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Background  During the late stage of keratinocyte differentiation, corneocytes gain a strong protein–lipid structure: the corneocyte envelopes ( CE ), composed of the inner corneocyte protein envelope ( CPE ) and the outer corneocyte lipid envelope ( CLE ). The hydrophobicity of  CE s depends on the covalent attachment of linoleoyl‐acyl‐ceramides by transglutaminases ( TG ). These ceramides are processed by a range of other enzymes, including 12R‐lipoxygenase (12R‐ LOX ), before the covalent attachment of the free  ω ‐hydroxyceramides to the  CPE  surface to form the  CLE . The mechanical strength of  CE  is obtained with the formation of isodipeptide bonds by TG. The increase in hydrophobicity and rigidity leads to  CE  maturation which supports the integrity and mechanical resistance of the stratum corneum ( SC ).    Objectives  The aim of this work was to develop and validate a novel enzyme activity assay for 12R‐ LOX  in tape strippings of photo‐exposed ( PE ) cheek and photo‐protected ( PP ) post‐auricular  SC  of healthy Chinese volunteers ( n  = 12; age 25 ± 3 years).    Results  A fluorescence‐based assay was developed with ethyl linoleic acid as the substrate and a polyclonal antibody against 12R‐ LOX  as an inhibitor. The specificity was shown by the lack of effect by a  LOX  inhibitor ( ML 351) and an epidermal‐type lipoxygenase 3 ( eLOX 3) antibody on the acquired 12R‐ LOX  activity. Reduced 12R‐ LOX  activity was observed in the outer compared to the inner  SC  layers. Moreover, dramatically lower activity was shown in the  PE  vs.  PP  samples. Furthermore, the enzyme activity has a positive correlation ( r  = 0.94 ± 0.03) with  CE  maturity, in particular hydrophobicity, and a negative correlation ( r  = −0.96 ± 0.01) with transepidermal water loss ( TEWL ).    Conclusion  This novel enzyme assay revealed a lower 12R‐ LOX  activity in tape strippings from  PE  cheek for the first time. This finding is in line with less mature  CE s and higher  TEWL  compared to  PP  post‐auricular samples. This study indicates a strong link between 12R‐ LOX  activity and  CE  maturation and  SC  integrity.

12R‐lipoxygenase activity is reduced in photodamaged facial stratum corneum. A novel activity assay indicates a key function in corneocyte maturation