Immunochemical Studies Of The Reaction Of Human Desarginyl-Fibrinopeptide B (Desarg-Fpb) With Anti-Fpb Sera

Fibrin II formation may be essential in thrombosis. Measurement of free FPB is the only direct index of fibrin II formation but is complicated by the fact that carboxypeptidase B rapidly cleaves the COOH-terminal arginine from FPB in human plasma. To facilitate the assay of Desarg-FPB as an index of in vivo FPB release, the immuno-chemical reactivity of Desarg-FPB with anti-FPB sera has been studied. As previously reported, Desarg-FPB was considerably less effective (0.7-17%) than FPB in inhibiting the binding of an ¹²⁵I-FPB analog by five of six rabbit antisera studied in detail. Surprisingly, Desarg-FPB was 4 to 27 times.more effective than FPB in inhibiting the binding of an ¹²⁵I-Desarg-FPB analog by the six anti-FPB sera. For example, in the case of antiserum R-28, FPB was 145 times more effective than Desarg-FPB in inhibiting the binding of the ¹²⁵I-FPB analog but Desarg-FPB was 27 times more effective than FPB in inhibiting the binding of the ¹²⁵I-Desarg-FPB analog. These findings indicate that the FPB and Desarg-FPB analogs are bound by different antibody populations. We suggest that carboxypeptidase B converts some molecules of FPB-protein conjugates to Desarg-FPB derivatives during the immunization of rabbits. From a practical point of view, the difference in reactivity of the different antisera has enabled the rational selection of anti-FPB sera for use in the assay of Desarg-FPB and its distinction from FPB. With the use of an appropriate antiserum, Desarg-FPB levels have been measured in clinical blood samples and the sensitivity is such that distinction can be made between levels in normal individuals and in disease states.