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A- and B-Crystallins in Diabetic Lenses
1 From the Department of Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, Little Rock, Arkansas; and the 2 Department of Chemistry, University of Nebraska, Lincoln, Nebraska.
PURPOSE. To investigate the influence of diabetes on the cleavage of C-terminal amino acid residues of 
A- and B-crystallins in human and rat lenses.
METHODS. The human lenses were diabetic or age-matched control lenses from donors 57, 59, 69, and 72 years of age. Lenses were also obtained from streptozotocin-induced diabetic rats. Individual lens crystallins in water-soluble fractions were separated by gel-permeation chromatography. The high (H)- and low (L)-molecular-weight fractions were analyzed by electrospray ionization mass spectrometry.
RESULTS. A typical mass spectrum of A-crystallin from human lenses showed intact unmodified A-crystallin, truncated A1-172, and monophosphorylated A-crystallin. Diabetic lenses showed nearly twofold higher levels of A1-172 than did the control lenses. Also, the H fraction consistently showed significantly higher levels of A1-172 than the L fraction. Human B-crystallin showed no evidence of C-terminal truncation. Rat A-crystallin had five C-terminal–truncated components, most of which showed substantial increases in diabetes. Truncated A1-162 appeared only in the diabetic rat lenses, suggesting specific activation of m-calpain in diabetes. B-crystallin had only one C-terminal–truncated component, B1-170, which also showed increased levels in diabetes.
CONCLUSIONS. These data suggest that diabetic stress causes either enzymatic or nonenzymatic cleavage of peptide bonds between specific C-terminal amino acid residues. Such truncated -crystallins appear to contribute to an increased level of the H fraction generally present in diabetic lenses. Loss of A-crystallin chaperone activity seems to be related to truncation of the C-terminal amino acid residues.
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