PURIFICATION AND CHARACTERIZATION OF THE HUMAN EPIDERMAL FATTY-ACID-BINDING PROTEIN - LOCALIZATION DURING EPIDERMAL-CELL DIFFERENTIATION IN-VIVO AND IN-VITRO

Epidermal fatty acid-binding protein (E-FABP) was isolated from human skin and purified to homogeneity. Its molecular mass was estimated to be 15 kDa and the pi of non-denaturing protein was 5.6. Scatchard-plot analysis revealed one class of binding site for oleic acid with a K-d of 0.46 mu M. Structure-binding relation experiments revealed a high affinity of E-FABP for stearic acid which decreased on reduction of th e number of carbon atoms or introduction of double bonds into the fatt y acid chain. Squalene, cholesterol and retinoic acid isomers showed n o affinity, suggesting that E-FABP displays high specificity for fatty acids. E-FABP is a scarce cytosolic protein (0.065% of total protein) . Only trace amounts could be detected in normal human skin but up to 42.5 +/- 3.4 pmol/mg of protein was found in a non-malignant defect of keratinocyte differentiation (psoriatic lesions). E-FABP levels were low in cultured human keratinocytes grown under proliferation-stimulat ing conditions but increased about 2-fold on induction of differentiat ion by Ca2+. Immunohistochemical localization showed cytosolic stainin g in differentiated cells of normal and psoriatic skin, suggesting a l ink between E-FABP and keratinocyte differentiation. The presence of E -FABP in tissues other than skin (heart, intestine and adipose tissue) excludes its specific role in fatty acid metabolism in epithelial cel ls or its involvement in skin lipid-barrier function.