Biochemical characterization of crystallins from frog lenses

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Abstract
Lens crystallins were isolated from the homogenate of frog (Rana catesbeiana) eye lenses by gel permeation chromatography and characterized by gel electrophoresis, amino acid analysis and circular dichroism. Four well-defined fractions corresponding to .alpha./.beta.-, .beta.-, frog 39.5 kDa and .gamma.-crystallins comprising the relative weight percentages in the total soluble cytoplasmic proteins of 18%, 15%, 14% and 48% respectively were obtained. The native molecular masses for each purified fraction were determined to be 432, 207, 40 and 23 kDa, respectively. The polypeptide compositions as determined by SDS-gel electrophoresis revealed the typical subunit structures of mammalian crystallins with the exception of 39.5 kDa monomeric crystallin, which has not been shown in other classes of vertebrate lenses. The spectra of circular dichronism indicate a predominant .beta.-sheet structure in all four fractions, which also bears a resemblance to the secondary structure of mammalian crystallins. Comparison of the amino acid compositions of frog crystallins with those of mammalian and fish crystallins suggests that .gamma.-crystallin from the frog is more closely related to that of porcine than fish crystallins, and the frog 39.5 kDa, frog .beta.- and lamprey 48 kDa crystallins are probably mutually interrelated.