Keratin cross-linking and epidermal transglutaminase. A review with observations on the histochemical and immunochemical localization of the enzyme.

Enzymes known as transglutaminases mediate cross-linking of polypeptide chains by epsilon-(gamma-glutamyl) lysine bonds. Such bonds stabilize structural proteins of many tissues; transglutaminases specific for these tissues have been identified. A calcium- and sulfhydryl-dependent transglutaminase with a molecular weight of 55,000 has been purified from bovine snout epidermis and used to elicit a specific antiserum to the enzyme. Sites of epidermal transglutaminase activity have been localized in the cytoplasm of upper malpighian and granular cells by two complementary methods. When thin-tissue sections were incubated with a fluorescent lysine analog(dansyl cadaverine) and calcium, tissue acceptor sites became fluorescent. Localization was confirmed by fluorescein-conjugated antibody labeling of the enzyme in situ. These observations indicate that epidermal transglutaminase cross-links epidermal proteins during the final stages of keratinization.