Stereospecific distribution

Table 4.3. Composition and Stereospecific Distribution of Fatty Acids in Milk Fat Trigylcerides from Bimonthly Samples of Maleny Butter.

Parodi, P. W. 1979. Stereospecific distribution of fatty acids in bovine milk fat trigylcer-ides. J. Dairy Res. 46, 75-81. 

The situation is reversed in evaluation of the active center stereospecificity distribution. Any method covered in Section 4.1 is more or less suitable to reveal this distribution, if a procedure to separate the diversely stereoregular polymer fractions can be realized. Then the tags can be monitored in the fractions and the C value calculated therefrom. The task of obtaining stereospecificity distribution is difficult for methods based on the catalyst poison consumption data (Sect. 4.2). 

In neutral oils and fats, the fatty acids are not usually randomly distributed among different positions on the glycerol backbone and are associated in particular patterns. As an example, saturated fatty acids such as palmitic and stearic acids are associated with the sn- and sn-3 positions of soybean oil, albeit at higher proportions in the sn- position. However, the reverse is observed at high content of saturated fatty acids. Linoleic acid is preferably in the sn-2 position, whereas oleic acid is randomly distributed among the three positions. Linolenic acid is primarily at sn-2 followed by sn- and sn-3 positions. The stereospecific distribution of fatty acids has a marked effect on the oxidative stability of the resultant oils, and their presence at the sn-2 position helps their stability (19). 

Oxidative stability of edible oils depends primarily on their fatty acid composition and, to a lesser extent, in the stereospecific distribution of fatty acids in the triacyl-glycerol molecules. The presence of minor components in the oils also affects their oxidative stability. A detailed discussion of oxidative processes in fats and oils is provided elsewhere in this series. Oxidation may occur via different routes and includes autoxidation, photo-oxidation, thermal oxidation, and hydrolytic processes, all of which lead to production of undesirable flavor and products harmful to health. Flavor and odor defects may be detected by sensory analysis or by chemical and instrumental methods. However, chemical and instrumental procedures are often employed in the processing and during usage of edible oils. Indicators of oxidation are those that measure the primary or secondary products of oxidation as well as those from hydrolytic processes or from thermal oxidation, including polymers and polar components (15). 

Harp and Hammond (45) explored the stereospecific distribution of acyl groups on the three positions of the glycerol molecule for soybean triacylglycerols with a wide range in fatty acid composition. They found that the amount of an acyl group... 

TABLE 5. Stereospecific Distribution of Acyi Groups in the Triacyigiycerois, Phospha-tidyichoiine, Phosphatidyiethanoiamine, and Phosphatidyiinositoi of a Typicai Soybean (45, 46). 

The primary phosphatides of soybean oil are phosphatidylcholine, phosphatidyl-ethanolamine, and phosphotidylinositol, which generally make up 55.3%, 26.3%, and 18.4% of the total phosphatides, respectively (50). The stereospecific distribution of the acyl groups in these phospholipids for a typical soybean lipid is shown in Table 5. In all the phospholipids, the saturated acyl groups are concentrated in the... 

Triacylglycerols (TAG) are the primary neutral lipids in soybean oil. Due to the high concentration of unsaturated fatty acid in soybean oil, nearly all the TAG molecules contain at least two unsaturated fatty acids, and di- and trisaturates are essentially absent (List el al. 1977). In natural oils and fats, the fatty acids are not usually randomly distributed among the three hydroxyl groups of glycerol but are associated in particular patterns. Several theories of regiospecific distribution exist (Litchfield 1972), but the 1,3-random, 2-random theory is most widely accepted. The stereospecific distribution of fatty acyl groups in soybean oils... 

Table 2.4 Fatty acid composition (mole %) and stereospecific distribution of neutral and polar lipids of a commodity soybean...

Soybean oil is a polyunsaturated or linoleic type of oil that is highly susceptible to lipid oxidation. The rate of lipid oxidation depends primarily on the fatty acid composition and only secondarily on the stereospecific distribution of the fatty acyl groups, as described earlier. The mechanism of lipid oxidation and lipid hydroperoxide breakdown has been discussed thoroughly by Frankel (1998). 

Wang, T., Hammond, E.G. and Fehr, W.R. (1997) Phospholipid fatty acid composition and stereospecific distribution of soybeans with a wide range of fatty acid compositions. J. Am. Oil Chem. Soc., 74, 1587-1594. 

