The evolutionary sequence and quantities of different antigenic determinants of calf lens alpha crystallin

Immunochemieal cross-reactions between calf lens alpha crystallin and homologous proteins from different vertebrate classes were used for the study of antigenic determinants which, in the evolution of this protein, were derived from common ancestral forms.

Determination of the sequence in which the different antigenic determinants evolved was based on estimation of the relative number of antigenic determinants shared by calf alpha crystallin with alpha crystallin from other vertebrate classes. For this estimation, samples of antiserum to calf lens alpha crystallin were absorbed with homologous lens proteins from each of the different vertebrate classes.

The unabsorbed antiserum reacted with homologous proteins from all vertebrate classes. Each of the absorbed antisera showed a taxonomically different extent of residual cross-reaction. The species used for absorption were then ordered according to an increasing taxonomic extent of residual cross-reactions of the absorbed anti-calf antiserum, indicating a sequence of species with decreasing numbers of antigenic determinants shared with the mammalian protein. This sequence represents the increasing phylogenetic distances between the species used for absorption and the species used to produce the antiserum. This immunochemical determination showed that the phylogenetic sequence in which antigenic determinants present in the calf alpha crystallin molecule were formed, progressed from the primitive aquatic vertebrates agnatha, elasmobranchii and actinopterygii to the primitive land vertebrates and then to amphibia, reptilia, aves and mammalia.

For the estimation of the relative amount (%) of determinants derived from different ancestral forms and transferred in evolution to calf alpha crystallin,¹²⁵I bound to a calf alpha crystallin immunoadsorbent after saturation with total¹²⁵I anti-calf alpha crystallin Fab fragments was compared to¹²⁵I bound to the same amounts of calf alpha crystallin after saturation with¹²⁵I Fab fragments of phylogenetically restricted specificity. Of all antigenic determinants in present-day calf alpha crystallin. approximately 42% were found also in the homologous lamprey protein. These determinants originated with the primitive agnatha which started the vertebrate subphylum 450 million years ago. Only about 15% originated with the mammalia, some 200 million years later, rellecting the slow evolutionary change of this protein molecule.