Metabolism from tissue analysis

The above analysis predicts relative (not absolute) contributions of the different enzymes to overall hepatic metabolism. Naturally, the relative contributions of the different enzymes will vary from tissue to tissue depending on the enzyme composition in the tissue. For example, if the... 

PK modeling can take the form of relatively simple models that treat the body as one or two compartments. The compartments have no precise physiologic meaning but provide sites into which a chemical can be distributed and from which a chemical can be excreted. Transport rates into (absorption and redistribution) and out of (excretion) these compartments can simulate the buildup of chemical concentration, achievement of a steady state (uptake and elimination rates are balanced), and washout of a chemical from tissues. The one- and two-compartment models typically use first-order linear rate constants for chemical disposition. That means that such processes as absorption, hepatic metabolism, and renal excretion are assumed to be directly related to chemical concentration without the possibility of saturation. Such models constitute the classical approach to PK analysis of therapeutic drugs (Dvorchik and Vesell 1976) and have also been used in selected cases for environmental chemicals (such as hydrazine, dioxins and methyl mercury) (Stem 1997 Lorber and Phillips 2002). As described below, these models can be used to relate biomonitoring results to exposure dose under some circumstances. 

Figure 3 An example of the use of GC-MS for metabolic profiling. Left A section of the total ion chromatogram from the analysis of TMS-derivatized aqueous tissue extracts from the liver of PPAR- null mouse. Metabolites are identified from exact retention times and comparison of corresponding mass spectra with those in the NIST database. 97 metabolites were quantified. Right Summary of metabolite differences in the tissues of the PPAR-a null mouse. Red- increased relative to control, blue- decreased relative to control. The increased/decreased width of certain arrows reflects relative increased/decreased concentrations across these pathways, respectively.

Various estimates have been made of the whole body content, based initially on direct cadaver analysis and more recently by isotope dilution techniques (Kennedy et al., 1978). Values between 1-2 g for an adult are quoted. Since the greatest amount of zinc is found inside cells, the average tissue concentration is higher than that found in body fluids. Zinc is present in all metabolically active tissue and some results obtained from tissue obtained at autopsy of accident victims, are shown in Table 5. These were obtained using neutron activation analysis (Smith, 1967) and by flame MS (Lyon et al., 1989, Martin et al., 1992). 

Bhushan (1991) on amino acids and their derivatives their new review includes references through 1994. Jain (1996) has reviewed studies on the applications of TLC to amino acid analysis of biological fluids and tissues. Such analyses are important in making diagnoses of inborn errors of amino acid metabolism. It was noted by Jain (1996) that TLC has proved useful to screen and quantify abnormal amounts of free amino acids in blood and urine samples. Shalaby (1996) in his chapter on TLC in food analysis has provided some protocols on amino acid analysis following the hydrolysis of protein samples. Fried and Haseeb (1996) in their chapter on TLC in parasitology have provided some information on the analysis of free pool amino acids from tissues of parasites and from the hemo-lymph of mosquitoes infected with Plasmodium (the malaria organism). 

The vial equilibration method is the most common in vitro method for determining partition coefficients for volatile or semivolatile materials and has been used most successfully for volatile organic solvents (Gargas et al., 1988). Tissues are harvested from the species of interest and incubated with the test compoxmd imtil equilibrium is reached between the tissue and the headspace in the vial. The blood/air or tissue/air partition coefficients are given by the ratio of the concentrations of the chemical in the blood or tissue relative to its concentration in the headspace. Tissue-blood partition coefficients are calculated from the respective tissue/air and blood/air values. A number of operational equations have been derived to calculate these ratios xmder specific experimental conditions. Time to steady state is critical and should be optimized for the test compoxmd. Metabolism of the compound in exposed tissue samples must be controlled. Analysis is performed by gas chromatography in a verified linear range. Human tissues can be obtained from tissue bank organizations to provide species specificity to models developed with human data. To estimate... 

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