Extracellular peroxides, activation

The peroxides that activate these enzymes are produced internally by th biota or, alternatively, are produced externally, mainly by photochemical proc esses in the sea (72). Gschwend et al. (71), having found very little halocarboi in the interior tissues of macroalgae that produce these compounds, conclude" that much of the activity must be located in the surface tissues. This findin is consistent with the idea that extracellular peroxides are involved in th activation of these haloperoxidases. Moffett and Zafiriou (73) and Cooper am Zepp (74) provided evidence that peroxidases associated with aquatic particle in fresh water and coastal waters account for a large fraction of the decay o hydrogen peroxide, although it was not shown that haloperoxidases were re sponsible for the observed activity. 

Choi H.W. Kim Y.J. Lee S.C. Hong J.K. Hwang B.K. (2007) Hydrogen peroxide generation by the pepper extracellular peroxidase CaP02 activates local and systemic cell death and defense response to bacterial pathogens / / Plant Physiology. V. 145. P. 890-904. 

Extracellular peroxidases are produced by Streptomyces chromofuscus, with the capability to decolorize azo dyes associated to ligninolytic activity in aerobiosis. Azo dyes are converted to cationic radicals, which are subjected to nucleophilic attack by water or hydrogen peroxide molecules, producing reactive compounds that undergo redox reactions that result in a more stable intermediate [37]. 

Lignin peroxidase activity, (i.e., peroxide-dependent oxidation of veratryl alcohol at pH 3) was not detected over the 30 days tested, while laccase appeared at day 7. Culture medium from day 7 onwards could also oxidize veratryl alcohol to aldehyde with concomitant conversion of oxygen to hydrogen peroxide. This activity, which was optimal at pH 5.0, was named veratryl alcohol oxidase (VAO). The extracellular oxidative enzyme activities (laccase and veratryl alcohol oxidase) could be separated by ion-exchange chromatography (Figure 2). Further chromatography of the coincident laccase and veratryl alcohol oxidase (peak 2), as described elsewhere (25) resulted in the separation of two veratryl alcohol oxidases from the laccase. 

The structural integrity of the cell membrane is irreversibly damaged by the process of membrane lipid peroxidation. The damaged membrane becomes leaky and extracellular calcium enters the cell. This in turn activates calcium-dependent phospholipases and protein kinases, subsequently leading to fatty acid cleavage and other biochemical alterations within the cell. Ultimately this leads to damage or death of the cell. 

Pepicelli O., Fedele E., Bonanno G., Raiteri M., Ajmone-Cat M. A., Greco A., Levi G., and Minghetti L. (2002). In vivo activation of A -methyl-D-aspartate receptors in the rat hippocampus increases prostaglandin E2 extracellular levels and triggers lipid peroxidation through cyclooxygenase-mediated mechanisms. J. Neurochem. 81 1028-1034. 

Perhaps the main peroxide-induced alterations, within cells and tissues, are those that affect calcium and sodium homeostasis. Na, K-ATPase, which is considered as the core of the sodium pump , is strongly affected by peroxides, and especially by lipid hydroperoxides [137-139]. This implies that oxidative stress will usually be associated with cellular edema . Alternatively, activation of the Na, K-ATPase of vascular endothelia, such as the blood-brain barrier, will result in extracellular edema on the antiluminal side of the endothelium, due to massive influx of sodium ions [119]. 

Calcium homeostasis is also affected by an overload in peroxides. In some instances, this may be due to activation of so-called calcium channels. In the presence of extracellular calcium, exposure of smooth muscle cells to 0.3 mM H2O2 was shown to induce a rapid increase in intracellular calcium concentration, followed by a decrease to a new constant level approximately twice higher than the initial one [140]. Subsequent treatment of the cells with Ca2+-channel blockers, with disulfide-reducing agents or with antioxidants such as trolox, prevented the stabilization of intracellular calcium at the high steady-state concentration. These results suggest that an increased disulfide/thiol ratio activates voltage-dependent calcium channels of the outer cell membrane. 

Cell culture studies have shown that aldehydic products derived from ethanol metabolism and lipid peroxidation can increase collagen mRNA levels and enhance the expression of connective tissue proteins. Acetaldehyde is able to increase the production of several extracellular matrix components. Studies also show that hepatic stellate cells, which are the primary source of extracellular matrix, become readily activated under conditions involving enhanced oxidative stress and lipid peroxidation. 

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