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Application of the immunoblotting technique to the study of single protein species in complex biological fluids: A model study with alpha‐2‐macroglobulin

Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis, and the immunoblotting technique, were applied to structural analysis of a single protein (alpha‐2‐macroglobulin, α‐2‐M) in human serum or plasma without prior purification of this protein. The use of immunoglobulin fractions of four monospecific polyclonal antibody preparations to α‐2‐M in the immunoblotting step showed marked unspecific binding to the proteins fixed onto nitrocellulose. Immunospecific purification of the antibodies by two different methods, and the introduction of biotinylated antibodies and avidin‐peroxidase, removed the background binding. When reduced proteins were fixed onto nitrocellulose a quantitative loss of α‐2‐M antigen reactivity was observed, compared to identical nonreduced samples. The degradation pattern of α‐2‐M in serum or plasma corresponded to the pattern observed with purified α‐2‐M. Thus, the described immunoblotting technique, applied directly to serum or plasma, could detect all not heat‐denaturated α‐2‐M products as seen on Coomassie‐stained polyacrylamide gels when analyzing purified α‐2‐M.