In vitro and in vivo fate of saturated and unsaturated fatty acids during development of insects

Abstract

1. 1. In vitro elongation, saturation and desaturation reactions of fatty acids using labelled 10:0, 12:0, 14:0, 18:0, 18:1 and 18:2 have been investigated with homogenates of larvae and pharate adults of the Dipterous Ceratitis capitata. 2. 2. Larval homogenates desaturate and elongate the labelled substrates according to their chain length. Direct unsaturation and elongation of fatty acids by pharate adult homogenates are insignificant. Unsaturated fatty acids are scarcely hydrogenated to stearic acid by larval homogenates; pharate adult homogenates does not exhibit any saturating activity. 3. 3. Larval and pharate adult homogenates show a different pattern of incorporation of labelled fatty acids into the main classes of lipids. As a general rule, labelled fatty acids are preferentially incorporated into triacylglycerols by larval homogenates; C 18 fatty acids exhibit a different incorporating pattern according to the degree of unsaturation. Labelled fatty acids remain mainly as free fatty acids in the presence of pharate adult homogenates. This different behaviour of larval and pharate adult homogenates can not be explained through differences in the fatty acid activating capacity of both stages of development. 4. 4. In vivo experiments of feeding larvae with 16:0, 18:0, 18:1 and 18:2 have been also carried out. Transformations of these fatty acids are followed through the different stages of development of the insect. The observed elongation activity is in general agreement with the in vitro results. Palmitic and stearic acids are increasingly desaturated during the larval stage of development, that is also in agreement with the in vitro results. A de novo synthesis from labelled acetate, obtained from oxidative degradation of labelled fatty acids, may explain the presence in the lipid fraction of shorter-chain labelled fatty acids than the substrates. 5. 5. Incorporation of 18:0 and 18:2 into the main classes of lipids was also determined in the in vivo experiments. An incorporation of free fatty acids into triacylglycerols during larval and pharate adult stages of development is noticed. Incorporation of 18:2 into phospholipids was higher than that of 18:0. In the main, the in vivo results of incorporation do not reproduce the in vitro experiments, as far as the triacylglycerols-free fatty acids relationship is concerned.