The purpose of this study was to determine whether normal human epidermis could produce leukotriene B4 (LTB4) from leukotriene A4 (LTA4) ex vivo, and to localize this LTA4-hydrolase activity. Epidermis obtained by suction blister technique incubated with human polymorphonuclear cells, resulted in a 54% increase in LTB4 formation when compared to polymorphonuclear cells incubated alone. Furthermore, human epidermis transformed exogenous LTA4 into LTB4, and this reaction obeyed Michaelis-Menten kinetics with an apparent Km of 6 microM. Subcellular fractionation of homogenized epidermis localized the LTA4-hydrolase activity mainly in the 105,000 x g supernatant fraction (cytoplasmic fraction). This activity was inhibited by two inhibitors of LTA4-hydrolase (bestatin and captopril). Western blot analysis of the 105,000 x g fraction of homogenized epidermis and cultured keratinocytes supported the presence of a LTA4-hydrolase. Thus, normal human epidermis possesses LTA4-hydrolase activity which can transform exogenous LTA4 and polymorphonuclear cell-derived LTA4 into LTB4. The identification of LTA4-hydrolase in the cytoplasmic fraction of human epidermis indicates that epidermal cells may play a more active role in the enzymatic process leading to formation of the proinflammatory compound LTB4 than previously expected.