CHEMICAL MODIFICATIONS AND DISSOCIATION CHARACTERISTICS OF TYROSINE AND TRYPTOPHAN RESIDUES IN ALPHA-CRYSTALLIN

A quantitative estimation of surface accessibility of aromatic residue s in cr-crystallin from goat lens has been accomplished by chemical mo difications using different specific reagents having varying sizes. Re sults of modification of tyrosine residues with N-acetylimidazole and tetranitromethane when combined with those of ionization studies carri ed out with hydroxyl ions having the smallest size reveal different cl asses of tyrosine residues in the native protein: 78+/-2 residues have been found to be easily available for modification; among the rest, 9 4+/-2 residues appear to be comparatively less exposed to the reagents while 28+/-2 residues are found to be completely unavailable for modi fication in the native protein and are modified only when the protein is denatured. Modification of tryptophan residues with H2O2 also indic ates different classes of these residues available for oxidation at di fferent concentrations of the oxidant. 34+/-2 residues of tryptophan a re found to be easily oxidized at a lower concentration of H2O2 during the first phase of the reaction. The remaining tryptophan residues ap pear to be less exposed to the reagent. This is also corroborated from the studies of reactivities of these residues towards another specifi c but bulkier reagent, 2-hydroxy-5-nitrobenzyl bromide. These surface exposed aromatic residues in cr-crystallin may be considered to be vul nerable to in vivo oxidative modifications forming insoluble aggregate s which may finally contribute to the formation of cataract.