Cell-free systems conjugation reactions

As can be seen from those simple examples, chemical monitoring of VOCs based on determination of their trace amounts (from parts per million [ppm] to parts per trillion [ppt]) with the use of modem analytical methods is necessary. It is a consequence of the ease with which VOCs permeate biological barriers (air-circulatory system, the internal and external activities of both plant and animal cells), and the relative ease with which they undergo conjugation reactions (enzymatic oxidation reactions, auto-oxidation, initiation, propagation, and termination). Therefore, new metabolites are created. These metabolites not only determine phenomena connected with the chemistry of the atmosphere but also are responsible for the vital functions of organisms (e.g., the uncontrolled de novo reaction that occurs in the presence of free radicals) (Fig. 14.2) [4]. 

Lipid peroxidation is probably the most studied oxidative process in biological systems. At present, Medline cites about 30,000 publications on lipid peroxidation, but the total number of studies must be much more because Medline does not include publications before 1970. Most of the earlier studies are in vitro studies, in which lipid peroxidation is carried out in lipid suspensions, cellular organelles (mitochondria and microsomes), or cells and initiated by simple chemical free radical-produced systems (the Fenton reaction, ferrous ions + ascorbate, carbon tetrachloride, etc). In these in vitro experiments reaction products (mainly, malon-dialdehyde (MDA), lipid hydroperoxides, and diene conjugates) were analyzed by physicochemical methods (optical spectroscopy and later on, HPLC and EPR spectroscopies). These studies gave the important information concerning the mechanism of lipid peroxidation, the structures of reaction products, etc. 

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