EQUIVALENT RECOGNITION OF FREE AND ACT-COMPLEXED PSA IN A MONOCLONAL-POLYCLONAL SANDWICH ASSAY IS CONFERRED BY BINDING-SPECIFICITY OF THE MONOCLONAL-ANTIBODY

The Bayer Immune 1(TM) PSA Assay measures free and ACT-complexed PSA o n an equimolar basis, although it uses a monoclonal antibody (MM1) for capture and polyclonal antibodies for detection. Competitive inhibiti on studies using antibodies directed at various epitopes on PSA and PS A-ACT demonstrated that the capture antibody, MM1, does not bind to fr ee PSA simultaneously with antibodies against Epitope E which is expos ed only in free PSA. Affinity studies showed that the affinity constan ts of MM1 for both free PSA and PSA-ACT are similar. One explanation f or the properties of MM1 is that it precludes the binding of antibodie s to Epitope E due to steric hindrance. Alternatively, the binding of MM1 causes a conformation change within the free PSA molecule, so that Epitope E is altered in a way that causes a loss of binding affinity. The unusual properties of MM1 are responsible for the equimolar respo nse of this monoclonal-polyclonal sandwich assay for free and ACT-comp lexed PSA. (C) 1998 Wiley-Liss, Inc.