Human Epidermal Transamidase

The possible presence of ϵ-(γ-glutamyl)lysine covalent bonds in human epidermal proteins prompted a study of transamidase activity in human hair-free epidermis. Callus contains an enzyme which catalyzes the incorporation of radioactive putrescine into α-casein. The enzyme is active without prior treatment with exogenous proteolytic enzymes. The putrescine incorporation is calcium dependent and inhibited by iodoacetamide. The enzyme was partially purified (50-fold over starting material), and has an apparent molecular weight between 50,000 daltons and 55,000 daltons by agarose 0.5m gel filtration. The apparent molecular weight is unaltered by chromatography in the presence of 11 mm CaCl2, a condition known to dissociate plasma transglutaminase (Factor XIII) into its ultimate subunits. The enzyme is active over a wide pH range up to pH 10.4. The Km for putrescine varies by 1-fold over the pH range 6.0 to 10.2, although enzyme activity increases at least 20-fold over the same pH range. The human epidermal transamidase is similar to the guinea-pig hair follicle transglutaminase and cow snout transamidase in its ability to cross-link fibrin