Use of protein A-containing Staphylococcus aureus as an immunoadsorbent in radioimmunoassays to separate antibody-bound from free antigen

Heat killed, formalinized Cowan I strain Staphylococcus aureus (PASA) was examined for its utility as a solid-phase immunoadsorbent in various RIA procedures. Specific antibody could be adsorbed to PASA and then reacted with radiolabeled and unlabeled hormone, or alternatively, PASA could be added to antibody-hormone mixtures in order to separate antibody-bound from free antigen. Both methods compared favorably with the more standard double-antibody method in each system tested. Antibodies produced by rabbits, guinea pigs, monkeys, and humans were precipitated equally by PASA and double antibody, whereas antibody raised in sheep was precipitated much less by PASA than by double antibody. Antibody bound to PASA was found to reach equilibrium of binding more slowly than unbound antibody for both rapid (TRH) and slow (rPRL) reactions. Binding of either free or complexed IgG by PASA, however, was extremely rapid, reaching equilibrium in less than 30 sec. This rapid binding was utilized in kinetic experiments which would not have been possible with slower-reacting precipitating agents. The versatility permitted by this property constitutes one of the most useful characteristics of PASA. Other important attributes of PASA include its convenience, economy, and wide applicability.