Reaction of plasmic degradation products of fibrinogen in the radioimmunoassay of human fibrinopeptide A

A radioimmunoassay (RIA) technique has been devised for the measurement of human fibrinopeptide A (FPA). The system utilizes rabbit antiserum to native human FPA and a synthetic fibrinopeptide, with tyrosine substituted for phenylanine in amino acid position 8. The test detects native human FPA at a concentration of 0.1 ng/ml, but does not cross react with human fibrinopeptide B or with fibrinopeptides A from canine, porcine, or bovine fibrinogen. Fibrinogen and chemical or plasmic degradation products with 2 moled of FPA per mole react fully in this test system. This includes the large-molecular-weight intermediate fragments X and Y and the NH2-terminal disulfide knot, and indicates that this antibody recognizes and reacts with FPA in the presence of the contiguous peptide structures present in fibrinogen. Fragment E, which is derived from the NH-2-terminal portion of fibrinogen, loses most of its FPA content after its liberation from its precursor derivative and reacts to a lesser extent in the RIA than do fragments X and Y. This correlated with the recovery of FPA-positive material from ultrafilitrates of extensive but not partial plasmic digests of fibrinogen. Although FPA immunoreactivity liberated from fibrinogen does not necessarily reflect thrombin activity and/or fibrin formation, only extensive plasmic degradation yields peptide material which reacts in this RIA system. This should not be a serious limitation to the application of the RIA in the detection of venous thrombosis.