The structure of polyenoic odd- and even-numbered fatty acids of mullet (Mugil cephalus)

Mullet oil contains more than 25% straight-chain odd-numbered fatty acids. Odd- and even-numbered components of chain lengths C₁₅ to C₂₀ were isolated and their structures determined. The vinylmethane rhythm prevails in all polyun-saturated acids. Numerous homologs have their double bonds in identical positions, relative to the carboxyl group, as for example, 舥^9,12- and 舥^6,9,12-C₁₅, -C₁₆, -C₁₇ and -C₁₈ acids. The terminal structures which are characteristic for oleic, linoleic, etc., families are not found in the unsaturated odd-numbered acids. The results show: that the proximal structure has greater influence than the terminal structure on the biosynthesis of unsaturated odd-numbered acids; that chain lengths of 17 and 18 carbon atoms with double bonds in position 9 are crucial for synthesis of the polyenoic C₁₉ and C₂₀ acids; that chain lengths C₁₅ and C₁₆ with double bonds in position 9 are suitable for desaturation but that they are not suitable for desaturation after elongation. These specifications bring all acids of mullet into a rational order and reflect their possible interconversions. Presently, such classification has only limited predictive value in regard to the physiological properties of polyunsaturated acids. However, the new definitions for grouping the polyunsaturated fatty acids lead to interesting working hypotheses.