The epidermis expresses cyclooxygenases, lipoxygenases, and cytochromes P450, 
which utilize arachidonic acid to generate a diverse array of lipid mediators 
affecting epidermal cellular differentiation and functions. Recent studies show 
that mouse epidermis expresses CYP2B19, a keratinocyte-specific epoxygenase that 
generates 11,12- and 14,15-epoxyeicosatrienoic (EET) acids from arachidonate. We 
studied CYP2B19-dependent metabolism in mouse epidermal microsomes, 
reconstituted in the presence of [1-(14)C]arachidonic acid. The majority of the 
(14)C products formed independently of NADPH, indicative of robust epidermal 
cyclooxygenase and lipoxygenase activities. We studied two NADPH-dependent 
products generated in a highly reproducible manner from arachidonate. One of 
these (product I) coeluted with the CYP2B19 product 14,15-EET on a 
reversed-phase high-performance liquid chromatography (HPLC) system; there was 
no evidence for other regioisomeric EET products. Further analyses proved that 
product I was not an epoxy fatty acid, based on different retention times on a 
normal-phase HPLC system and failure of product I to undergo hydrolysis in 
acidic solution. We analyzed purified epidermal (14)C products by liquid 
chromatography negative electrospray ionization mass spectrometry. Structures of 
the NADPH-dependent products were confirmed to be 12-oxo-5,8,14-eicosatrienoic 
acid (I) and 12-hydroxy-5,8,14-eicosatrienoic acid (II). This was the first 
evidence for a 12-hydroxy-5,8,14-eicosatrienoic acid biosynthetic pathway in 
mouse epidermis. Epidermal microsomes also generated 12-hydroperoxy, 12-hydroxy, 
and 12-oxo eicosatetraenoic acids from arachidonate, possible intermediates in 
the 12-hydroxy-5,8,14-eicosatrienoic acid biosynthetic pathway. These results 
predict that hydroxyeicosatrienoic acids are synthesized from arachidonate in 
human epidermis. This would have important implications for human skin diseases 
given the known pro- and anti-inflammatory activities of stereo- and 
regioisomeric hydroxyeicosatrienoic acids.