The stability of fibrinopeptide B immunoreactivity in blood.

The stability of fibrinopeptide B immunoreactivity as measured by radioimmunoassay was studied in buffer solutions, normal human plasma and blood. Fibrinopeptide B immunoreactivity was stable in buffer, but rapid temperature-dependent loss of immunoreactivity occurred in plasma and whole blood. Carboxypeptidase B (porcine pancreas) inactivated fibrinopeptide B immunoreactivity at a rate of 3.1 × 10^?10 mol/ml/min/unit at pH 7.4, 0.15 M Tris-NaCl and 24°. The loss of immunoreactivity was inhibited with 0.01 M o-phenanthroline. O-phenanthroline (0.01 M) completely inhibited the loss of fibrinopeptide B immunoreactivity in blood and other carboxypeptidase inhibitors including EDTA, hippuryl-L-arginine and hippuryl-L-lysine slowed the rate loss of immunoreactivity. Proof that the loss of immunoreactivity resulted from loss of Arginine (Bβ 14) was obtained by demonstrating that Bβ 1–13 immunoreactivity was stable in whole blood. The results of these studies indicate that the loss of fibrinopeptide B immunoreactivity in blood results from the cleavage of Arginine (Bβ 14) by carboxypeptidase B, that the enzyme is effectively inhibited by 0.01 M o-phenanthroline and that Arginine (Bβ 14) is a critical component of one set of antigenic determinants of fibrinopeptide B.