The molecular weight and the amino acid and carbohydrate compositions of free secretory component (SC)

Free secretory component was isolated from human milk whey using gel filtration and immunosorbent techniques. The purified protein was shown to be homogeneous in the ultracentrifuge and on cellulose acetate, polyacrylamide gel and immunoelectrophoresis.

From three different preparations the molecular weight was determined using Archibald's meniscus method. A mean value of 66,000±4500 was found. The S⁰_20,w value was calculated to be 4.3 S±0.17 and the D⁰₂₀ value was 5.66±0.08. From these data the molecular weight was calculated to be 67,000±3000.

The amino acid composition revealed the absence of methionine from the molecule.

The carbohydrate analysis showed the absence of N-acetylgalactosamine and a rather high fucose content. These results are discussed in relation to the carbohydrate composition of secretory IgA and serum IgA.