Measurement of the incorporation of orally administered arachidonic acid into tissue lipids

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Abstract
The applicability of a stable isotope method to monitor the mixing of dietary arachidonic acid with endogenous arachidonic acid in tissue lipids was evaluated. Rats were fed octadeuterated arachidonic acid during a 20-day period, and the entry of the dietary acid into lipid esters of various tissues was examined by gas chromatographymass spectrometric (GC-MS) analysis of their fatty acids. The rats were maintained on a fat-free diet from weaning until 63 days old to enhance the ratio of the dietary acid to endogenous arachidonate. Three separate forms of eicosatetraenoic acid in the tissue lipids could be distinguished by GC-MS: octadeuterated arachidonic acid (recent dietary origin), unlabeled arachidonic acid (maternal origin) and unlabeled, 4,7,10,13-eicosatetraenoic acid (originating from palmitoleic acid). The total eicosatetraenoic acid in the tissue lipids contained about 90% arachidonate from recent dietary origin in lung, kidney, heart and fat, 70% in muscle and liver and 27% in brain. The n−7 isomer of eicosatetraenoic acid was estimated to make up 6% or less of the total eicosatetraenoic acid in lung, kidney, brain, muscle and heart tissue lipids, but it comprised around 15% of the total eicosatetraenoic acid in liver. The unlabeled arachidonic acid of maternal origin thus comprised only about 10% of the eicosatetraenoic acid in all tissues examined except muscle and brain, where it was 24% and 70% of the eicosatetraenoic acid, respectively. The relative amounts of the three forms of eicosatetraenoic acid are consistent with a limited access of dietary arachidonate to the brain tissue and with a competition between the dietary n−6 isomer and the endogenous n−7 isomer for esterification in the liver. Because most muscle mass would have formed after weaning, the high proportion of maternal arachidonate in the muscle lipids suggested that maternal arachidonate may have been displaced from other tissues to muscle, from which it equilibrated slowly with dietary arachidonate acid. The combination of deuterated arachidonic acid and GC-MS analysis thus furnished more detailed information about the composition and origin of eicosatetraenoic, acid in tissue lipid esters than that previously available from radiotracer studies or GC-MS analyses alone.