Metabolism of α-linolenic acid during incubations with strained bovine rumen contents: products and mechanisms

Description of alpha-linolenic acid (cis-9, cis-12, cis-15-18 : 3, ALA) metabolism in the rumen is incomplete. Ruminal digesta samples were incubated with ALA and buffer containing water or deuterium oxide to investigate the products and mechanisms of ALA biohydrogenation. Geometric Delta 9,11,15-18 : 3 isomers were the main intermediates formed from ALA. An increase in the n + 1 isotopomers of Delta 9,11,15-18 : 3 was due to H-2 labelling at C-13. Isomers of Delta 9,11,13-18 : 3, cis-7, cis-12, cis-15-18 : 3 and cis-8, cis-12, cis-15-18 : 3 were also formed. No increase in n + 1 isotopomers of Delta 7,12,15-18 : 3 or Delta 8,12,15-18 : 3 was detected. Enrichment in n + 2 isotopomers of 18 : 2 products indicated that ALA metabolism continued via the reduction of 18 : 3 intermediates. Isomers of Delta 9,11,15-18 : 3 were reduced to Delta 11,15-18 : 2 labelled at C-9 and C-13. ALA resulted in the formation of Delta 11,13-18 : 2 and Delta 12,14-18 : 2 containing multiple H-2 labels. Enrichment of the n + 3 isotopomer of Delta 12,15-18 : 2 was also detected. Metabolism of ALA during incubations with rumen contents occurs by one of three distinct pathways. Formation of Delta 9,11,15-18 : 3 appears to be initiated by H abstraction on C-13. Octadecatrienoic intermediates containing cis-12 and cis-15 double bonds are formed without an apparent H exchange with water. Labelling of Delta 9,11,13-18 : 3 was inconclusive, suggesting formation by an alternative mechanism. These findings explain the appearance of several bioactive fatty acids in muscle and milk that influence the nutritional value of ruminant-derived foods.