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. 

Unwanted radicals in biological systems must be destroyed before they have an opportunity to cause damage to cells. Cell membranes, for example, are susceptible to the same kind of radical reactions that cause butter to become rancid (Section 26.3). Imagine the state of your cell membranes if radical reactions could occur readily. Radical reactions in biological systems also have been implicated in the aging process. Unwanted radical reactions are prevented by radical inhibitors—compounds that destroy reactive radicals by creating unreactive radicals or compounds with only paired electrons. Hydroquinone is an example of a radical inhibitor. When hydroquinone traps a radical, it forms semiquinone, which is stabilized by electron delocalization and is, therefore, less reactive than other radicals. Furthermore, semiquinone can trap another radical and form quinone, a compound whose electrons are all paired. 

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. 

Spin trapping is an often-used technique in the study of possible radical production in biological systems (for reviews see Kalyanaraman, 1982 Mason, 1984 Mottley and Mason, 1989), particularly by the detection and monitoring of spin adducts of the hydroxyl and hydroperoxyl ( OOH) radicals in view of their relation to possible damage mechanisms. This is a large area of research which it is not possible to cover in a limited review, and the treatment will therefore be restricted to a discussion of the electron transfer properties of biochemical systems (for a review on the application of the Marcus theory to reactions between xenobiotics and redox proteins, see Eberson, 1985) and... 

Abstraction reactions in biological systems, in particular the site of radical attack in proteins (oxidative damage), continue to be a matter of great interest, primarily owing... 

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. 

Common error alert The hydroxyl radical ( OH) is very high in energy. Although it is involved in certain very important reactions in biological systems and in atmospheric chemistry, its intermediacy in a synthetic reaction should be viewed with skepticism. 

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. ... 

By far the greatest yield of chemiluminescent reactions in biological systems is produced by oxidation reactions involving the chemical species of oxygen, O2, H2O2, 02, HO and HO. It is possible that all chemiluminescent emissions with the exception of dissociation reactions are the result of precursor radical recombination reactions ... 

Reaction 28 is too slow to be Important in biological systems (43b). However, we have recently shown that the reaction of the superoxide anion radical with organic hydroperoxides, eq i, is fast (45b). Since PUFA forms hydroperoxides in vivo both by an enzymatic path and by autoxidation, it appears that reaction i could be responsible for the HO radicals observed 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 ... 

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