NOVEL INHIBITORS AGAINST THE TRANSGLUTAMINASE-CATALYZED CROSS-LINKINGOF LENS PROTEINS

Post-translational modifications by transglutaminase may contribute to the remodeling of cellular architecture in the development of lens fi ber cells, and there is evidence that the enzyme may also play a role in cataract formation. It catalyses hydrolytic deamidations as well as amide exchanges on select glutamine side chains at endo positions in a small subset of proteins of the lens. N-epsilon(gamma-glutamyl)lysin e crosslinks, the characteristic hallmarks of transglutaminase activit y, were identified in polymers isolated from human cataract. Following up on our earlier studies relating to the inhibition of protein cross linking by the Ca2+-activated transglutaminase in the lens, we have no w examined the effects of 2-[(2-oxopropyl)thio]imidazolium derivatives , recently described as active site-directed inhibitors for this famil y of enzymes. First, we have shown that the compounds at concentration s of 1-2 mu M were effective in blocking the transamidating activities of partially purified lens transglutaminase. Then we focused on their efficacy in preventing the formation of the ca. 55 kDa beta crystalli n dimers in the whole lens tissue. The production of these dimers, cro sslinked by N-epsilon(gamma-glutamyl)lysine isopeptide bridges, is an early sign of transglutaminase action in rabbit lens, and it can be re adily documented by the SDS-PAGE analysis of proteins remaining in the soluble phase after brief exposure of the homogenate to Ca2+. The new compounds proved to be potent inhibitors of transglutaminase also in this preparation, preventing the crosslinking event at ca. 1 mu M conc entration. Moreover, even when applied at a 1,000-fold greater concent ration (2 mM), they did not interfere with the action of calpain which , similarly to the activation of the transglutaminase system, is trigg ered by the addition of Ca2+. The high selectivity of the new compound s for differentially blocking only the transglutaminase and not the ca lpain of the lens, is all the more remarkable because these two enzyme s share several mechanistic and structural similarities. (C) 1998 Acad emic Press Limited.