Covalently bound lipids of human stratum corneum.

In the present study, we demonstrate that human stratum corneum contains covalently bound lipids accounting for 1.4% of the dry weight of the tissue. The major component (53.3% of the total by weight) is a ceramide (CER-A) consisting of 30 through 34-carbon omega-hydroxyacids amide-linked to sphingosine. The other bound lipids in human stratum corneum include fatty acids (12.7%), omega-hydroxy acids (9.4%), and a second, more polar, omega-hydroxyacid-containing ceramide (CER-B, 24.8%). The predominant omega-hydroxyacids in both ceramides, as well as the free hydroxyacid fraction, are the 30-carbon saturated and 32- and 34-carbon monoenoic species. The bound fatty acids consist largely of 14 through 22-carbon saturated species, but significant proportions of monoenoic species and linoleic acid are also present. Psoriatic scale contains a similar total concentration of the same covalently bound lipids, but the proportions of the individual bound lipids are different from those found in normal stratum corneum. It is suggested that the principal function of the covalently bound lipids in human stratum corneum is the formation of a lipid envelope on the outer surface of the keratinized cells.