The Effect of Diabetes and Dietary Ascorbate Supplementation on the Oxidative Modification of Rat Lens βL Crystallin

The level of characteristic markers of protein oxidative modification (tryptophan oxidation and sulfhydryl group loss as well as carbonyl and bityrosine formation) and glycation (AGEP formation) have been measured in β_L crystallin purified from the lenses of control, diabetic, and ascorbate-supplemented diabetic animals. These markers were also determined following the application of an in vitro graded oxidative insult. Prior to the application of stress, diabetic lens crystallins, in comparison with control, exhibited a higher content of bityrosine and AGEPs, a lower level of nonoxidized tryptophan, and a loss of sulfhydryl groups. After exposure to the oxidative insult there was a stress-proportional increase of the parameters in all β_L crystallins, irrespective of their source. The effects were most pronounced in the diabetic, in which the already-elevated indicators of oxidative damage were further increased. Dietary supplementation of the diabetic group with ascorbate had a marked effect in preventing β_L crystallin modification in vivo, alleviating the loss of sulfhydryl groups and the oxidation of tryptophan, partially preventing the formation of AGEP and carbonyl groups, but not affecting the formation of bityrosine. Supplementation also inhibited the increase in susceptibility of diabetic β_L crystallin to in vitro oxidative stress, preventing sulfhydryl group loss as well as carbonyl and AGEP group formation. The results are discussed in relation to the proposal that diabetes renders lens crystallins more susceptible to oxidative stress and that this may be a causative factor in cataractogenesis. The possible role of ascorbate in the inhibition, or attenuation, of cataractogenesis is examined.