Age-dependent changes in the heat-stable crystallin, beta Bp, of the human lens

The present study examined the effects of aging and cataractogenesis on the biochemical properties of the uniquely heat-stable lens crystallin, beta basic principle polypeptide (beta Bp). Using the techniques of SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and Western-blot immunoassay, we analyzed cortical and nuclear lens sections from normal lenses of individuals aged 0 to 91 years for beta Bp content in the soluble fraction, and retention of heat stability with aging. In addition, we compared the characteristics of beta Bp in cataractous lenses with those of normal lenses of approximately the same age. While beta Bp is synthesized in new cortical cells throughout life, the beta Bp of the nucleus, which had been laid down early in life, decreased significantly in both absolute concentration and in its proportion of the total soluble protein fraction during the normal aging process. In addition, posttranslational changes in the nuclear soluble beta Bp result in a gradual loss of approximately 3000 d in the apparent mass of the beta Bp molecule; i.e., as a result of the aging process, the single heat-stable band of an apparent mass of 26 kd on SDS-PAGE of the young lens nucleus becomes two heat-stable bands, one at 26 kd and one at 23 kd. Normal lenses up to 91 years of age always retain some of the 26 kd subunit, whereas lenses with severe nuclear cataracts had only the 23 kd subunit in the soluble fraction.