Enhanced C-terminal truncation of alphaA- and alphaB-crystallins in diabetic lenses.
OBJECTIVE
To investigate the influence of diabetes on the cleavage of C-terminal amino acid residues of alphaA- and alphaB-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 (alphaH)- and low (alphaL)-molecular-weight fractions were analyzed by electrospray ionization mass spectrometry.


RESULTS
A typical mass spectrum of alphaA-crystallin from human lenses showed intact unmodified alphaA-crystallin, truncated alphaA(1-172), and monophosphorylated alphaA-crystallin. Diabetic lenses showed nearly twofold higher levels of alphaA(1-172) than did the control lenses. Also, the alphaH fraction consistently showed significantly higher levels of alphaA(1-172) than the alphaL fraction. Human alphaB-crystallin showed no evidence of C-terminal truncation. Rat alphaA-crystallin had five C-terminal-truncated components, most of which showed substantial increases in diabetes. Truncated alphaA(1-162) appeared only in the diabetic rat lenses, suggesting specific activation of m-calpain in diabetes. alphaB-crystallin had only one C-terminal-truncated component, alphaB(1-170), 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 alpha-crystallins appear to contribute to an increased level of the alphaH fraction generally present in diabetic lenses. Loss of alphaA-crystallin chaperone activity seems to be related to truncation of the C-terminal amino acid residues.
D000368:Aged; D000818:Animals; D003920:Diabetes Mellitus / Q000378:metabolism*; D003921:Diabetes Mellitus, Experimental / Q000378:metabolism*; D006801:Humans; D007908:Lens, Crystalline / Q000378:metabolism*; D008875:Middle Aged; D010446:Peptide Fragments / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012016:Reference Values; D021241:Spectrometry, Mass, Electrospray Ionization; D038201:alpha-Crystallins / Q000378:metabolism*
D010446:Peptide Fragments; D038201:alpha-Crystallins