Polyenoates—

Polyunsaturated (polyethenoid, polyenoic) acids, containing two or more double bonds. 

Eicosanoids These compounds, derived from eicosa- (20-carbon) polyenoic fatty acids, comprise the prostanoids, leukotrienes (LTs), and lipoxins (LXs). Prostanoids include prostaglandins (PGs), prostacyclins (PGIs), and thromboxanes (TXs). 

Certain long-chain unsaturated fatty acids of metabolic significance in mammals are shown in Figure 23-1. Other C20, C22, and C24 polyenoic fatty acids may be derived from oleic, linoleic, and a-flnolenic acids by chain elongation. Palmitoleic and oleic acids are not essential in the diet because the tissues can introduce a double bond at the position of a saturated fatty acid. 

Fatty acids of plant, animal, and microbial origin usually consist of an even number of carbon atoms in the straight chain. The number of carbon atoms of fatty adds in animals may vary from 2 to 36, whereas some microorganisms may contain 80 or more carbon atoms. Also, fatty adds of animal origin may have one to six ds double bonds, whereas those of higher plants rarely have more than three double bonds. Fatty adds also may be saturated, monounsaturated (monoenoic), or polyunsaturated (polyenoic) in nature. Some fatty acids may consist of branched chains, or they may have an oxygenated or cyclic structure. 

Schnurr et al. [22] showed that rabbit 15-LOX oxidized beef heart submitochondrial particles to form phospholipid-bound hydroperoxy- and keto-polyenoic fatty acids and induced the oxidative modification of membrane proteins. It was also found that the total oxygen uptake significantly exceeded the formation of oxygenated polyenoic acids supposedly due to the formation of hydroxyl radicals by the reaction of ubiquinone with lipid 15-LOX-derived hydroperoxides. However, it is impossible to agree with this proposal because it is known for a long time [23] that quinones cannot catalyze the formation of hydroxyl radicals by the Fenton reaction. Oxidation of intracellular unsaturated acids (for example, linoleic and arachidonic acids) by lipoxygenases can be suppressed by fatty acid binding proteins [24]. 

The health impairing and toxic elfects of oxidation of lipids are due to loss of vitamins, polyenoic fatty acids, and other nutritionally essential components formation of radicals, hydroperoxides, aldehydes, epoxides, dimers, and polymers and participation of the secondary products in initiation of oxidation of proteins and in the Maillard reaction. Dilferent oxysterols have been shown in vitro and in vivo to have atherogenic, mutagenic, carcinogenic, angiotoxic, and cytotoxic properties, as well as the ability to inhibit cholesterol synthesis (Tai et ah, 1999 Wpsowicz, 2002). 

Linoleic (C18 2) and linolenic (C18 3) acids cannot be synthesized by mammals and must be supplied in the diet, i.e. they are essential fatty acids (linoleic is the only true essential acid). These two polyenoic acids may then be elongated and/or further desaturated by mechanisms similar to stearic - oleic, to provide a full range of polyenoic acids. A summary of these reactions is given in Figure 3.12a, b. 

Smith et al. (1978) have described a procedure for the GLC determination of cis and trans isomers of unsaturated fatty acids in butter after fractionation of the saturated, monoenoic, dienoic, and polyenoic fatty acid methyl esters by argentation TLC. Total trans acids were much higher, as measured by infrared spectrophotometry than by GLC, probably because some of the acids could have two or more of the trans bonds designated as isolated by infrared spectrophotometry. Enzymatic evaluation of methylene-interrupted cis, cis double bonds by lipoxidase resulted in lower values than those obtained by GLC. The authors mention that the lipoxidase method is difficult, requiring considerable skill, and suggest that their method is suitable for the determination of the principal fatty acids in complex food lipids such as bovine milk fat. 

Polyenoic acids also give rise to malondialdehyde, a reactive mutagenic compound, which can be reduced... 

Prostaglandins are not stored by cells but are synthesized in response to external stimuli. Arachi-donic acid and other polyenoic acids are present in relatively small amounts (e.g., 1% of total plasma... 

Normally, the method of choice for the analysis of complex mixtures of polyenoic fatty acids such as those derived from fish oils is capillary gas chromatography with prechromato-graphic derivatization and mass spectrometric detection. However, GC is impractical for the purification of the large amounts of polyenoic fatty acids required for biological and clinical studies. Moreover, the temperatures required in GC may cause degradation of oxidized long-chain polyunsaturated fatty acids that are present as minor components of the mixture. 

HPLC, little degradation of polyenoics occurs, and methods can be easily scaled up for semipreparative or preparative use. 

As indicated above, there are many possible oxidation products of the different polyenoic acids. It is probably naive to ascribe the effects of dietary intervention reported thus far to such metabolites. Carefully controlled clinical studies will be needed before these questions can be satisfactorily answered. However, subjects on diets containing highly saturated fatty acids clearly show increased platelet aggregation when compared with other study groups. Such diets (eg, in Finland and the USA) are associated with higher rates of myocardial infarction than are more polyunsaturated diets (eg, in Italy). 

