Stereospecific analysis of the major triacylglycerol species containing γ-linolenic acid in evening primrose oil and borage oil

Abstract

Evening Primrose oil (EPO) and borage oil (BO) are used frequently in nutritional and clinical studies involving a disease condition with an impaired or inadequate Δ 6-desaturase enzyme activity. This impairment may be bypassed by supplementation with γ-linolenic acid (GLA, 18:3n-6), an intermediate metabolite of linoleic acid (LA, 18:2n-6). The major individual triacylglycerol (TG) species comprising all potential positional isomers (molecular species) from both EPO and BO were separated, isolated by reversed-phase HPLC and subjected to HPLC stereospecific analysis as naphthylethyl urethane derivatives. The method of analysis is useful since only small quantities of the individual TG species are required and prior experimentally demanding fractionation steps are eliminated. Over 90% of the important clinical fatty acid, GLA, present in EPO and over 80% of the GLA in BO have been identified and quantified in the molecular species of their respective analyzed TG fractions. Generally, within the individual GLA-containing TG species from both oils, GLA is distributed asymmetrically among the three positions, preferentially at the sn-2 and sn-3 positions, although more so for the TG species in BO. The positional isomers of the diacid TG species were determined directly from the stereospecific analysis. For the triacid TG species computer aided linear regression was used to determine the positional isomers. The predicted positional isomeric distributions for the individual TG species calculated using the 1-random, 2-random, 3-random distribution theory from the stereospecific analysis of the native oils were in good agreement with the experimentally determined values. In contrast to other seed oils the pairs of individual TGs possessing chirality do not exist as racemic mixtures in either oil.