Some Metabolic Aspects

Whereas we have no intention to describe in this review the various aspects of halobacterial metabolism, we would like to mention several unique features of their metabolic system. The conversions of the two 2-oxoacids (pyruvate and oxoglutarate) to their corresponding acyl-CoA thioesters are crucial steps in the two pathways described above. In most eukaryotes and aerobic eubacteria these reactions are catalyzed by the 2-oxoacid dehydrogenase multienzyme complexes that use NAD+ as the final electron acceptor. These complexes are 

The molar masses of the 2-oxoacid ferredoxin oxidoreductases are 200,000-300,000 g/mol and they are composed of four subunits of the kind a2p2. It has been shown that halobacteria have only these systems of 2-oxoacid ferredoxin oxidoreductases. The two enzymes of H. halobium (pyruvate and oxoglutarate) were isolated and characterized by Kerscher and Oesterhelt (1981a). These systems proved to be thiamin diphosphate-containing iron-sulfur proteins. The relative stability of the halobacterial enzymes enabled detailed analysis of the various steps of the catalytic cycles (Kerscher and Oesterhelt, 1981b), demonstrating two distinct steps of one-electron transfer reactions. 

Whereas the electron acceptors in the anaerobic organisms are the bacterial-type ferredoxins that contain [4Fe-S] clusters as the redox center, in the case of the halobacteria the electrons are transferred to [2Fe-S] ferredoxins. These ferredoxins were isolated from two different halobacteria and their amino acid sequences were determined (Hase et al., 1977, 1980) and shown to be highly homologous to the chloroplast (and cyanobacterial) ferredoxins. The implications of these perplexing findings for the question of the molecular evolution of the system is discussed in detail in Kerscher and Oesterhelt (1982). 

The absence in halobacteria of the oxoacid dehydrogenase complexes creates another puzzle. In most known systems, the role of the enzyme lipoamide dehydrogenase is to reoxidize the lipoic acid that is involved in the oxidation of the oxoacids in the oxoacid dehydrogenase complexes. This enzyme was nonetheless found in H. halobium and purified to homogeneity by Danson et al. (1986). What, then, is its function It is likely that lipoamide dehydrogenase assumes a different role in halobacteria. Another reducing system unique to 

Another unique property of at least some of the halobacteria is the ability to grow phototrophically by employing the light-driven proton pump bacteriorhodopsin. The proton gradient that is produced is used directly to generate ATP (Hartmann et al., 1980 Oesterhelt and Kripphal, 1983). Photoassimilation of CO2 by halobacteria was shown by Danon and Caplan (1977) and Oren (1983). In vivo C02 fixation was demonstrated by Javor (1988) and the existence of the enzyme ribulose-bisphosphate carboxylase activity in several halobacteria was shown by Altekar and Rajagopalan (1990). 

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Diaz-Mayans J, Laborda R, Nunez A. 1986. Hexavalent chromium effects on motor activity and some metabolic aspects of Wistar albino rats. Comp Biochem Physiol 83C(1) 191-195. 

Peters, R.A. (1972). Some metabolic aspects of fluoroacetate especially related to fluorocitrate. In Carbon Fluorine Compounds. A Ciba Foundation Symposium, pp. 55-70. Associated Scientific Publishers., Amsterdam. 

Berry LJ, Smythe DS. 1965. Some metabolic aspects of tolerance to bacterial endotoxin. J. Bacteriol. 90 970-77... 

In-vitro models can provide preliminary insights into some pharmacodynamic aspects. For example, cultured Caco 2 cell lines (derived from a human colorectal carcinoma) may be used to simulate intestinal absorption behaviour, while cultured hepatic cell lines are available for metabolic studies. However, a comprehensive understanding of the pharmacokinetic effects vfill require the use of in-vivo animal studies, where the drug levels in various tissues can be measured after different dosages and time intervals. Radioactively labelled drugs (carbon-14) may be used to facilitate detection. Animal model studies of human biopharmaceutical products may be compromised by immune responses that would not be expected when actually treating human subjects. 

Miller JA, Miller EC (1983) Some historical aspects of N-aryl carcinogens and their metabolic activation. Env Health Persp 49 3-12... 

Schoor, W.P. 1984. Benzo(a)pyrene metabolism in marine fish and some analytical aspects of its metabolites. Pages 391-396 in K.L. Hoover (ed.). Use of Small Fish in Carcinogenicity Testing. Natl. Cancer Inst. Mono. 65. Bethesda, MD. 

Nonetheless, the topological and stoichiometric analysis of metabolic networks is probably the most powerful computational approach to large-scale metabolic networks that is currently available. Stoichiometric analysis draws upon extensive work on the structure of complex reaction systems in physical chemistry in the 1970s and 1980s [59], and can be considered as one of the few theoretically mature areas of Systems Biology. While the variety and amount of applications of stoichiometric analysis prohibit any comprehensive summary, we briefly address some essential aspects in the following. 

This chapter lays down the conceptual framework for understanding the quantitative and temporal aspects of drug-drug interactions, hereafter called drug interactions for simplicity. Emphasis is placed primarily on the pharmacokinetic aspects, partly because pharmacokinetic interactions are the most common cause of undesirable and, to date, unpredictable interactions and also because most of this book is devoted almost exclusively to this aspect and indeed to one of its major components, drug metabolism. Some pharmacodynamic aspects are also covered, however, for there are many similarities between pharmacokinetic and pharmacodynamic interactions at the molecular level and because ultimately one has to place a pharmacokinetic interaction into a pharmacodynamic perspective to appreciate the likely therapeutic impact (1-5). 

This chapter will discuss some blopharmaceutical aspects of enzyme absorption, although most studies have been performed using other proteins or peptides. Small molecules are predominantly metabolized in the liver, but proteins and peptides are degraded by enzymes present in most compartments of the body. 

Due to the complexity of the metabolism, investigation of metabolic networks necessitates powerful analytical methods, both experimentally and mathematically. Figure 11 illustrates some of the metabolic aspects, e.g., metabolic channeling and reversible reactions, that complicate the analysis of metabolic networks. 

Peck, H.D., 1966. Some Evolutionary Aspects of Inorganic Sulfur Metabolism. Lectures on theoretical and applied aspects of modern microbiology. University of Maryland,... 

Nomenclature is also a subject of confusion, although of much lesser importance than the metabolic aspects discussed in the previous paragraph. As examples of potential confusion some terms used in Europe are briefly discussed. A similar confrontation of definitions could also be made for other countries. The nomenclature adopted in 1965 (18)by the European Communities presents no problem for prodrugs ... 

The Vinca alkaloids metabolism and transport in the producing plant cells and in the treated animal cells illustrate some interesting aspects of how evolution can be winding and parsimonious in the solutions it creates. 

The kinetics of enzyme-catalyzed reactions will be discussed followed by a listing of the different systems of importance in the metabolism of exogenous organic compounds. The objective will be to develop a background sufficient to project the metabolites one might expect to form from an organic compound. Some unique aspects of plant systems will be discussed followed by an analysis of degradation processes in soil. 

The above experiments indicate that the intestinal microflora is an important variable in qualitative and quantitative bile acid metabolism. In the following sections, some specific aspects of this interaction will be examined in detail. 

Both metabolism sensors and affinity sensors can be designed using electrochemical transducers for the signal generation, as will be discussed in detail below. However, before evaluating specific features of electrochemical biosensors, some general aspects of the qualification of biosensor properties have to be discussed. 

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