Structural studies of α-crystallin

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Abstract
[alpha]-Crystallin has been isolated from the cortex of ox lens by isoelectric precipitation followed by chromatography on DEAE-cellulose. The amino acid composition is in agreement with that reported for [alpha]-crystallin prepared by a different method. There is one thiol group/20,000 g. of protein (20,000 is the order of magnitude of the sub-unit molecular weight), and disulphide bonds are absent. The thiol group has been alkylated with radioactive iodoacetate in the presence of urea. Partial acid hydrolysis of the alkylated protein gives, according to the conditions, mainly three radioactive peptides or nearly exclusively one radioactive dipeptide. The dipeptide is N-seryl-(S-carboxymethyl)cysteine, Ser-CMCys. The two other peptides are probably the tripeptides related to Ser-CMCys. The simplest interpretation of these results is that the sequence around the cysteine residue is a common structural feature of the sub-units of [alpha]-crystallin.