Isolation of free secretory component (S.C.) from human milk, determination of its molecular weight

Tomasi et al. (1968) isolated human secretory component (S.C.) after reduction and alkylation of 11 S secretory IgA. In an ultracentrifugal study a molecular weight of 58, 000 has been found for this protein. We investigated free S.C. as it occurs in human milk. The accurate determination of the elution volume after Sephadex G 200 gelfiltration revealed a molecular weight of 120, 000 suggesting the occurrence of a dimer form of the native molecule. A reproducible method has been developed to prepare monospecific rabbit anti S.C. antisera which dont require any absorption. Isolation of free S.C. has been performed using insolubilised anti S.C. antiserum as an immunoadsorbent. Contamination with lactoferrin, which is a big problem in the purification of free S.C., could be prevented by selective adsorption to insolubilised albumin. The isolated free S.C. was found to be homogenous in polyacrylamide gelectrophoresis, in the ultracentrifuge and in immunoelectrophoresis. A sedimentation coefficient S 20, ω 0 = 4.5 has been found. The molecular weight determined by the Archibald method amounted to 74.000 ± 6000. The resulting data have been discussed. The conclusion can be drawn that free S.C. occurring in human milk is not a dimer form of the S.C. which is obtained after reductive cleavage of 11S secretory IgA.