Fig. 7.4. Stereospecific distribution of fatty acyl groups in commodity soybean triacylg-lycerides (TAG), phospahtidylcholine (PC), phosphatidyletanolamine (PE), and phospha-tidylinositol (PI). 16 0, palmitic acid 18 0, stearic acid 18 1, oleic acid 18 2, linoleic acid 18 3, linolenicacid (Harp Hammond, 1998 Wang et al., 1997 Hammond et al., 2005).

Class composition, fatty acyl stereospecific distribution, and molecular species composition of PLs in normal soybeans and seeds with modified fatty acid composition were studied by Wang et al. (1997) and Wang and Hammond (1999). The PI... 

Phospholipid (PL) fatty acid composition and stereospecific distribution of 25 genetically modified soybean lines having a wide range of compositions were determined by GC and phospholipase hydrolysis (Wang et al., 1997). PL class proportions were affected by changes in overall fatty acid composition. PL fatty acid composition was changed with oil fatty acid modification, especially for palmitate, stearate, and linolenate. 

Stereospecific Distribution of Fatty Acids in Glycerols of Yolk... 

Polybutadiene samples were fractioned to obtain the stereospecificity distribution of AC. All the extracted fractions are sufficiently narrowly dispersed (M,yM < 2) and their yield was at least 95-96% (Table 3.9). There is satisfactory correlation between the average MW data of initial samples obtained from the MW of fractions and gel-permeating chromatography data, confirming successful fractionation [102]. 

The triglycerides of corn oil follow the normal distribution anticipated for a vegetable oil (Table 3.142). The stereospecific distribution of the fatty acids (de la Roche et ai, 1971) is influenced by the overall fatty acid composition. The triglyceride carbon number distribution is given in Table 3.143. 

The stereospecific distribution of the fatty acids (Sanders 1980 Treadwell and Young, 1982) shows that the greatest concentration of 16 0 and 18 0 is at the sn-1 position and that the 20, 22 and 24 acids occur mainly at sn-3. The entire amount to 20 0 was found at the sn-3 position. See also Table 3.144. The effect of season and location has been studied (Sanders, 1982). As the seed matures the overall level of 18 1 increases (Pattee etal, 1969 Sanders, 1980). 

As with most vegetable oils the saturated acids hardly occur at the 2-position of the triglyceride. The glyceride composition calculated by the 1,3-random, 2-random distribution agrees well with experimentally determined values. Typical figures are given in Table 3.142. The stereospecific distribution of fatty acids in soya do not show strong differences in the composition of the 1- and 3-positions (Table 3.144). 

Stereospecific Distribution of Fatty Acids in Triacylglycerols. It has been carried out by an adaptation of the procedures devised by Brockerhoff (1967) and Christie Moore (1969). 

Regarding the stereospecific distribution of fatty acids in triacylglycerols in relation to the positional distribution in phosphoglycerides, it can be stated that oleic acid is randomly distributed in the three positions of triacylglycerols from eggs... 

To obtain further information on the participation of the various positions of the glycerol moiety in the acyltransferase mechanism, the stereospecific distribution of fatty acids in triacylglycerols obtained after incubating insect homogenates with labelled fatty acids was investigated. 

The stereospecific distribution of fatty acids in TAG molecules of DHA, EPA and EPA+DHA-enriched structured lipids synthesized in our laboratory was determined. Tables V and VI report the positional distribution of fatty acids in structured lipids examined. The results of this study showed that DHA was randomly distributed over all three positions (34.6% at snA 33.5% at sn-2 and 35.9% at snA>) of the TAG molecules of DHA-enriched borage oil (Table V). In DHA-enriched evening primrose oil, however, this fatty acid was mainly occupied by the sn 2 position (38.2%), followed by snA (33.1%) and sn- (24.5%) positions (Table VI). It should be noted that these DHA-enriched structured lipids were prepared using Novozym-435 from Candida antarctica as the biocatalyst. The positional specificity of Novozym-435 depends on the type of substrates used in various reactions. In some reaction systems, this enzyme behaves as a nonspecific lipase whereas in other systems it exhibits 5W-1,3 regiospecificity (77). Based on the reaction conditions enq)loyed in this study, Novozym-435 functions as a nonspecific lipase. 

The stereospecific distribution of fatty acids in the native borage and evening primrose oils have previously been reported 18). In native borage oil, GLA was distributed asymmetrically and preferentially located at the sn 2 and snA positions 18). In native evening primrose oil, GLA was concentrated in the snA position 18). Linoleic acid (LA 18 2n-6) was fairly evenly distributed in all positions of native evening primrose oil, but was preferentially located in the snA position of native borage oil 18). The results of our study showed that in DHA-enriched borage oil, GLA was mainly located in the sn-2 (18.4%) and snA... 

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