DEGRADATION OF DIFFERENTIALLY OXIDIZED ALPHA-CRYSTALLINS IN BOVINE LENS EPITHELIAL-CELLS

There is a growing consensus that altered proteins are more susceptibl e to degradation than native proteins. The enhancement of degradation of damaged proteins may be of significance since it prevents the accum ulation of damaged proteins in cells. Several proteolytic pathways hav e been discovered in the lens. These include ATP-independent, ATP-depe ndent and ATP/ubiquitin-dependent proteolytic pathways. However, the e xtent of involvement of these proteolytic pathways in degradation of d amaged proteins is not well described. alpha-Crystallin was oxidized b y exposure to 0.03-3.2 mol (OH)-O-. (mel protein)(-1). Modifications t o the oxidized alpha-crystallin and proteolytic susceptibility of the oxidized alpha-crystallin were studied. Exposure to > 0.32 mol(.)OH pe r mole of subunit produced aggregates and fragments of alpha-crystalli n. Changes in isoelectric points of the proteins were observed after e xposure to 0.64 mol (OH)-O-. (mel protein)(-1). The extent of loss of tryptophan and sulfhydryl groups was related to the level of (OH)-O-.- exposure. Carbonyl content increased progressively with increasing oxi dation. When incubated with a supernatant of bovine lens epithelial ce lls, the (OH)-O-.-modified proteins were proteolytically degraded up t o three times faster than untreated alpha-crystallin. ATP stimulated t he degradation of native alpha-crystallin and alpha-crystallin which w as exposed to 1.6 mol (OH)-O-. (mel subunit protein)(-1) (alpha 1.6). Sixty-seven per cent and 100% of the ATP-dependent degradation of nati ve alpha-crystallin and alpha 1.6 was ubiquitin-dependent, respectivel y. The data indicate that alpha-crystallins oxidized by O-. are recogn ized and degraded rapidly by cytoplasmic proteolytic systems in bovine lens epithelial cells. Both ATP-independent and ATP/ubiquitin-depende nt proteolytic pathways are involved in the degradation of native and oxidized alpha-crystallin. (C) 1995 Academic Press Limited