Biochemistry of transglutaminases and cross-linking in the skin

Transglutaminase is a calcium-dependent enzyme found widely in nature. It catalyzes the formation of epsilon-(gamma-glutamyl)lysine bonds that participate in processes varying from fibrin clot formation to epidermal cell envelope formation. Epidermal transglutaminase is localized to the granular layer of the epidermis. It catalyzes the covalent cross-linking of a soluble cytoplasmic substrate into large polymers to form the cornified envelope that lines the inner membrane of keratinocytes in the stratum corneum. The soluble precursor from epidermis has been named keratolinin, and from keratinocyte culture, it has been named involucrin. Hair follicle transglutaminase is biochemically and immunochemically distinct from its epidermal counterpart. It has been localized to the inner root sheath and medulla of the hair follicle. The substrate of hair follicle transglutaminase has been poorly defined but appears to be rich in the amino acid citrulline. Transglutaminase has been shown to be an important marker of normal differentiation. There is a rise in its activity at the time of keratinization, and transglutaminase activity has been shown to be greatly decreased in basal cell epithelioma and in psoriasis. Keratinocyte cell culture has proven most helpful in delineating the processes of normal differentiation and keratinization, since the formation of the cell envelope in culture appears to parallel the formation in vivo.