Characterization of lens proteins. III. A group of labile rat lens proteins involved in cataracts

Abstract In the fractionation of soluble rat lens proteins by Sephadex G-200, superfine, a group of labile proteins, designated F-II, has been obtained. The soluble protein of normal rat lens contains 17% F-II which is not found in that of severe cataract (5+) lens induced by galactose feeding. F-II is prone to irreversible precipitation by a variety of laboratory techniques, such as concentration, lyophilization and freeze-thaw. The apparent molecular weight of the F-II has been estimated to be 240 000 daltons from the position of its elution from the Sephadex G-200, superfine, column. The amino acid composition of the F-II has been found similar but not identical to the β 1 fraction of the β-crystallin. F-II consists of a number of polypeptides with slight variations in surface charge having pI in the range pH 6·3–7·1, distinct from other crystallins. The extinction coefficient of F-II was E 280 nm 1% = 19·4. The amino terminals of the fraction appeared to be blocked, but analyses with carboxypeptidase Y indicated the presence of a carboxyl terminal sequence, -(Val, Ile)-Tyr-Phe-Leu-Ser(OH). When the F-II was further fraction-ated on DEAE-cellulose, the first four major subfractions were found in regions where rat β-crystallin fractions were eluted. The last major subfraction was eluted in the α-crystallin region. All fractions appear to be closely related in structure as they showed nearly identical amino acid composition and identical carboxyl terminal. By a freeze-thaw process, F-II and β-crystallin fractions have shown various degrees of precipitation but the α- and γ-crystallin fractions are unaffected. Attempts are in progress to determine the relationship of this group of labile proteins to the “cold precipitable protein” for the purpose of understanding the cold cataract phenomenon.