The time course of lipid biosynthesis in pig epidermis

In order to examine the time course of lipid metabolism in the epidermis, pairs of sites on each of two pigs were given seven intradermal injections of 10 microCi of [1-14C]-acetate at 1, 3, 6, 12, 24, 36, and 48 h before the animals were killed. A second time course was obtained with two other pigs at 1 h and 3.5, 7, 10.5, 14, 17.5, and 21 d after labeling. The epidermis over each injection site was recovered, and the lipids were extracted and analyzed by thin-layer chromatography and by autoradiography. Virtually all of the radioactivity in aliphatic chains was found first in phospholipids and was then transferred to glucosylceramides, and then into ceramides, between 1 and 7 d after labeling. Labeling of free fatty acids showed a similar time course. Most of the radioactivity acquired by acylglucosylceramide was transferred to hydroxyceramide, bound to the corneocyte envelope, between 2 and 10 d after labeling. The hydroxyacid and sphingosine moieties of the hydroxy-ceramide had equal specific activity. The results indicate that the aliphatic lipids of the stratum corneum are synthesized de novo in the epidermis via phospholipids. Cholesterol, however, was synthesized directly from acetate within hours of labeling, and its esters acquired label 3 to 7 d later.