Associative mechanism metabolism

The first involves various feedback and feed forward control mechanisms associated with metabolic pathways. Here a chemical present in the cell, usually an end product of a metabolic sequence, will influence the activity of an enzyme at the beginning of the pathway. This is usually achieved by the presence of an allosteric enzyme whose properties are significantly changed by effector molecules (Fig. 5.31). 

The two alternative strategies of metabolism explored in active and sluggish fish therefore apply both to the divergence of energy metabolism and to the closely associated plastic metabolism, which provides normal functioning of energy transformation mechanisms. Active fish are characterized by more... 

RIO. Rossi, P., Romeo, D., and Patriarca, P., Mechanism of phagocytosis-associated oxidative metabolism in polymorphonuclear leukocytes and macrophages. J. Reticuloen-doth. Soc. 12, 127-149 (1972). 

Zeevalk, G.D. and Nicklas, W.J. (1991) Mechanisms underlying initiation of excitotoxicity associated with metabolic inhibition. J. Pharmacol. Exp. Ther. 257 870-878. 

Transport across cell membrane Due to chemical similarities between certain heavy metal and macronutrients (e.g. ionic charge, ionic radius) the transport across microbial cell can occur using the mechanism used to convey metabohcally important ions, e.g. K, Mg and Na. This mechanism is not associated with metabolic activity. 

Allopurinol (Zyloprim) reduces the production of uric acid, thus decreasing serum uric acid levels and the deposit of urate crystals in joints. The exact mechanism of action of colchicine is unknown, but it does reduce the inflammation associated with the deposit of urate crystals in the joints. This probably accounts for its ability to relieve the severe pain of acute gout. Colchicine has no effect on uric acid metabolism. 

The microsomal ethanol oxidizing system is another mechanism of ethanol metabolism. CYP2E1 may be an important enzyme in the metabolism of ethanol in heavy drinkers, who may have a 10-fold increase in activity. Two aUehc variants in the gene cl and c2) are associated with differing enzymatic activity. Approximately 40% of Japanese have the more active c2 allele, which is rare in individuals of European heritage (Sun et al. 2002). It is not believed to be a risk or protective factor in the development of alcohohsm, although current studies are examining its relationship to a variety of ethanol-related diseases. 

Senaratna, T. McKenzie, B.D. (1986). Loss of desiccation tolerance during seed germination A free radical mechanism of injury. In Membranes, Metabolism and Dry Organisms, ed. A.C. Leopold, pp. 85-101. Ithaca, N.Y. Comstock Publishing Associates. 

Resistance mechanisms associated with changes in toxicokinetics are predominately cases of enhanced metabolic detoxication. With readily biodegradable insecticides such as pyrethroids and carbamates, enhanced detoxication by P450-based monooxygenase is a common resistance mechanism (see Table 4.3). 

A cell subjected to a stress of any kind can potentially exhibit a wide range of responses. Severe stress may lead to cell death and, ultimately, to cell lysis imposition of less severe conditions may result in a metabolically perturbed system, which may either revert to its initial state or adapt in some way to the imposed conditions. Figure 10 shows a hypothetical scheme, presented by Prokop and Bajpai [12], for the signal-response cascade associated with hydrodynamic shear stress. The signal reception/transduction mechanisms are, as yet, poorly understood. While Fig. 10 can be applied to any biological system, Namdev and... 

A number of genetic diseases that result in defects of tryptophan metabolism are associated with the development of pellagra despite an apparently adequate intake of both tryptophan and niacin. Hartnup disease is a rare genetic condition in which there is a defect of the membrane transport mechanism for tryptophan, resulting in large losses due to intestinal malabsorption and failure of the renal resorption mechanism. In carcinoid syndrome there is metastasis of a primary liver tumor of enterochromaffin cells which synthesize 5-hydroxy-tryptamine. Overproduction of 5-hydroxytryptamine may account for as much as 60% of the body s tryptophan metabolism, causing pellagra because of the diversion away from NAD synthesis. 

By interfering with any one of the many phases associated with these second messenger pathways, toxins may alter channel gating. For example, the blue green algal toxins, aplysiatoxin, and lyngbyatoxin bind to and activate protein kinase C in a manner similar to phorbol esters (73). They also stimulate arachidonic acid metabolism (74). The coral toxin, palytoxin, also stimulates arachidonic acid breakdown albeit by an unknown mechanism (74) and affects other biochemical activities of the cell (see chapters by Fujiki et al., Wattenberg et al., and Levine et al., this volume). 

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