Biosynthesis of Fatty Acids by Subcellular Fractions of Newborn Mouse Skin*

Homogenates of newborn mouse skin were centrifuged at 900 × g, 15,000 × g, and 100,000 × g. Sediment from the last two centrifugations, as well as the 100,000 × g supernatant fraction incorporated [1-14C]-acetyl CoA and [1, 3-14C]-malonyl CoA into fatty acids which were analyzed by chromatographic procedures and scintillation counting. All three fractions, especially the supernatant fraction, utilized malonyl CoA in labeling saturated acids. The particulate fractions used both malonyl CoA and acetyl CoA in labeling unsaturated acids. Several cofactor requirements were established. Coenzyme A was markedly inhibitory. When oxygen was replaced by nitrogen, incorporation of both precursors was increased by 50%.Comparison of the incorporation of [1-14C]-acetyl CoA into fatty acids of the 15,000 × g and 100,000 × g pellet fractions was studied with and without oxygen. Results showed that chain elongation predominated as the mode of labeling of unsaturated acids. This was confirmed by estimating the extent of carboxyl carbon labeling. The principal chain extensions were: 16: 1→ 18: 1 → 20: 1, 18: 2 → 20:2, and 20:4 → 22:4. Desaturase activity was not observed, even in the 100,000 × g (microsomal) pellet fraction.The distribution of radioactivity among polyunsaturated acids of whole skin in after exposure to [14C]-acetate was compared to the pattern observed with a 15,000 × g pellet fraction after incubation with [14C]-acetyl CoA. Results indicated that 18:2ω;6 (linoleic acid), 20:3ω;6, and other were probably labeled in whole skin by a retroconversion mechanism which was inoperative under the conditions used with the subcellular fractions