Phylogeny of Immunoglobulin Structure and Function

Giant grouper serum was shown to contain 16 S (∼3 mg per ml of serum) and 6.4 S (∼6 mg per ml of serum) immunoglobulins. The 16 S immunoglobulin had a molecular weight of ∼700,000, a relatively high hexose content, and was composed of approximately equimolar amounts of H and L polypeptide chains. It is suggested that this molecule resembles immunoglobulin M on the basis of polypeptide chain properties but that it is most likely a tetramer instead of the typical 19 S pentamer of immunoglobulin M. The 6.4 S immunoglobulin appeared structurally to represent a fragment of the 16 S molecule; the major difference being that the 6.4 S H chain was missing ∼30,000 daltons. In attempting to fit the class Osteichthyes into a scheme of vertebrate immunoglobulin evolution, it was suggested that the primitive form of polymeric immunoglobulin M may be a tetramer of subunits held together by both disulfide and noncovalent bonds.