Cataracts are a leading cause of blindness throughout the world and are the third leading cause of legal blindness in the United States. The formation of water-insoluble covalently-crosslinked protein aggregates appears to be a major cause of the lens opacification which occurs in the disease. A protein polymer characteristically found in human cataracts recently was shown to contain significant amounts of epsilon (gamma - glutamyl) lysine isopeptide bonds. These linkages are formed by transgluaminases and are typically involved in cross-linking proteins into large molecular weight complexes. The overall goal of our research is to define the molecular processes leading to lens opacification during cataract formation. To provide a basis for better understanding these pathological processes, the more specific goal of this pilot project will be to define the role of the lens transglutaminase in normal tissue. Since many tissues have both particulate and soluble transglutaminases, the distribution of the transglutaminase in lens will be defined by subcellular fractionation procedures. The relationship of the lens transglutaminases to each other, if there is more than one, and to the liver epithelial transglutaminases, which have been well characterized, will be determined by comparing the kinetic and physical properties of the enzymes. Endogenous transglutaminase substrates and products in lens will be identified by enzyme- mediated labeling procedures and by isolating covalently crosslinked protein complexes. The presence of fibronectin in the large molecular weight complex and its role as a transglutaminase substrate in lens will also be investigated. These studies on the role of the transglutaminase in normal lens will serve as a basis for future investigations into the pathological role of the enzyme during cataract formation.
