CARBOHYDRATE-MEDIATED RECOGNITION OF A CIRCULATING PLACENTAL ALKALINEPHOSPHATASE-IMMUNOGLOBULIN-M COMPLEX

We detected an abnormal alkaline phosphatase (AP) electrophoretically in the serum of a patient with rheumatoid arthritis, who had a macromo lecular AP linked with immunoglobulin M (IgM) bearing a kappa light ch ain. The IgM isolated from the AP-IgM complex in the patient's serum r eacted apparently with all of the AP isozymes tested, i.e. those origi nating in the liver, bone, intestine and placenta, but the alpha-manno sidase-treated IgM from the patient's serum bound to placental AP (PAP ) alone. This suggests that untreated IgM recognizes multivalent epito pes of the AP and that the complex of AP with alpha-mannosidase-treate d IgM is a specific antibody-antigen complex. In order to investigate further the multivalent binding capacity for the PAP-untreated IgM com plex, we prepared a monoclonal antibody (MoAb) against PAP and identif ied it as an IgM with a kappa light chain. The binding affinities and their circulating half-lives of the synthetic complexes of PAP and res pective MoAbs were examined with and without treatment with several gl ycosidases. The untreated MoAb bearing IgM had binding affinity for al l of the AP isozymes tested, while alpha-mannosidase-treated IgM attac hed only to PAP, the same as the IgM isolated from the PAP-IgM complex in the patient's serum. The circulating clearance of the PAP-IgM comp lex in rabbits was faster than either component alone. In addition, th e PAP-IgM complex treated with alpha-mannosidase was found to have the shortest half-life of all of the complexes of PAP and Igs treated wit h the several glycosidases tested. These results suggest that the form ation of the PAP-IgM complex as an enzyme-linked antibody and the clea rance of the complex in vivo are dependent on the sugar moieties of th e Igs.