Vertebral epidermal transamidases

Abstract

The α-helical fibrous proteins (keratin) of the stratum corneum undergo changes in their physicochemical properties which are not solely explained on the basis of disulfide bond formation. The precursor of the stratum corneum protein, prekeratin, and the final product keratin were studied in the hair-follicle-free skin of calf snout epidermis. All of the lysines of prekeratin can be cyanoethylated while keratin molecules has 7–8 nmoles lysine per mg protein not available to cyanoethylation. Keratin from the human stratum corneum has 8–9 nmoles/g, protein which cannot be cyanoethylated. Direct examination of these proteins failed to detect the presence of N- ε-acetyllysine groups or ε-(γ-glutamyl)lysine groups as the explanation of the blocked lysines. The hair-follicle-free epidermis of several vertebrate species contains transamidases similar to the transglutaminases which form ε-(γ-glutamyl)lysine crosslinks, in fibrin, hair follicle, and seminal fluid. The epidermal transamidase has a molecular weight between 40 000 and 60 000, is stimulated by reducing agents and calcium, and has most of its activity in the highermost levels of the epidermis. A semipurified preparation of the epidermal transamidase was able to form γ dimers and α polymers from Factor XIII-free fibrin. A transamidase substrate, [1,4- 3H 2]putrescine, when injected into mouse skin and studied autoradiographically, was localized in the stratum corneum. [1,4- 14C 2]putrescine incorporation into epidermal proteins was inhibited by 50 mM iodoacetamide. The incorporation of [1,4- 14C 2]putrescine was highest in very insoluble protein fractions. The exact role of transamidases in epidermal metabolism is not yet identified.