Differential susceptibility of rabbit sIgA subclasses to trypsin cleavage: characterization of the fragments obtained from the g subclass.

Rabbit secretory IgA (sIgA) was digested with trypsin at 37 degrees C for 30 min and four fractions were isolated by gel filtration. These fragments were characterized by ultracentrifugation, and by their antigenic properties as undigested sIgA, Fab and Fe (Two Fc fragments differing in their content of secretory component were obtained.) The IgA-f subclass molecules were resistant to cleavage and were found in the undigested sIgA fraction; the IgA-g subclass molecules were cleaved into Fe and Fab fragments. The g allotypic determinants of IgA-g molecules were found on both the Fc fragments and the Fab fragments. The Fc and Fab fragments obtained from trypsin-digested sIgA were compared by means of antigenic properties and peptide maps with the Fc and Fab fragments obtained from papain-digested sIgA; no differences attributable to the alpha-chain were found. Thus, papain and trypsin cleave the alpha-chain of IgA-g molecules at similar but not necessary identical positions.