Polyenoic fatty acids (acids with more than one double bond in the chain) play a leading role where total unsaturation is observed to change. According to current concepts, the structure and functioning of cell membranes are maintained through the agency of a binary film composed of phospholipids and cholesterol. The phospholipids consist of polar, outwardly directed heads and fatty acid tails immersed in the membrane (Fox, 1972 Singer and Nicholson, 1972 Chapman, 1973). In this binary film, there are inclusions of protein molecules. 

Within each of two Black Sea species, anchovy (warm water) and sprat (cold water), both the concentrations and absolute amounts of phospholipids fluctuate within similar limits, but do not change during the annual cycles in the same tissues. This contrasts with, for example, the considerable differences between the phospholipid contents of red and white muscle or between that of either of them and liver (Shchepkina, 1980a Shchepkin and Minyuk, 1987). The content of polyenoic acids in the phospholipids of anchovy is higher than that in the sprat (Yuneva, 1990) possible explanations will be given in Chapter 3. 

Studies by Johnston and Roots (1964), Roots (1968) and Kreps (1981) have revealed an increased ratio between the plasmalogenic and diacyl forms of phosphatidyl ethanolamine in oceanic fish from low-temperature waters. During cold adaptation, the ratios between the main phospholipid fractions alter the relative proportion of phosphatidyl choline decrease and phosphatidyl ethanolamine, phosphatidyl serine and sphingomyelin, all of which contain large amounts of polyenoic acids, increase (Caldwell and Vemberg, 1970 Miller etal, 1976 Wodke, 1978 Hazel, 1979 Brichon et al., 1980 van den Thillart and de Bruin, 1981 Zabelinsky and Shukolyukova, 1989). 

In nature, fish apparently acquire polyunsaturated lipids in one of two ways. The first of them conforms with the concept of Saigent and Henderson (1980), Watanabe (1982) and Henderson et al. (1985), that some species of fish do not need to synthesize long-chain polyenoic acids, since they occur in phytoplankton, which are eaten by zooplankton which in turn are food for fish. Takahashi et al. (1985) described the situation as unsaturated fatty acids being transferred from plant organisms to phytoplankton-eating fish to predatory fish. 

On the other hand, some fish are able to synthesize long-chain polyenoic fatty acids (Kayama et al., 1963) from shorter carbon chains. Docosohexaenoic acid is laid down in coho salmon in quantities related to the size of the fish, rather than to its availability in the diet (Tinsley et al., 1973). Rainbow trout fed on 18 2 and 18 3 fatty acids can produce 20 3, 22 5 and 22 6 fatty acids in substantial quantities (Owen et al., 1975), but these workers noticed that the capacity of marine flatfish to elongate or desaturate the carbon chains was more limited. They found that 70% of the radioactivity of labelled 18 3 appeared later in the 22 6 fatty acid of rainbow trout, but that turbot converted only 3-15% of labelled precursors into polyunsaturated fatty acids of longer chain length. It was suggested that turbot in the wild probably received adequate polyunsaturated acids in their diet, which the fish therefore did not need to modify. The elongation of the carbon chains and the creation of more double bonds is also only slight in Atlantic cod, another marine teleost, presumably for the same reason (Ross, 1977). 

In Chapter 2, section 2.1, the importance of lipids in temperature adaptation was noted, and the versatile role of phospholipids, cholesterol and polyenoic... 

As mentioned earlier, the polyenoic fatty acids from marine fish are mostly from the linolenic series (m3). These are eicosopentaenoic (C20 5o>3) and docosohexaenoic (C22 6o>3) acids, the latter usually being the more abundant. Yakovleva (1969) reported that in Black Sea fish die ratio between fatty acids of the 3 and 6 series also displayed a close correlation with the natural mobility levels. 

Rabinovich and Ripatti (1990) have shown that docosohexaenoic acid has conformational properties which keep its physico-chemical and, possibly, functional characteristics effective over a wide temperature range. This ensures the adaptation of cell membranes to changes of metabolic activity. Fluctuation in locomotory activity is one factor responsible for these changes. From their studies of the sea cucumber, Cucumaria frondatrix, Kostetsky et al. (1992) concluded that polyenoic acids of linolenic affinity did not exhibit a direct relatonship with temperature adaptation. In contrast to this, Zabelinsky et al. (1995) claim that C20 5o>3 (not C22 6a>3) and Cl8 1 are the fatty acids of key importance for temperature adaptation in marine fish. 

Figure 33 Seasonal dynamics of lipid unsaturation in anchovy. (After Shulman, 1978a Yuneva, 1990.) Continuous line, iodine value broken line, total polyenoic acids.

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