Free sphingosines in porcine epidermis.

Sphingosines and phytosphingosines serve as intermediates in the synthesis of ceramides and glucosylceramides, which are prominent components of mammalian epidermis. In the present study, we have investigated the possibility that free sphingoid bases also may be present in epidermal tissue. Samples of pig epidermis were trypsinized to separate the stratum corneum from the unkeratinized portion of the epidermis. After drying, the lipids were extracted and analyzed by thin-layer chromatography using ninhydrin to detect free amino groups. Both the stratum corneum and the unkeratinized epidermal material contained a ninhydrin-positive material with the same mobility as the sphingosine standard. Quantitation of the chromatograms by photodensitometry indicated that free sphingosine bases account for 0.44% by weight of the total stratum corneum lipid and 0.09% of the lipid in the viable portion of the epidermis. To further identify this material, it was treated with 1-fluoro-2,4-dinitrobenzene, which resulted in the production of an intensely yellow N-2,4-dinitrophenyl derivative with the same mobility as N-2,4-dinitrophenylsphingosine on thin-layer chromatography. Oxidation of the isolated dinitrophenyl derivative with lead tetraacetate produced a mixture of aldehydes which were analyzed by gas-liquid chromatography. This analysis indicates that the free sphingoid bases from the stratum corneum consist of a mixture of mainly 16- through 20-carbon sphingenines and sphinganines, the most abundant components being d17:0, d17:1, d18:1 and d20:1. The production of these free sphingosine bases may be significant in regulating epidermal differentiation.