Metabolism comparisons

More recently [135], 72 and 73 were also found in the sea hare Stylocheilus longicauda but, surprisingly, only dehalogenated 73 was present in the algal food source, Lyngbya majuscula. This metabolic comparison suggested an uncommon in vivo bromination of 73, but further experiments need to ascertain whether L. majuscula is the unique food source of S. longicauda. 

Mundy WR, Freudenrich T, Shafer TJ, et al. 1995. In vitro aluminum inhibition of brain phosphoinositide metabolism Comparison of neonatal and adult rats. Neurotoxicology 16 35-44. 

If metabolism in animal models is extensive or the generated metabolite(s) is shown to have toxic effects, in vitro metabolism studies using isolated P-450 isozymes, tissue homogenates containing the microsomal fraction, hepatocytes, and liver slices are commonly conducted to determine if the extent of metabolism and the metabolite profile is similar for animals and humans. The results from these in vitro metabolism comparison studies can be used to select the animal models for definitive development studies that have similar metabolism profiles to humans. 

Figure 31.4 illustrates an interspecies difference in paclitaxel metabolism (9). The principal metabolite formed in humans was not produced by rat microsomes. This example illustrates the potential of in vitro studies to discover interspecies differences in metabolism. In most cases, it is no longer necessary to wait for in vivo Phase I studies to discover such differences, and certainly not advisable. Regulatory authorities around the world have encouraged early consideration of inter species metabolic comparisons. 

Von Weymam, L.B. and S.E. Murphy (2003). CYP2A13-catalysed coumarin metabolism Comparison with CYP2A5 and CYP2A6. Xenobiotica 33, 73-81. 

Naritomi Y, Terashita S, Kagayama A, Sugiyama Y. Utility of hepatocytes in predicting drug metabolism comparison of hepatic intrinsic clearance in rats and humans in vivo and in vitro. Drug Metab Dispos 2003 31 580-588. 

Battiston L, Tulissi P, Moretti M, Pozzato G. Lansoprazole and ethanol metabolism comparison with omeprazole and cimetidine. Pharmacol Toxicol (1997) 81, 247-52. 

Pastink, M. L, Teusink, B., Hols, R, Visser, S., De Vos, W. M., Hugenholtz, J. (2009). Genome-scale model of Streptococcus thermophilus LMG18311 for metabolic comparison of lactic acid bacteria. Applied and Environmental Microbiology, 75, 3627-3633. 

This LC-MS experimental approaches used in NEF metabolite identification are also routinely employed for in vitro metabolism comparisons across species, in which liver microsomes or hepatocytes from humans and animal species are used. In addition, liver microsomal incubations followed by metabolite identification and quantitative estimation using liquid chromatography-ultraviolet/ mass spectrometry (LC-UV/MS) is an approach commonly taken to determine metabolic soft spots, where a major metabolic reaction takes place (Table 6.10). Use of UV detection allows for quantitative analysis of major metabolites in the absence of chemical standards, with the assumption that the metabolic reaction did not disturb the molecule s UV chromophore. 

The World Wide Web has transformed the way in which we obtain and analyze published information on proteins. What only a few years ago would take days or weeks and require the use of expensive computer workstations can now be achieved in a few minutes or hours using personal computers, both PCs and Macintosh, connected to the internet. The Web contains hundreds of sites of Interest to molecular biologists, many of which are listed in Pedro s BioMolecular Research Tools (http // www.fmi.ch/biology/research tools.html). Many sites provide free access to databases that make it very easy to obtain information on structurally related proteins, the amino acid sequences of homologous proteins, relevant literature references, medical information and metabolic pathways. This development has opened up new opportunities for even non-specialists to view and manipulate a structure of interest or to carry out amino-acid sequence comparisons, and one can now rapidly obtain an overview of a particular area of molecular biology. We shall here describe some Web sites that are of interest from a structural point of view. Updated links to these sites can be found in the Introduction to Protein Structure Web site (http // WWW.ProteinStructure.com/). 

Yamamoto T, Egashira T, Yoshida T, et al. 1982. Comparison of the effect of an equimolar and low dose of fenitrothion and methylparathion on their own metabolism in rat liver. J Toxicol Sci 7 35-41. 

Parker, R. J., Priester, E. R., and Sieber, S. M. (1982). Comparison of lymphatic uptake metabolism, excretion, and biodistribution of free and liposome-entrapped (l cjcytosine beta-D-arabinofuranoside following intraperitoneal administration in rats, Drug. Me tab. Dispos., 10, 40-46. 

Computational methods including both metabolism databases and predictive metabolism software can be used to aid bioanalytical groups in suggesting all possible potential metabolite masses before identification by mass spectroscopy (MS) [116,117]. This approach can also combine specialized MS spectra feature prediction software that will use the outputs from databases and prediction software and make comparisons with the molecular masses observed... 

Data from both in vivo and in vitro systems showed PbTx-3 to have an intermediate extraction ratio, indicating in vivo clearance of PbTx-3 was equally dependent upon liver blood flow and the activity of toxin-metabolizing enzymes. Studies on the effects of varying flow rates and metabolism on the total body clearance of PbTx-3 are planned. Finally, comparison of in vivo metabolism data to those derived from in vitro metabolism in isolated perfused livers and isolated hepatocytes suggested that in vitro systems accurately reflect in vivo metabolic processes and can be used to predict the toxicokinetic parameters of PbTx-3. 

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. 

Dekant W, Schulz A, Metzler M, et al. 1986b. Absorption, elimination and metabolism of trichloroethylene A quantitative comparison between rats and mice. Xenobiotica 16 143-152. 

Callaghan AV, LM Gieg, KG Kropp, JM Suflita, LY Young (2006) Comparison of mechanisms of alkane metabolism under sulfate-reducing conditions among two bacterial isolates and a bacterial consortium. Appl Environ Microbiol 72 4274-4282. 

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