Both the structural and chaperone-like properties of lens alpha-crysta
llins have been implicated in maintaining lens transparency. Modificat
ions of lens alpha-crystallins may lead to formation of cataract by af
fecting the close-packing of the crystallins or by reducing the chaper
one-like activity of the cc-crystallins. A previously unreported modif
ied alpha B-crystallin, whose molecular weight is 72 u greater than un
modified alpha B-crystallin, has been isolated from human lenses by si
ze exclusion chromatography, reversed phase HPLC and ion exchange HPLC
. Approximately one nanomole of this modified alpha B-crystallin was o
btained from each of five human eye lenses. Molecular weight determina
tions of peptides produced by digestion with trypsin or endoproteinase
Asp-N showed that the modification is in the C-terminal region of alp
ha B-crystallin. The fragmentation pattern of peptides from the C-term
inal region, analysed by tandem mass spectrometry, located the modific
ation of the E-amino group of the C-terminal lysine. The elemental com
position of this modification, determined from its exact mass, is C3H4
O2. Because this modification decreases the net charge of alpha B-crys
tallin by one unit, and because the C-terminus has been implicated in
the chaperone activity attributed to alpha B-crystallin, this modifica
tion at Lys 175 may have a significant role in cataractogenesis. (C) 1
997 Academic Press Limited.