The origin of hydroxy-cyclohexenone fatty acids from skin barrier protein and relevance to covalent binding of ceramides.

Lipid constituents of the skin permeability barrier include a portion of ceramides and fatty acids covalently bound to the barrier protein. The covalent binding requires enzymatic oxidation of linoleate (C18:2) esterified to skin-specific acylceramides, forming a reactive 9,10-epoxy-11E-13-keto derivative. Barrier proteins treated with alkali release the bound lipids and as described recently, including two prominent cyclic linoleate derivatives, C18 hydroxy-cyclohexenone fatty acids. Herein we addressed the origin of these cyclic products by alkali treatment of potential precursors. A UV-based assay indicated the rates of Michael adduction of 9,10-epoxy-11E-13-keto to cysteine are two orders of magnitude faster than for a typical unsaturated keto fatty acid, and 10-fold faster for the dihydroxy analog, rationalizing their biosynthesis for protein adduction. Alkali treatment degraded the epoxy-ketone and its cysteinyl (glutathione) adduct to multiple UV-absorbing products, although not including the hydroxy-cyclohexenones. By contrast, these derivatives were prominently produced from KOH treatment of the 9,10-dihydroxy-13-ketone or its glutathione adduct. As further evidence of the origin of the hydroxy-cyclohexenones, LC-MS quantitation showed a 90% reduction following KOH treatment of epidermis from mice deficient in Srd9c7, the dehydrogenase in the linoleate oxidation pathway. Taken together, the results confirm the hydroxy-cyclohexenones as derivatives of the linoleate oxidations in the skin barrier pathway and identify the dihydroxy-ketone as a component of the covalently-bonded lipids, and critical to integrity of the epidermal barrier.