Metabolic activation, organic

The nephron and its related cells perform diverse physiological functions. It is the major organ of excretion and homeostasis for water-soluble molecules because it is a metabolically active organ, it can concentrate certain substances actively. In addition, its cells have the potential to bioconvert chemicals and metabolically activate a variety of compounds. There are a number of other processes described in the following that establish the potential for cellular injury. Specific physiological characteristics are localized to specific cell types. This makes them susceptible to, and the target for, chemicals. 

The bowel, one of the largest and most metabolically active organs, contains bacteria that may change the chemical composition of the human body. In renal failure the altered bacterial flora cause the accumulation of aliphatic amines in the gut (09, S25). Bacteria transform part of the choline in the gut to trimethylamine, which is reabsorbed and then either oxidized or demethylated to dimethylamine in the liver (S24). Dimethylamine enters the circulation and is excreted in the bile and urine. The trimethylamine and dimethylamine in the exhaled air of uremic patients may contribute to the classic fishy breath, which can be improved by hemodialysis or by gut sterilization with nonabsorbable antibiotics (S23, S25). The overall role of these compounds as uremic toxins, however, remains to be defined. 

The nervous system is a complex system that, structurally, has two components the central nervous system and the peripheral nervous system. Between these two there is a complicated network of signals, that are responsible for all the activities, senses, speech, memories and emotions that a human can have. The human brain, being the most metabolically active organ in the body, requires large amounts of energy in order to develop its functions. 

Metabolic rate (M) The rate of transformation of chemical energy into heat and mechanical work by aerobic and anaerobic metabolic activities w ithin an organism, usually expressed per unit area of the total body surface, in met or W m -. 

This discrepancy is due to die fact that other products such as formate, are formed in very small amounts as byproducts of the metabolic routes leading to L-phenylalanine and polymer synthesis. Of course, part of die glucose is also used for die metabolic activities in the micro-organism necessary to maintain the cells in a viable state, this is termed the maintenance energy requirement... 

Cell growth and metabolic activities are similarly described as a simple chemical reaction. It is also necessary to establish a definite formula for dry cell matter. The elemental composition of certain strains of microorganism is defined by an empirical formula CHaO/3Ns. The general biochemical reaction for biomass production is based on consumption of organic substrate, as shown below. Substrate oxidation is simplified in the following biochemical oxidation ... 

The varying metabolic activities of bacteria and their response to immediate environmental factors have been exploited in the design of special diagnostic and selective media. Recipes for these run into many hundreds such media are used in hospital and public health laboratories for identifying organisms found in samples believed to be contaminated by them, and as an aid to diagnosis and treatment. In addition they are used to detect contaminants in pharmaceutical products (British Pharmacopoeia 1993). A few examples will be given to illustrate the principle. 

In media selective for enterobacteria a surface-active agent is the main selector, whereas in staphylococcal medium sodium and lithium chlorides are the selectors staphylococci are tolerant of salt concentrations to around 7.5%. Mannitol salt, Baird-Parker (BP) and Vogel-Johnson (VJ) media are three examples of selective staphyloccocal media. Beside salt concentration the other principles are the use of a selective carbon source, mannitol or sodium pyruvate together with a buffer plus acid-base indicator for visualizing metabolic activity and, by inference, growth. BP medium also contains egg yolk the lecithin (phospholipid) in this is hydrolysed by staphylococcal (esterase) activity so that organisms are surrounded by a cleared zone in the otherwise opaque medium. The United States Pharmacopeia (1990) includes a test for staphylococci in pharmaceutical products, whereas the British Pharmacopoeia (1993) does not. 

Organisms in natural ecosystems may not be actively dividing but may, nonetheless, be metabolically active. This may be particularly important for ultramicro marine bacteria in their natural habitat. 

For many exogenous organisms, many of these issues will already have been established, so that only complementation will be required for indigenous organisms determination of their presence, number, and metabolic activity by conventional laboratory procedures. It may be valuable to set up enrichment cultures using site material to provide suitable material for laboratory and microcosm studies. 

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