Previous studies have established differential susceptibility of the rabbit sIgA subclass to proteolytic digestion; the sIgA-f sublcalss molecules are resistant and the sIgA-g subclass molecules are susceptible to proteolytic digestion. In this investigation rabbit sIgA was reduced and alkylated, digested with papain and subsequently subjected to gel filtration on Sephadex G-200. Antigenic analysis, by a quantitative radio-precipitation assay, and physio-chemical analysis revealed that one fraction from gel filtration contained an Fc2α fragment of sIgA-f subclass molecules; this Fc2α fragment also contained some determinants of the f-allotypic specificities. These studies indicate that the differential susceptibility of the rabbit sIgA subclass to proteolytic digestion may be related to differences in intercahin disulfice bonds.
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