Lipid hydroperoxides autoxidation

As a reasonable biogenetie pathway for the enzymatic conversion of the polyunsaturated fatty acid 3 into the bicyclic peroxide 4, the free radical mechanism in Equation 3 was postulated 9). That such a free radical process is a viable mechanism has been indicated by model studies in which prostaglandin-like products were obtained from the autoxidation of methyl linolenate 10> and from the treatment of unsaturated lipid hydroperoxides with free radical initiators U). 

When fats are heated, autoxidation occurs and lipid hydroperoxides are produced (64,68,69). 

It is interesting to consider the concentrations of free radicals that result from lipid hydroperoxides in an in vitro model system. For example, my group has been studying the autoxidation of linoleic acid in SDS micelles at 37°C. We initiate the autoxidation by the decomposition of an initiator, as shown in Equation 3. 

Upon exposure to air, animal and vegetable fats and oils become rancid (i.e., develop color changes and a musty, rank taste and odor). Here, the hydrogen atoms of the —CH2—groups located between alternating double bonds (i.e., —CH=CH—CH2—CH=CH—) of a polyunsaturated phospholipid or fatty acid (LH) are very susceptible to abstraction by free radicals. This process can then lead to a general reaction known as autoxidation, which results in the formation of a lipid hydroperoxide (LOOH) and the generation of a new free radical hence, an autocatalytic reaction results (lipid peroxidation). 

Hydroperoxides have geometric as well as positional isomers on lipid chains. When the hydrogen is abstracted at an allylic carbon, the double bond shifts one carbon to a position (3 to the abstraction site, and it reforms in the trans rather than cis configuration. The fran5,c/i-conJugated diene structure is retained whether oxygen adds or not, and provides the first detectable intermediate in lipids during autoxidation (238). 

As oxidation normally proceeds very slowly at the initial stage, the time to reach a sudden increase in oxidation rate is referred to as the induction period (6). Lipid hydroperoxides have been identified as primary products of autoxidation decomposition of hydroperoxides yields aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, known as secondary oxidation products. These compounds, together with free radicals, constitute the bases for measurement of oxidative deterioration of food lipids. This chapter aims to explore current methods for measuring lipid oxidation in food lipids. 

Undesirable changes in nutritional quality of foods are initiated by the autoxidation or enzymic oxidation of unsaturated lipids to lipid hydroperoxides. Lipid hydroperoxides and their products of decomposition can react with food components, such as amino acids, proteins and certain other biochemicals. These reactions and the potential role of hydroperoxides in causing mutagenicity are reviewed. 

The susceptibility of lipids to autoxidation thus depends on the availability of ally lie hydrogens and their relative ease to react with peroxyl radicals to form hydroperoxides. A hybrid radical (A) is formed, with a partial free radical (5 ) at each end of the allylic system (8). Oxygen attack at each end of the allylic system produces a mixture of allylic 1- and 3-hydroperoxides (10). 

Chan, H.W.-S., and Coxon, D.T. Lipid hydroperoxides, in Autoxidation of Unsaturated Lipids, pp. 17-50 (1987) (edited by H.W.-S. Chan), Academic Press, London. 

Hydroperoxides are formed during processing, transportation and storage of fats and oils (Paulose and Chang, 1973 Billek et al, 1978). The formation of hydroperoxide as primary oxidation products may affect oxidation of lipids via autoxidation, enzymatic and photosensitized oxidation (Labuza, 1971). 

Finley, J.W. and Lundin, R.E. (1980). Lipid hydroperoxide induced oxidation of cysteine in peptides, in Autoxidation in food and biological systems, p. 223, M.G. Simic and M. Karel, eds. Plenum Press, New York... 

Lipid derived radicals (L, LO-, and LOO ) are produced during the initiation and propagation stages of the autoxidation of unsaturated lipids. The source of the primary radical in lipid autoxidation is still an open question. However, the reaction may be initiated by cleavage of preformed lipid hydroperoxides (LOOH) by transition metal ions, i ch form lipid alkoxy 1 radicals (LO ), or by metal-catalyzed cleavage ofhydrogen peroxide (HjOj), which forms hydroiQ l radicals ( OH, Fenton... 

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