Quantitation of Staphylococcal Protein A: Determination of Equilibrium Constant and Number of Protein a Residues on Bacteria

For quantitation of cell wall associated staphylococcal protein A a method was developed using 125I-labeled myeloma globulins. The maximal sensitivity of the method could be varied according to the particular experimental questions. Using the same reagent the number of protein A residues on Staphylococcus aureus, Cowan I, was estimated to 80,000/organism. In comparison with the blood group antigen A the protein A residue is four times as densely located on this staphylococcal strain. The equilibrium constants for the reactions between two myeloma globulins and staphylococcal protein A were found to be 4.0 × 107 and 4.5 × 107 liters/mol, respectively.