Structure and stability of the dityrosine-linked dimer of gamma B-crystallin

Oxidative damage to proteins leads to a variety of modifications such as ra cemization, carbonyl compound formation, new fluorophores, aggregation, cro sslinking and insolubility, several of which are markers of pathogenesis. A particular modification that has been associated with abnormal and patholo gical situations is the dityrosine crosslink in proteins, thought to be res ponsible for the reduced solubility and elasticity of proteins, and plaque formation. Dityrosine crosslinking has been suspected to occur in the cryst allins of the eye lens during cataract. We focus attention here on the gene ration, structure and conformational stability of such a dityrosine-linked protein of the eye lens. We find this crosslink to be readily generated pho todynamically in the presence of sensitizers. Among the crystallins, crossl inking occurs most readily in the gamma-crystallins under these conditions. We have isolated, purified and studied the properties of the dityrosine-li nked dimer of the eye lens protein gamma B-crystallin. While the dityrosine crosslink does not alter the secondary structure of the protein, it change s the tertiary structure in a subtle manner. This alteration destabilizes t he dimer, which denatures more readily than the parent monomer, and also ma kes it precipitate more readily, a point of relevance to cataractogenesis o f the eye lens, Comparison of these results with those reported on other di tyrosine-dimerized proteins suggests that while the conformation of these p roteins might not be altered in a major manner upon dityrosine linkage, the dimer is structurally less stable and displays reduced solubility, both of which are of pathological importance. (C) 1999 Academic Press.