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High D/L aspartic acid ratios are observed in heavy molecular weight aggregates and in water-insoluble protein extracted from whole lenses and nuclear and cortical regions. Purified a-, A-, and -y-crystallins have low D/L ratios. Fractionation of urea-solubilized material from the water-insoluble protein yields four molecular weight classes of proteins. Fractions representing crosslinked material or apparently degraded products have high D/L ratios. Racemization within lens proteins may contribute to formation of the water-insoluble fraction seen in aging lenses and cataracts. The racemization of aspartyl residues has recently been added to the list of age-related changes taking place in the proteins of the human lens (1). D-Aspartyl residues have been shown to accumulate at the rate of 1.25 X 10-3 yr-1 or 0.14% yr-1 in the central nucleus. The extent of racemization in yellow cataracts is equivalent to that observed in age-matched normal lenses, but brunescent cataracts exhibit up to a 2-fold increase in D/L aspartic acid ratios over comparably aged normal lenses or yellow cataracts.The rationale for investigating lens proteins came from our studies of calcified proteins in human teeth (2, 3). Detectable amounts of D-aspartic acid accumulate during the human lifespan as a result of the spontaneous chemical racemization reaction. These findings suggested that the aspartyl residues in any metabolically stable protein maintained at mammalian body temperature for several decades or longer would be subject to racemization. Because the proteins in the nucleus of the human lens are among the most stable in the body, we decided to test our hypothesis with the lens system.Numerous studies have shown that structural changes are taking place in lens proteins during the normal aging process and in cataracts. The proportion of water-insoluble protein increases relative to water-soluble protein (4, 5). The increase of insoluble protein is more marked in cataracts (6). When the water-insoluble residue is extracted with 7 M urea, the ureainsoluble portion is greater in cataracts (6) and is derived primarily from the lens nucleus (7). There is some evidence that more exposed thiol groups occur in cataractous proteins, and these proteins may have a greater susceptibility to tryptic digestion (8). All of these observations indicate that changes in native conformation of the lens proteins are occurring in cataract development and in aging.We have previously suggested that alterations in protein conformation will result from racemization of aspartyl residues (9). If racemization is to any degree responsible for the changes in the properties of lens proteins, then D-aspartic acid should

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