The Specific Distribution of Fatty Acids in the Glycerides of Animal and Vegetable Fats

Abstract

The structure of naturally occurring triglycerides has been studied over a number of years with little agreement about the distribution of the fatty acids, except as to the prevalence of mixed glycerides. Several types of arrangement, for example, even, random, partial random, and restricted random, have been proposed and defended. Hilditch (I) and Bhattacharyya (2) have recently reviewed the bases of these various theories. One reason that a number of theories developed has been the lack of suitable methods for the isolation and characterization of glycerides. It is possible, with fairly quantitative yields, to separate the triglycerides of a natural fat into glyceride classes, i.e. trisaturated, disaturated, monosaturated, and triunsaturated. However, only with a limited number of fats has it been possible to separate major proportions of single glycerides which can be identified by thermal and x-ray diffraction techniques. For instance, it has been shown that 2-oleoyl distearin comprises approximately 60 per cent of kokum butter (3, 4), that 2-oleoyl dipalmitin is the principal single triglyceride of stillingia tallow (5), and that 2-oleoyl palmitoyl stearin is present to the extent of 50 per cent in cocoa butter (6-8). Thus, the conclusively identified disaturated glycerides of vegetable oils are exclusively 2-oleoyl disaturated. On the other hand, lard has a very high proportion of 2-palmitoyl stearoyl “olein,” clearly in contrast with beef and mutton tallow (9, 10). Thus, the evidence, although scattered, is indisputable for the occurrence in several different organisms of specific glycerides in proportions far in excess of expectations for random distribution. Nevertheless, reports continue to appear containing data which are interpreted as supporting an element of randomness in glyceride organization (11, 12). In 1956 Mattson and Beck (13) demonstrated the specificity of pancreatic lipase for the cleavage of ester linkages at the 1 and 3 positions of triglycerides and suggested that this offered a new tool for study of triglyceride structure, a tool with certain obvious advantages over previous means. This method of enzymatic hydrolysis has been applied by Savary, Flanzy, and Desnuelle (7) to fractions of eight natural glycerides. On the basis of iodine value and, in some instances, identification of the fatty acids, they concluded that the fatty acids were not distributed at random. In the present study the enzymatic technique, applied to twelve vegetable fats and seven animal fats, has yielded strong evidence for a high degree of specificity in fatty acid distribution in the glycerides of natural fats.