Racemization in human lens: evidence of rapid insolubilization of specific polypeptides in cataract formation

After early life, the dry weight of normal human lenses increases at a relatively constant rate with time. Transformation from soluble to insoluble material appears to occur at a comparable rate, resulting in a constant amount of soluble material. However, in cataract the insolubilization rate is accelerated. These observations are supported by determination of D-aspartic acid/L-aspartic acid ratios. The abundance of D-aspartic acid increases with aging at a constant rate in the insoluble fraction of normal lenses but does not change in the soluble fraction. However, in cataractous lenses there is a significant decrease in the ratio in the insoluble fraction. Examination of polypeptides isolated from reduced and alkylated soluble and insoluble cataractous lens proteins as well as other data suggest the following additional conclusions: (i) the 10,000-dalton polypeptide in the insoluble fraction is derived in part from degradation of an already insoluble precursor; and (ii) the lowered abundance of D-aspartic acid in the insoluble fraction of cataractous lenses is primarily due to the rapid insolubilization of the 43,000- and 20,000-dalton range components.