Physicochemical characterization of lens crystallins from the carp and biochemical comparison with other vertebrate and invertebrate crystallins

Lens crystallins were isolated from the homogenate of carp (Cyprinus carpio) eye lenses by gel permeation chromatography and characterized by gel electrophoresis, immunodiffusion, amino acid analysis, circular dichroism, and protein sequence analysis. Three well-defined fractions corresponding to α/β-, β-, and γ-crystallins were obtained in relative weight percentages of 26, 22, and 52%. The native molecular masses of the purified fractions were determined to be 410, 60, and 20 kDa, respectively. The polypeptide compositions as determined by SDS gel electrophoresis revealed the substantial presence of β-crystallin polypeptides in the α-crystallin fraction; this is also evident in the fractionation of amphibian crystallins but is not common in the case of higher classes of vertebrates. The circular dichroism spectra indicate a predominant β-sheet structure in all three fractions, albeit with some contribution of α-helical structure in the γ-crystailin, the amino acid composition of which bears a resemblance to that of squid crystallin. Sequence comparison of carp γ-crystallin with frog and calf γ-crystallins indicates a high degree of homology in their N-terminal segments despite the dissimilarity of amino acid compositions and weak immunological cross-reactivity.