Site of alkaline phosphatase attachment in alkaline phosphatase-immunoglobulin G complexes.

Sera containing the rare alkaline phosphatase-immunoglobulin G complex were studied to try to determine the type of interaction involved. Pepsin and papain digestion of immunoglobulin G showed that alkaline phosphatase was attached to the F(ab')2 region of the immunoglobulin molecule and not to the Fc region. Sialic acid did not play a role in this attachment. Attempts to generate the complex in vitro using polyclonal immunoglobulin, and attempts to dissociate the complex is an immune complex in vitro, were both unsuccessful. It is concluded that the complex is an immune complex formed by antibody-antigen reaction in the circulation, and consists of two molecules of monovalent alkaline phosphatase associated with one molecule of divalent immunoglobulin G.