Distribution of unusual fatty acids in the triacylglycerols of sea buckthorn mesocarp oil

Abstract

The structure of unusual fatty acid (FA) components of triacylglycerols (TAGs) of mature sea buckthorn (Hippophae rhamnoides L.) mesocarp oil was determined by GLC and MS, and the positional-species composition (PSC) of these TAGs was estimated using the methods of TAG chemical deacylation, TLC, GLC, and computer calculation. It was shown that the unusual FAs comprised n-cis-Δ9-hexadecenoic, n-cis-Δ9,12-hexadecadienoic (palmitolinoleic), and n-cis-Δ11-octadecenoic (cis-vaccenic) acids. The hexadecenoic acid predominated in the oil, and in its distribution in TAGs, it was similar to the total FAs differing from them only in some prevalence in the triunsaturated TAGs and in the TAGs with a shorter acyl chain, as well as in the sn-2 position of TAGs. Palmitolinoleic (16:2) acid comprised only 5% of total FAs, and it was exclusively concentrated in the sn-2 position of TAGs. As regards its distribution between various positional types and forms of TAGs, the 16:2 acid was similar to oleate and total FAs. As compared to the total TAGs, the TAGs with 16:2 acid were characterized by a lower FA chain length as well as by a highest unsaturation. The TAGs with vaccenic acid (V-TAGs) considerably exceeded O-TAGs, i.e., the TAGs containing oleic acid, another 18:1 positional isomer, both in their content in the total TAGs and in their unsaturation. In the composition of positional types and fractions of various unsaturation, O-TAGs were similar to the total TAGs, while V-TAGs were characterized by a very unusual structure, viz., a very high triunsaturated TAG level and an extremely low concentration of 1,3-disaturated-2-monounsaturated TAGs. In addition, oleic acid, like most other unsaturated FAs, was incorporated predominantly in the sn-2 position of TAGs, while vaccenic acid, being also unsaturated, was nevertheless by 90% concentrated in the sn-1,3 positions of V-TAGs. Unusual FAs were related to each other in the mechanism of their biosynthesis. In fact, hexadecenoic acid biosynthesis produced by palmitic acid desaturation, was, on the one hand, further desaturated forming palmitolinoleic acid, and, on the other hand, converted to vaccenic acid via C2 elongation.