Free-radical reactions in biological systems

Pryor, W. A. (1976). The role of free radical reactions in biological systems. In Pryor, W. A. (ed.), Free Radicals in Biology, Vol. 1. Academic Press, New York. 

Pruell RJ, Lake JL, Davis WR, Quinn JG (1986) Uptake and depuration of organic contaminants by blue mussels Mytilus edulis) exposed to environmentally contaminated sediment. Mar Biol 91 497-507 Pryor W A (1976) The role of free radical reactions in biological systems. In Pryor WA (ed) Free radicals in biology, Vol 1. Academic Press, New York, pp 1-49 Quattrochi LC, Lee RF (1984a) Microsomal cytochromes P-450 from marine crabs. Comp Biochem Physiol 79C 171-176... 

This chapter considers the nutritional consequences of polyunsaturated lipids in the diet on free radical reactions in biological systems and diseases. The term lipid peroxidation is now used broadly to include both non-enzymatic and enzymatic oxidative reactions of free fatty acids, phospholipids, triacylglycerols, cholesterol and cholesteryl esters, lipoproteins, proteins,... 

Leibovitz, B., and Siegel, B. V. 980) Aspects of free radical reactions in biological systems aging, J, Gerontology 35 45-56. 

Substantial LPO occurs only when cellular defense mechanisms have been weakened or overcome by prolonged oxidative stress. Hence understanding of the balance between free radical generation and antioxidant defense systems is critical to the understanding and control of free radical reactions in biology and medicine. ... 

These studies show that Cu(III)-peptide complexes have relatively low electrode potentials and suggest that Cu(III) may be a far more common oxidation state than had previously been thought possible. Furthermore, the decomposition reactions of Cu(III)-peptides indicate that two-electron transitions to give Cu(I) species are possible. Two-electron redox reactions in biological systems are intriguing because high energy, free radical intermediates are avoided. However, as yet we know very little about possible Cu(I) complexes. This oxidation state is poorly characterized in aqueous solution, and studies with various model complexes are needed. 

The foundations for the edifice had been laid when I compiled my review on inorganic free radicals in solution ten years ago (23), in the sense that the basic concepts had been realized and kinetic studies based on sound energetics offered very great scope for further investigation in many fields polymerization and autoxidation reactions, photochemistry and radiation chemistry of aqueous systems, and even reactions in biological systems. 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

It is unfortunate that typical concentrations of free-radical species present in biological systems are only at the limit of e.s.r. detection sensitivity and, of course, there are major technical difficulties in studying whole animals in this manner. Therefore, the most successful e.s.r. experiments have adopted the approach of spin trapping in which very reactive and thus transient radical species are converted to long-lived adducts via reaction with a trap such as a nitrone, e.g. Equation 1.1 ... 

One of the important possible mechanisms of MF action on biological systems is the influence of free radical production. Chemical studies predict that MFs may affect free radical reactions through the radical pair mechanism [201]. A reaction between two free radicals can generate a free radical pair in the triplet state with parallel electron spins. In this state free radicals cannot recombine. However, if one of the electrons overturns its spin, then free radicals can react with one another to form a diamagnetic product. Such electron spin transition may be induced by an alternative MF. 

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. 

These authors supposed that the repairing function of antioxidants may be even more important compared to scavenging reactive free radicals. However, although Reactions (3) and (4) may occur in biological systems, it is very difficult to estimate their importance. First of all, there is always a competition between the repairing Reaction (4) and the reaction of the biomolecule free radical R with dioxygen (Reaction (5)) ... 

Both vitamin E and vitamin C are able to react with peroxynitrite and suppress its toxic effects in biological systems. For example, it has been shown [83] that peroxynitrite efficiently oxidized both mitochondrial and synaptosomal a-tocopherol. Ascorbate protected against peroxynitrite-induced oxidation reactions by the interaction with free radicals formed in these reactions [84]. 

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