A radioimmunoassay for total human cathepsin B.

A radioimmunoassay for human cathepsin B is described, which demonstrates the usefulness of protein A-bearing Staphylococcus aureus as an immunosorbent in a system where the primary antibody IgG has a low binding affinity for protein A. The removal of bound antigen from the incubation mixture is achieved by the use of a rabbit second step antiserum which confers high binding affinity for protein A to the primary immune complex. This method, as employed in the assay for human cathepsin B, is very reproducible and economical for large numbers of samples. The use of a monospecific antiserum to human cathepsin B and slightly alkaline assay conditions allow the determination of total cathepsin B protein in tissue fluids which is not possible by enzyme activity determination.