Free sulfhydryl groups of rabbit secretory IgA

The reaction of sIgA with 5,5'dithiobis (2-nitrobenzoic acid) or with 14C-iodoacetamide revealed an average of 1 mol -SH per mol sIgA under nondenaturing conditions, whereas an average of 7.5 mol -SH was found under denaturing conditions. These -SH groups were found in both the sIgA-f and sIgA-g subclasses; in the sIgA-g subclass, the free -SH groups were found in the Fab alpha portion but not in the Fc alpha portion of the molecule. To determine whether the noncovalently bound polypeptide chains in sIgA contained free -SH groups, sIgA was alkylated with 14C-iodoacetamide and subjected to gel filtration in 6 M guanidine-HCl. This treatment liberated several different polypeptides that were identified as secretory component (SC), L chain dimers, and L chain monomers; measurement of radioactivity incorporated in each of these noncovalently bound chains showed that SC and the L chain dimers did not contain free -SH groups but that an average of 1 -SH group per L chain monomer was found. Of the covalently bound polypeptide chains, J chain and SC did not have detectable -SH groups, whereas the L chains and alpha-chains had 0.3 and 1.2 mol -SH per mol protein, respectively. The identification of free -SH groups in the Fab portion of the sIgA-g subclass and their preferential localization within only some of the L chains reflect the structural heterogeneity of sIgA; these -SH groups presumably arise during biosynthesis.