couplings in localized 1H NMR spectra. ]. Magn. Reson. 171, 213-224. 

Hedli, C.C.. Snyder, R. Witmer, C.M. (1990) Bone marrow DNA adducts and bone marrow cellularity following treatment with benzene metabolites in vivo. In Witmer, C.M., Snyder, R.R.. Jollow, D.J., Kalf, G.F., Kocsis, J.J. Sipes, LG, eds, Biological Reactive Intermediates IV. New York, Plenum Press, pp. 745-748... 

To fully understand the actual mechanism of action of dietary flavonoids either as antioxidants, modulators of cell signalling, or inflammatory pathways, it is important to detect and identify their metabolites in vivo as well as to study the consequences of interaction of these circulating metabolites with cells. Animal models and human studies should also be conducted to identify the in vivo mechanism of action of dietary flavonoids. Results of such studies are crucial in the evaluation of their potential as cardiovascular protective agents and may eventually lead to specific advice regarding intake of foods and beverages rich in flavonoids and attempts to alter and enhance flavonoid content of a range of foods. 

Zhang, H., Zhu, M., Ma, L., He, H., Humphreys, W. G., and Sanders, M. (2006). Combining MDF and peak match techniques for comprehensive and selective detection of drug metabolites in vivo. In Proceedings of the 54th ASMS Conference on Mass Spectrometry and Allied Topics, Seattle, WA. 

It has been shown by using NMR spectroscopy in conjunction with isotopelabelling studies that there is a significant degree of deacetylation followed by reacetylation (futile deacetylation) of paracetamol metabolites in vivo in the rat [58]. If this also occurs in humans, then it may help to explain the observed incidence of nephrotoxicity of paracetamol in that the process would result in levels of the potent nephrotoxin 4-aminophenol in vivo. Confirmation of the levels of futile deacetylation in individual metabolites of isotopically labelled paracetamol in man has been achieved by using directly coupled HPLC-NMR... 

The enzymic oxidative deamination of simple phenethylamines is exemplified by the reported bio transformations of mescaline (146) (114, 115) and ephedrine (148) (116). Mescaline is metabolized to 3,4,5-trimethoxy-phenylacetic acid by tissue homogenates of mouse brain, liver, kidney, and heart (114,115). 3,4,5-Trimethoxybenzoic acid is also formed as a minor metabolite. The formation of jV-acetylmescaline (147), a significant metabolite in vivo, was not observed in the in vitro studies. Both D-(—)-and L-(+)-ephedrine have been incubated with enzyme preparations from rabbit liver norephedrine (149), benzoic acid, and 1-phenyl-1,2-propanediol were characterized as metabolites (116). The D-(—)-isomer was the better substrate, being more rapidly converted. Similar results were previously reported with rabbit liver slices as the source of enzyme (153,154). The enzymic degradation of the side chain of /i-phenethylamines has been extensively investigated with nonalkaloid substrates such as amphetamine (151) and jV-methylamphetamine (150) (10,155-157), and the reader is referred to these studies for a more comprehensive coverage of this aspect of the subject. 

Distribution of [ C]-DMH Metabolites In Vivo as a Function of Dietary Protein. The distribution of metabolites was studied in mice given subcutaneous injections of [AHC] DMH. Most of the radioactivity exhaled as azomethane (AM) was collected within 1 hr, and production of this metabolite wa completed by 3 hr (Figure 2). In contrast, the expiration of CO was negligible during the first hour and ceased after the fifth hour. 

Following the in vitro discovery that a particular metabolic pathway is mediated by a single CYP isoform and that such metabolism is a major route of elimination, it is not uncommon to speculate that appropriate assessment of the formation of the metabolite in vivo could serve as a phenotypic trait measure (see Chaps. 3 and 7). Moreover, knowledge of the disposition of the drug and metabolite may indicate a putative quantitative trait for this purpose, e.g., urinary MR. However, considerably greater effort and information is, in fact, required before such a trait value can be accepted as a valid measure of the enzyme s metabolic activity. Unfortunately, several of the earlier-developed in vivo probes were not rigorously evaluated prior to their application, and interpretation of differences/changes in their trait values is therefore not easy. 

Kettunen MI, Grohn OH, Kauppinen RA (2004) Quantitative Tlrho NMR spectroscopy of rat cerebral metabolites in vivo effects of global ischemia. Magn Reson Med 51 875-880... 

Hedli CC, Snyder R, Witmer CM. 1990. Bone marrow DNA adducts and bone marrow cellularity following treatment with benzene metabolites in vivo. Adv Exp Med Biol 283 745-748. 

Tuk, B., Van Oostenhmggen, M.F., Herhen, V.M.M., Mandema, J.W., Danhof, M. (1999). Characterization of the pharmacodynamic interaction between parent drug and active metabolite in vivo midazolam and alpha-OH-midazolam. J. Pharmacol. Exp. Then 289 1067-74. 

Note A G, the actual free-energy change, has been calculated from A G° and known concentrations of reactants imder typical physiologic conditions. Glycolysis can proceed only if the A G values of all reactions are negative. The small positive A G values of three of the above reactions indicate that the concentrations of metabolites in vivo in cells rmdergoing glycolysis are not precisely known. 

Ciccoli and coworkers have shown that incubation of rat erythrocytes with hydroxy lated metabolites of aniline, dapsone (39) and the corresponding hydroxylamines brings about enhanced release of iron and methemoglobin formation. This did not occur with parent compounds. That xenobiotics are effective only after biotransformation to metabolites in vivo is supported by acute intoxication of rats with aniline or 39 and marked increase in the erythrocyte content of free iron and of methemoglobin144. The potent toxicity of aniline-derived aminophenylnorharman in the liver of the gpt delta transgenic mouse has been demonstrated145,146. 

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