Occurrence, function and biosynthesis of wax esters in marine organisms

Abstract

Wax esters occur as a major lipid-type in at least 30 species of marine animals, distributed among 17 orders and 3 phyla. They are of limited usefulness as a chemotaxonomic character, since only in two suborders, the calanoid copepods, Calanoidei, and the toothed whales, Odontoceti, do the wax esters occur in all members so far examined. In bony fishes their occurrence in muscle correlates better with mesopelagic habitat and vertical migration patterns than with taxonomy. Homologs with 21 to 44 total carbon atoms have been reported, but the usual range for the wax esters in copepods and fish is C₃₀–C₄₂. In fishes the muscle wax esters contain predominantly one and two double bonds per molecule, while in roe lipids up to 65% of the homologs contain three or more double bonds. The component alcohols are saturated and monounsaturated, with 16∶0 and 18∶1 as the usual major constituents. The fatty acids are more diverse, but 18∶1 is most often the main component, and 16∶1 and 20∶1 are frequent major constituents; polyunsaturated acids make up 1–12% in fish muscle and whale oils and up to 45% in fish roe wax esters. Possible functions of the wax esters are for buoyancy, as energy reserves and for thermal insulation. In vitro, various tissues of marine bony fishes synthesize wax esters from long chain alcohols and fatty acids, without activation. A competing pathway for the long chain alcohols in vivo is their catabolic oxidation to the corresponding fatty acids. The key to the accumulation of wax esters is to be sought in the metabolism of the long chain alcohols, their biosynthesis and esterification vs. their catabolism.