Positional specificity of gastric hydrolysis of long-chain n-3 polyunsaturated fatty acids of seal milk triglycerides

Long-chain n-3 polyunsaturated fatty acids (n-3 PUFA) of marine oils are important dietary components for both infants and adults, and are incorporated into milks following maternal dietary intake. However, little is known about the hydrolysis of these PUFA from milk triglycerides (TG) by lipases in suckling young. Seals, like humans, possess gastric lipase; however, the milk lipids of seals and sea lions are almost devoid of the readily hydrolyzable medium-chain fatty acids, and are characterized by a large percentage (10-30%) of n-3 PUFA. Gastric hydrolysis of milk lipids was studied in vivo in suckling pups of three species (the California sea lion, the harp seal and the hooded seal) in order to elucidate the actions and specificity of gastric lipases on milk TG in relation to fatty acid composition and TG structure. Regardless of milk fat content (31-61% fat) or extent of gastric hydrolysis (10-56%), the same fatty acids were preferentially released in all three species, as determined by their relative enrichment in the free fatty acid (FFA) fraction. In addition to 16:1 and 18:0, these were the PUFA of 18 carbons and longer, except for 22:6n-3. Levels of 20:5n-3 were most notably enriched in FFA, at up to five times that found in the TG. Although 22:6n-3 was apparently also released from the TG (reduced in the diglyceride), it was also notably reduced in FFA. Positional analysis of milk TG based on the products of Grignard hydrolysis revealed that these PUFA, including 22:6n-3, were preferentially esterified at the α-position of the TG, and that the fatty acids not released during gastric hydrolysis were located at the sn-2 position. The extreme reduction of 22:6n-3 and enrichment of 20:5n-3 in FFA is discussed. Results from this study are consistent with reports that gastric lipase acts stereo-specifically to release fatty acids at the α-positions (sn-3, sn-1). We conclude that the n-3 PUFA in milk are efficiently hydrolyzed by gastric lipase and that this has important implications for digestion of milks enriched in PUFA by neonates in general.