One of the characteristic reactions of lipids that are exposed to oxygen is the formation of peroxides. Indeed, among non-enzymic chemical reactions taking place in the environment at ambient temperatures, the oxidation of unsaturated compounds is, perhaps, the most important both from an industrial and a medical point of view. In biological tissues, uncontrolled lipid peroxidation causes membrane destruction and is increasingly regarded as an important event in the control or development of diseases (Sections 4.3 and 5.4.1). In food, oxidation (either enzymically or chemically catalysed)
can have desirable as well as adverse consequences (Sections 4.2.5 and 4.3).
In common with other radical chain reactions, lipid peroxidation can be divided into three separate processes - initiation, propagation and termination. During initiation a very small number of radicals (e.g. transition metal ions or a radical generated by photolysis or high-energy irradiation) allow the production of R • from a substrate RH:
Propagation then allows a reaction with molecular oxygen:
and this peroxide radical can then react with the original substrate:
Thus, the events form the basis of a chain-reaction process.
Free radicals such as ROO' (and RO', OH', etc., which can be formed by additional side-reactions) can react at random by hydrogen abstraction and a variety of addition reactions to damage proteins, other lipids and vitamins (particularly vitamin A). Compounds that react rapidly with free radicals can be useful in slowing peroxidation damage. Thus, naturally occurring compounds such as vitamin E are powerful anti-oxidants and tissues deficient in such compounds may be prone to peroxidation damage. Formation of lipid hydroperoxides can be readily detected by a number of methods of which the absorption of conjugated hydroperoxides at 235 nm is particularly useful.
Termination reactions may lead to the formation of both high and low molecular mass products of the peroxidation reactions. Depending on the lipid, some of the low molecular mass compounds may be important flavours (or aromas) of foods (Section 4.2.5). For example, short- to medium-chain aldehydes formed from unsaturated fatty acids may give rise to rancidity and bitter flavours on the one hand or more pleasant attributes such as those associated with fresh green leaves, oranges or cucumbers on the other hand. Fish odours are attributed to a ketone. Some relevant changes in quality or nutritional value of food are indicated in Chapter 4 (Fig. 4.11).
In general, it is considered desirable to reduce the initiation reaction as a means of controlling peroxidation. Apart from natural anti-oxidants, BHT (3,5-di-i-butyl-4-hydroxytoluene) is often used as a food additive. Metal binding compounds and phenolic compounds may also be inhibitory as well as the endogenous superoxide dismutase and glucose oxidase-catalase enzyme systems.
2.3.5 Peroxidation catalysed by lipoxygenase enzymes
The second kind of peroxidation is catalysed by the enzyme lipoxidase (or lipoxygenase). The enzyme was originally thought to be present only in plants, but it has now been realized that it catalyses very important reactions in animals (Section 2.4.6). The chief sources of the enzyme are peas and beans (especially soybean), cereal grains and oil seeds. It was originally detected by its oxidation of carotene and has been used extensively in the baking industry for bleaching carotenoids in dough.
All known lipoxygenases catalyse the following reaction:
That is, they catalyse the addition of molecular oxygen to a 1,4-cis,cis-pentadiene moiety to produce a 1-hydroperoxy-2,4-irans,cis-pentadiene unit.
When Theorell and his colleagues in Sweden first purified and crystallized soybean lipoxygenase in 1947, they reported that it had no prosthetic group or heavy metal associated with it. Such a situation would make lipoxygenase unique among oxidation enzymes. However, Chan in England and Roza and Franke in The Netherlands demonstrated the presence of one atom of iron per mole of enzyme by atomic absorption spectroscopy. The product of the enzymic reaction - a hydroperoxide - is similar to the products of purely chemical catalysis, but the lipoxidase reaction has a number of distinguishing features. The activation energy is smaller than that for chemical reactions, and the enzyme has very specific substrate requirements. In order to be a substrate, the fatty acid must contain at least two cis double bonds interrupted by a methylene group. Thus, linoleic and a-linolenic acids are good substrates for the plant enzymes while arachidonic acid, the major polyunsaturated fatty acid in mammals, is attacked by different lipoxygenases in their tissues. Like chemically catalysed peroxidation, the lipoxidase reaction involves free radicals and can be inhibited by radical trapping reagents such as the tocopherols. The reaction sequence shown in Fig. 2.35 represents the currently accepted pathway.
Lipoxygenases are widespread in plants. One effect of the enzymes in plant tissues is to yield
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