Physiological cascade reactions

Nucleotides participate in reactions that fulfill physiologic functions as diverse as protein synthesis, nucleic acid synthesis, regulatory cascades, and signal transduction pathways. 

Generation of superoxide radical under physiological conditions ultimately leads to the production of hydroxyl radical through a cascade of redox reactions. Initially, sn-peroxide disproportionates to generate hydrogen peroxide (Eq. 3, Scheme 8.36) Superoxide radical exists in equilibrium with its protonated form (H02, = 5). 

Fig. 2.3. Schematic representation of consecutive zymogen activation reactions (cascade). Following initiation by a physiological signal ( ), the zymogens X, Y, and Z are sequentially activated. The resulting peptidases (Xa and Ya) are inactivated by specific inhibitors (Ix and Iy) to limit their action (modified from [20a]).

Enzymic lipid peroxidation reactions are a common reaction in mammalian cells as response to stress from outside. In contrast to plants mammalian cell membranes contain arachidonic acid instead of linolenic acid. The latter is known to generate by LPO processes physiological potent compounds prostaglandines, prostacyclines, thromboxanes and leukotiienes [221]. In addition a great number of other LPO products derived not only from arachidonic acid but also from linoleic acid have been detected in blood, especially low density lipoprotein (LDL) and tissue samples of humans [222-227]. These are increased especially in diseases combined with cell degradation [228], indicating that cell death in plant and mammalian cells is connected with a similar cascade of events. 

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