Identification of alpha-fetoprotein polymers. Artifacts of the isolation procedure

By combining polyacrylamide gel electrophoresis and radioimmunoassay, a method has been developed to detect polymeric forms of alpha-fetoprotein (AFP) during the isolation procedure. The method is capable of detecting AFP polymers in the presence of other serum proteins in serum samples that contain as little as 100 ng of AFP. Using this method, we have found that AFP polymers do not exist in cord serum; if encountered, we feel they should be considered to be artifacts of the isolation procedure AFP polymers appear in 7.5% polyacrylamide gel as multiple bands. We believe the multiple bands to be AFP polymers and not serum protein contaminants for the following reasons: (1) the electrophoretic pattern is very similar to that of serum albumin; (2) the polymers do exhibit AFP activity, although less than the monomeric form does; (3) the molecular weight of the dimer is exactly twice that of the monomer; (4) in the presence of sodium dodecyl sulfate and mercaptoethanol, the polymers move to the same position as do the monomers in polyacrylamide gel electrophoresis; (5) the polymers cannot be removed by repeated use of an immunoadsorbent affinity column designed to remove contaminant serum proteins.