The protein contents and binding modes of immunoglobulin-amylase complexes were investigated. Amylase staining after immunoelectrophoresis permitted identification of IgA-lambda type in three cases, IgA-kappa type in two cases and IgG-kappa type in one case. The precipitin line of Fab fragment of IgA in four patient's sera after papain digestion was found to have amylase activity. Sucrose density gradient ultracentrifugation studies showed that these complexes were divided into three groups: Group I complexes were dissociated in acid pH, and at pH 7.0 the immunoglobulins, which were isolated at the acid pH, became rebound to serum amylase; Group II complexes were dissociated by acidification, but did not reform at pH 7.0; Group III complexes were unaffected by acidification. We suggest that the immunoglobulin-amylase complexes belonging to groups I and II result from an antigen-antibody reaction. Precipitin curves resulting from the addition of increasing quantities of the patient's serum containing 9 S or 11 S immunoglobulin-amylase complexes to specific antisera revealed that these complexes contained not only monoclonal IgA, but also alpha-1-antitrypsin (alpha 1 AT) and albumin. These IgA complexes could have bound non-specifically to alpha 1 AT and albumin, forming the larger macroamylase complexes.
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