Liquid-Phase Binding Assay of α-Fetoprotein Using a Sulfated Antibody for Bound/Free Separation

A rapid immunoassay using a sulfated antibody for bound/free separation in a liquid-phase binding assay is described. A first anti-α-fetoprotein monoclonal antibody was labeled with peroxidase (Fab'-POD) and a second monoclonal antibody was conjugated with polysulfated tyrosine peptide (Fab'-YS). The monoclonal antibodies and α-fetoprotein (AFP) were mixed, incubated, and analyzed directly by anion-exchange column chromatography. The amount of POD activity in the column effluent was determined fluorophotometrically. The bound (Fab'-POD + AFP + Fab'-YS) and free (Fab'-POD) forms of the conjugate were clearly and easily separated by ionic charge, and the free sulfated antibody (Fab'-YS) was not detectable fluorophotometrically. The elution position of the bound conjugate was adjusted by varying the length of the polysulfated tyrosine peptide. This method is convenient for antigen measurement because (1) only two modified antibodies are used in a buffer solution, (2) the concentration of antibodies and other assay conditions are easily set, (3) no solid phase is required, and (4) no washing is necessary.