Xenobiotic metabolism studies

Shift reagents have not been used extensively in xenobiotic metabolism studies however, they may find increased use for determination of isomer composition in studies on metabolic mechanisms because metabolic rates are often different for geometric and stereoisomers. Mizugaki et al. used NMR with Eu(fod)3 to determine structures of cis- and trans-3-alkenoic acids (22). Moser et al. used NK and a chiral shift reagent for analysis of the... 

The use of compounds enriched with l c is common in biosynthetic studies (26), but quite rare in xenobiotic metabolism studies. Hernandez et al. (27) studied the enzymic and non-enzymlc reaction of glutathione with benzo(a)pyrene 4,5-oxlde enriched with at the 4 and 5 positions. The oxide was prepared with an enrichment... 

Few applications of hWR with nuclei other than 1h and 13c have been reported for xenobiotic metabolism studies. Some studies in... 

As drugs are usually foreign chemicals, history of concern for the biotransformations of drugs leading to toxic metabolites formation is intrinsically hnked to the history of xenobiotic metabolism studies. The International Society for the Study of Xenobiotics (ISSX) website (http //www. issx.org) presents an overview of the field history where some key figures may be pointed out. 

Studies on rats had shown no toxicity of astaxanthin preparations. Dietary administration of astaxanthin has proved to significantly inhibit carcinogenesis in the mouse urinary bladder, rat oral cavity, and rat colon. In addition, it is reported to induce xenobiotic-metabolizing enzymes in rat liver. 

Donnarumma L, G de Angelis, F Gramenzi, L Vittozzi (1988) Xenobiotic metabolizing enzyme systems in test fish. III. Comparative studies of liver cytosolic glutathione S-transferases. Ecotoxicol Environ Saf 16 180-186. 

Although the radioactive isotope H has been extensively used for studies on the uptake of xenobiotics into whole cells, the intrusion of exchange reactions and the large isotope effect renders this isotope rather less straightforward for metabolic studies. Both deuterium H-labeled substrates, and oxygen and OH2 have, however, been extensively used in metabolic studies, since essentially pure labeled compounds are readily available and mass spectrometer facilities have become an essential part of structural determination. 

Egaas, E., J.U. Skaare, N.O. Svendsen, M. Sandvik, J.G. Falls, W.C. Dautennan, T.K. Collier, and J. Netland. 1993. A comparative study of effects of atrazine on xenobiotic metabolizing enzymes in fish and insect, and of the in vitro phase II atrazine metabolism in some fish, insects, mammals and one plant species. Comp. Biochem. Physiol. 106C 141-149. 

Miyamoto, J. 1988. Disposition and metabolism studies of xenobiotics in the environment fundamentals for safety assessment. Jour. Toxicol. Sci. 13 221-246. 

Comparative Metabolism. Since the liver is the major organ involved in the biotransformation of xenobiotics, primary hepatocyte cultures provide an excellent model for in vitro metabolism studies. Primary hepatocyte cultures provide useful tools with which to study the comparative metabolism of xenobiotics by both humans and laboratory animals. 

Guillouzo, A. (1986). Use of isolated and cultured hepatocytes for xenobiotic metabolism and cytotoxicity studies. In Isolated and Cultured Hepatocytes (Guillouzo, A. and Guguen-Guillouzo, C., Eds.). John Libbey, London, pp. 313-332. 

Metabolism - a final factor in need of comparative studies is the metabolism of xenobiotics. One obvious difference between mammalian and fish species is that their bodies usually function at temperatures at least 10°C different. This fact undoubtedly explains some differences in metabolic rate but even when in vitro incubations are run at optimal temperatures there is a 10 - 100 fold higher rate of mammalian vs. fish metabolism (14, 15). In other words, the level of the xenobiotic-metabolizing capacity, especially for oxidative pathways, of the poikilothermic animals is considerably lower than that of the homeothermic species. Elsewhere in this volume Dr. Bend has focused on this aspect of the handling of xenobiotics by fish (16). 

A simple cyclic diester of carbonic acid is ethylene carbonate (7.96), a chemical with a toxicity profile resembling that of ethylene glycol (7.97). Metabolic studies have confirmed that ethylene carbonate is hydrolyzed very rapidly to ethylene glycol (whose oxidation is discussed in Chapt. 2 in [7]) and C02. Indeed, rats given an oral dose of ethylene carbonate did not excrete the unchanged xenobiotic in detectable amounts, and blood levels of the diester were ca. 100-fold smaller than those of ethylene glycol [178],... 

Chaturvedi (1993) also examined the effect of mixtures of 10 pesticides (alachlor, aldrin, atrazine, 2,4-D, DDT, dieldrin, endosulfan, lindane, parathion, and toxaphene) administered by oral intubations or by drinking water on the xenobiotic-metabolizing enzymes in male mice. He concluded, The pesticide mixtures have the capability to induce the xenobiotic-metabolizing enzymes, which possibly would not have been observed with individual pesticides at the doses and experimental conditions used in the study. ... 

In vitro systems with varying levels of complexity have been used to study xenobiotic metabolism. These include purified proteins, subcellular fractions/cell lysates, intact whole cells, tissue slices and whole organ culture. Advantages and disadvantages of the approaches are summarized below. 

Tissue lysate (or homogenates), post-mitochondrial supernatants and microsomes offer several practical advantages for the study of xenobiotic metabolism. The principal advantages are that the human tissues provide a complete system containing all the enzymes in ratios found in vivo, and tissue fractions are stable in relatively long-term storage. Within the different types of tissue fractions, microsomes provide an enrichment of the membrane-bound enzymes, and post-mitochondrial supernatants provide a means to study both membrane-bound and soluble enzymes. Tissue fractions are easily prepared from a variety of tissues including human liver and can be cryopreserved for several years. This allows detailed characterization of the tissue prior to use with xenobiotics of unknown routes of metabolism... 

Whole cells (for example, primary hepatocytes or genetically engineered cells) provide the simplest intact system for the study of xenobiotic metabolism (Ratanasavanh et al., 1986 Daujat et al., 1991 Berry et al.,... 

A number of practical considerations affect the choice of experimental approaches to address a specific question. In general, these practical considerations tend to have a greater impact on applications to toxicology (where specific endpoints may not be measurable in all cell types) than studies of xenobiotic metabolism (where the principal requirement is enzyme activity). However, applications to toxicology should be considered when making a choice of an experimental approach as specific toxicological issues may develop from the metabolism studies or in other aspects of the safety assessment processes. 

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