INTRACHAIN AND INTERCHAIN DISULFIDE BRIDGES OF THE HUMAN J CHAIN IN SECRETORY IMMUNOGLOBULIN-A

All intra J chain disulfide bridges of human sIgA, the disulfide bonds between the J chain and the two IgA monomers, and one inter IgA monomer disulfide bridge were determined. sIgA was isolated from colostrum of healthy women and digested with IgA1-specific protease followed by cyanogen bromide cleavage. This procedure generated fragments of 140 kDa, 60 kDa, and 28 kDa. The 28-kDa polypeptide comprised the complete J chain covalently bound to two alpha1 chain octapeptides derived from the C-termini of two alpha1 chains. The 28-kDa fragment was digested with trypsin. The resulting peptides were purified by RP-HPLC, and subsequently characterized by amino-acid analysis, mass spectrometry, and gas phase sequencing. These data unequivocally show that the J chain cysteines C1-C6, C4-C5, and C7-C8 form intra chain disulfide bridges. The second (C2) and the third (C3) J chain cysteines are disulfide linked to two a chain cysteines (C17) joining the two IgA monomers of sIgA tail to tail. The remaining two a chains of the two monomers are directly bound to each other via their ultimate cysteines (C17-C17). A new model for the J chain in sIgA is presented.