Characterization of double bond position in free fatty acids of human sebum

Free fatty acids (FFAs) are essential components of skin surface lipids (SSLs), which consist of a blend of epidermal and sebaceous lipids. The sebum component of SSLs is abundant in seborrheic body areas, which are rich in sebaceous glands (SGs). Sebum FFAs exhibit unique characteristics, including a prevalence of C14-C18 chain lengths, terminal branching, and a double bond at the Δ6 position. Notably, over one-third of the sebaceous FAs contain a single double bond. Deregulated synthesis of monounsaturated FAs (MUFAs) can lead to changes in sebum composition, which contribute to the pathogenesis of inflammatory skin conditions such as acne vulgaris, rosacea, psoriasis, and seborrheic dermatitis. A comprehensive characterization of the different isomers of unbound MUFAs distinguished by the position of the carbon-carbon double bond, has not been achieved in human sebum. Therefore, we aimed to develop an analytical strategy to differentiate the various carbon-carbon double bond positions in FFAs. Our methodology combines the Paternò-Büchi reaction, utilizing 2-acpy, with liquid chromatography and tandem mass spectrometry (LC-MS/MS). This strategy was initially optimized on standard FFAs and subsequently applied to human sebum. The method enabled the characterization of seventeen MUFAs with C14-C18 chain lengths and two C18 polyunsaturated FAs, namely linoleic and sebaleic acid. The prevalence of sapienic acid (C16:1n-10) among MUFAs supports the dominance of the Δ6 desaturation pathway catalyzed by the FADS2 enzyme in human SG. Additionally, the assay proved comparable abundance of MUFAs and PUFAs in sebum from males and females.