Lemon sharks immunized with bovine serum albumin produced two molecular forms of antibodies detectable by passive hemagglutination of antigen-coated, tanned sheep erythrocytes. Throughout the course of immunization 2-ME-sensitive antibody was associated with a 19S immunoglobulin fraction (4–5 mg/ml serum) while late in the course of immunization antibody was found also associated with a 7S immunoglobulin fraction (7–8 mg/ml serum). No evidence for any anamnestic response was found in these animals. Naturally occurring hemagglutinins for sheep erythrocytes were found to be 2-ME-sensitive and present in the 19S immunoglobulin fraction.
These immunoglobulin fractions were readily purified by DEAE-cellulose chromatography and Sephadex G-200 gel filtration. Both immunoglobulin molecules yielded equimolar amounts of H and L polypeptide chains when subjected to extensive reduction and alkylation followed by gel filtration in 5 M guanidine-HCl. Antigenically reactive H and L chains were obtained by partial reduction and alkylation followed by gel filtration in 1 M propionic acid. The 7S and 19S immunoglobulin H chains were indistinguishable by fingerprints of tryptic digests, disc electrophoretic patterns, antigenic properties, and mass (molecular weight ∼70,000), thus suggesting these two molecules to belong to the same immunoglobulin class. The shark 19S and 7S immunoglobulin L chains were indistinguishable from each other by similar criteria and were different from the H chains. These L chains exhibited the electrophoretic heterogeneity of their mammalian counterparts.
The 7S (shark immunoglobulin) molecule was shown to have a molecular weight of ∼160,000 and to consist of 2H and 2L polypeptide chains (total mass ≅180,000). The 19S molecule was shown to have a molecular weight of 800,000–900,000; therefore, there were probably five 7S subunits per 19S molecule, comparable to mammalian γM. Other reasons for considering the 7S and the 19S lemon shark molecules to belong to a class of immunoglobulins comparable to the γM class of mammals are that they both have high carbohydrate contents, and H chains of mass similar to µ chains.
The lemon shark serum proteins with electrophoretic mobilities comparable to gamma G of mammals were not related to the immunoglobulins of this species. These proteins had no antibody activity and had no antigenic or chemical similarity to either the H chains, the L chains, or the intact immunoglobulin molecules from the lemon shark.