THE HYDROXYL RADICAL IN LENS NUCLEAR CATARACTOGENESIS

Cataract is the major cause of blindness; the most common form is age- related, or senile, cataract, The reasons for the development of catar act are unknown, Here we demonstrate that nuclear cataract is associat ed with the extensive hydroxylation of protein-bound amino acid residu es, which increases with the development of cataract by up to 15-foId in the case of DOPA, The relative abundance of the oxidized amino acid s in lens protein (assessed per parent amino acid) is DOPA > o- and m- tyrosine > 3-hydroxyvaline, 5-hydroxyleucine > dityrosine, Nigrescent cataracts, in which the normally transparent lens becomes black and op aque, contain the highest level of hydroxylated amino acids yet observ ed in a biological tissue: for example, per 1000 parent amino acid res idues, DOPA, 15; 3-hydroxyvaline, 0.3; compared with dityrosine, 0.05, The products include representatives of the hydroperoxide and DOPA pa thways of protein oxidation, which can give rise to secondary reactive species, radical and otherwise. The observed relative abundance corre sponds closely with that of products of hydroxyl radical or metal-depe ndent oxidation of isolated proteins, and not with the patterns result ing from hypochlorite or tyrosyl-radical oxidation, Although very Litt le light in the 300-400-nm range passes the cornea and the filter comp ounds of the eye, we nevertheless also demonstrate that photoxidation of lens proteins with light of 310 nm, the part of the spectrum in whi ch protein aromatic residues have residual absorbance, does not give r ise to the hydroxylated aliphatic amino acids. Thus the post-translati onal modification of crystallins by hydroxyl radicals/Fenton systems s eems to dominate their in vivo oxidation, and it could explain the kno wn features of such nuclear cataractogenesis.