Two distinct groups of immunoglobulin A(IgA) revealed by peptic digestion.

Serum IgA M-components, secretory IgA separated from colostrum, and IgA from serum of patients with cirrhosis of the liver were digested with pepsin at pH 4.1. The IgA M-components segregated into two groups on the basis of their relative rates of peptic digestion. Serum and colostral IgA were digested at a total rate intermediate to that of the two groups of IgA myeloma proteins. It appeared, however, that colostral IgA may have been initially more resistant to peptic digestion than serum IgA. The variability in the rate of peptic digestion was not related to electrophoretic mobility, light-chain type, or IgA subclass. Experimental conditions related to enzyme to substrate ratio or to the pH of the reaction mixture did not appear to explain the differences found.These findings indicate that (a) two groups of IgA proteins can be distinguished on the basis of susceptibility to proteolysis with pepsin, and (b) secretory piece confers, at most, only a minor increase in stability to the IgA molecule against the digestive action of pepsin.