Extracellular enzymatic systems

The role of extracellular enzymatic systems in the degradation of triiodinated aromatics compounds was demonstrated by the same authors. Degradation yields between 87% and 93% for diatrizoate, iodipamide, and acetrizoate, and between 68% and 73% for aminotrizoate and aminotriiodoisophthalate were observed in in vitro experiments with extracellular enzyme concentrate of T. versicolor in the... 

GSHPx, CAT and SOD, which normally protect cells from free-radical damage have not been detected in aqueous humour. It has therefore been su ested that damage by free radicals and hydrogen peroxide to the anterior segment is prevented by a non-enzymatic extracellular oxidoreduction system involving a constant supply of reduced glutathione to the aqueous fluid from the ciliary epithelium, cornea and lens (Riley, 1983). 

WRF presents an intracellular enzymatic system, cytochrome P450 monooxigenases, similar to those mammalian cells, that catalyzes a broad range of intracellular degradation reactions of released metabolites after the pollutants breaking by extracellular enzymes. 

One of the major features of solid tumors and even small deposits of tumor tissue is deficiency in the level of oxygen, because of an inadequate vascular supply. The adenosine elevation in response to hypoxia is not exclusive to tumor tissues, but, in this context, the adenosine elevation is localized to the tumor microenvironment, since the surrounding tissue is normally oxygenated. Adenosine is generated mainly by two enzymatic systems intra- or extracellularly localized 5 -nucleoti-dases and cytoplasmic S-adenosylhomocysteine hydrolase. The processes of adenosine elimination in the cell involve reactions catalyzed by adenosine deaminase and adenosine kinase (Shryock and Belardinelli 1997) yielding inosine or 5 -AMP,... 

By virtue of their size and charge, peptide molecules are not the ideal candidates for transfer into the systemic circulation following instillation in the nose. Among the many barriers to absorption that must be overcome are mucociliary clearance, extracellular enzymatic destruction, the lipophilic bilayer membrane of nasal epithelial cells, the potential for nasal epithelial cells to degrade any peptide molecules that cross the lipid bilayer, and the potential to establish futile cycles of endocytosis and exocytosis on the apical surface of polarized epithelial cells. Indeed, in the face of these multiple barriers, it seems all the more remarkable that any substantial absorption of peptide drugs from the nose has ever been observed. Despite these barriers, recent... 

The precise relationship between potential hydrolysis rates measured with externally added substrates and the rates at which complex microbial communities in marine systems hydrolyze and ultimately remineralize a spectrum of organic macromolecules actually available as substrates is unknown. Extracellular enzymatic hydrolysis is frequently regarded as the rate-limiting step in remineralization of organic carbon (e.g., King, 1986 ... 

A different picture emerges when formation of solid-state water in extracellular spaces is examined. Here, ice formation commonly occurs. Because extracellular fluids lack the complex membrane systems found within the cell, the potential for physical disruption of structures is much less than in cells. The tolerance of extracellular protein systems to increased solute concentration may also be greater than those of typical intracellular proteins, whose coordinated enzymatic functions generally are very sensitive to solute composition and concentration (chapter 6). 

Often extracellular enzymes in natural waters are induced by a low concentration of a necessary substrate. For example, some deaminases and phosphatases are induced in phytoplankton when inorganic nitrogen and phosphorus are scarce, so as to use organic nutrient sources. Further, the enzymatic system may be made more efficient if the substrate itself (e.g., the organic nutrient) becomes low. 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

Finally, Chap. 6 deals with the exploitation of biocatalysis in generating supramolecular polymers, a class of polymers where the monomers are connected via non-covalent bonds. This approach provides highly dynamic and reversible supramolecular structures, inspired by biological polymeric systems found in the intra- and extracellular space. A number of potential applications of enzymatic supramoleular polymerizations are discussed in the context of biomedicine and nanotechnology. 

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