Plasmic Degradation Products of Fibrinogen 759 Chromatography Electrophoresis

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