Nonenzymatic glycosylation of bovine lens crystallins. Effect of aging.

We have investigated nonenzymatic glycosylation of crystallins from calf and mature bovine lenses (2-6 years old). The lens homogenates were treated with 200-fold molar excess of [3H]NaBH4 and the incorporation of radioactivity was determined. The extent of glycosylation was more precisely determined from the 6 N HCl hydrolysate of [3H]borohydride-treated proteins by analyzing the glucitol-lysine adduct on a high pressure cation exchange column. We found that the [3H]NaBH4 incorporation and the amount of glucitol-lysine detected increased with age, particularly in HM alpha crystallin, a high molecular weight aggregate which accumulates with aging. This age-related increase in nonenzymatic glycosylation was also demonstrated by a comparison of crystallins isolated from the cortex and nucleus of a single lens. Nonenzymatic glycosylation of lens crystallins exemplifies a new form of post-transitional modification of long-lived proteins in vivo.