Metabolism systems, experimental

Comparative Toxicokinetics. In humans, the targets for trichloroethylene toxicity are the liver, kidney, cardiovascular system, and nervous system. Experimental animal studies support this conclusion, although the susceptibilities of some targets, such as the liver, appear to differ between rats and mice. The fact that these two species could exhibit such different effects allows us to question which species is an appropriate model for humans. A similar situation occurred in the cancer studies, where results in rats and mice had different outcomes. The critical issue appears to be differences in metabolism of trichloroethylene across species (Andersen et al. 1980 Buben and O Flaherty 1985 Filser and Bolt 1979 Prout et al. 1985 Stott et al. 1982). Further studies relating the metabolism of humans to those of rats and mice are needed to confirm the basis for differences in species and sex susceptibility to trichloroethylene s toxic effects and in estimating human heath effects from animal data. Development and validation of PBPK models is one approach to interspecies comparisons of data. 

Given the inherent limits of a purely computational approach to obtain an estimate of the flux distribution of a metabolic system, an experimental determination of metabolic fluxes is paramount to the construction and validation... 

Also when resorting to heuristic rate equations or other approximative schemes, the construction of detailed kinetic models necessitates quantitative knowledge about the kinetic properties of the involved enzymes and membrane transporters. Notwithstanding the formidable progress in experimental accessibility of system variables, detailed in Sections IV and VI, for most metabolic systems such quantitative information is only scarcely available. 

Experimental aquatic metabolism systems have taken one of four forms -- static water in ordinary aquaria, jars, or beakers static "model ecosystems" static outdoor ponds and continuously-flowing systems (Table 1). A very rough comparison of their advantages is shown in Table II. For example, while the static aquaria doubtless are by far the... 

Indications or suspicions of effects on organ systems and functions that are especially vulnerable during development and maturation in early life (in particular the nervous, reproductive, endocrine, and immune systems and also the metabolic pathways) Experimental data on such effects in young animals are not available... 

Chrysene was mutagenic to Salmonella typhimurium in the presence of an exogenous metabolic system. It induced sister chromatid exchanges in one mouse study and chromosomal aberrations in one hamster study. Chrysene is metabolically activated to a 1,2-diol-3,4-epoxide that is mutagenic and carcinogenic in experimental animals and forms covalent adducts with DNA. ... 

Predictive models can be better produced when recombinant cytochrome data are available, an experimental technique which may increase the probability of obtaining consistent and predictive models, since one protein is involved in the metabolic reaction. The most accurate data to describe the rate and affinity of the ligand towards an enzyme are the kinetic parameters Vmax and Km. Nevertheless, the calculation of these parameters is time consuming. A less precise parameter is the determination of the compound percentage remaining in a cytochrome incubation after a certain period of time. These metabolic data are less accurate and can only be used to classify the compounds in a metabolic system as stable or unstable. This type of data was the basis for a predictive model of metabolic stability towards CYP3A4 [35]. 

The above-reported chemical reactions proceed under conditions that are compatible with an origin of life under the locally and temporally coherent conditions of a volcanic flow system. Therefore, the discovered reactions may well be components of the metabolic system of the pioneer organism. As additional components come into experimental view, the theory is expected to evolve. So far we have addressed the notions of growth and reproduction as aspects of one unitary chemical system. We now show that this unitary system is also the physical basis for the earliest mechanism of evolution and that it constitutes in fact the evolutionary Aiflage for the emergence of the cellular and genetic features of extant forms of life. 

This chapter contains a brief discussion of allometry, physiological pharmacokinetics, and the use of in vitro systems to predict drug metabolism in experimental animals and human study participants. 

The objective of this study is to identify the major metabolites of the drug in question. For this study, the drug in question is incubated with an appropriate in vitro metabolic system to allow the formation of metabolites. Metabolites are then identified using analytical chemical approaches. The in vitro experimental system of choice is human hepatocytes, with high-performance liquid chromatography/mass spectrometry (HPLC/MS) or tandem mass spectrometry (HPLC/MS/MS) as the most convenient analytical tool to identify the metabolites. 

The mode of action of protopine on rabbit platelet aggregation was investigated in the metabolic system of adenosine 3 ,5 -cyclic monophosphate (cyclic AMP) in vitro experimental models. The inhibitory activity of the alkaloid on adenosine S -diphosphate induced platelet... 

Quant, P.A. (1993) Trends Biochem. Sci. 18,26-30. Experimental application of top-down control analysis to metabolic systems. 

The formation of protein and DNA adducts may be studied in vitro by incubation of the macromolecule with a reactive intermediate or with the parent compound in the presence of a metabolizing system, or in vivo in experimental animals. In most studies, radiolabels are used that may be analyzed by liquid scintillation counting but more recendy also by a specific MS-based technique (see section 5 of this chapter). 

It was subsequently discovered that lucanthone is metabolized in the body in part to hycanthone (30), a compound with enhanced schistomacidal activity. The relatively high biologic activity of lucanthone in experimental animals compared to man was subsequently attributed to the inefficient hydroxylating system present in man for this biochemical conversion.Microbiologic oxidation of lucanthone by fermentation with the fungus Aspergil-lus scelorotium affords hycanthone. ... 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

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