A peculiar immunoglobulin M (IgM) identified in eggs of chum salmon (Oncorhynchus keta)

A protein reacting with antibody against chum salmon serum IgM was found in salmon egg yolk. The protein tentatively named IgM-like protein was partially purified from egg yolk extract by removal of euglobulin from the extract by dialysis against a low ionic strength buffer following DEAE ion-exchange chromatography. The IgM-like protein was antigenically identical with serum IgM, showing a complete fusion of precipitin line of the IgM-like protein with that of serum IgM on double immunodiffusion with antiserum IgM. The molecular weight of intact IgM-like protein was assessed by 3% SDS-PAGE to be 495 kDa, smaller than that of serum IgM (750 kDa). Upon 10% SDS-PAGE with mercaptane and subsequent Western blotting with antiserum IgM, the IgM-like protein separated into three components with molecular weights of 68 kDa, 51.5 kDa, and 23 kDa. The 68 kDa, the most minor component, and the 23 kDa, the smallest molecular weight component, were identified, respectively, as H and L chain in view of their molecular weights identical to those of serum IgM. The 51.5 kDa, the main component, was also identified as H chain, since it reacted with antiserum IgM absorbed with purified L chain. From these results, it was concluded that the IgM-like protein in egg yolk, having a lower molecular weight than that of serum IgM, consists of H chain of smaller molecular weight as well as H and L chain of ordinary molecular weight.