Plasma fibrinopeptide B (B.beta.1-14 or FPB) immunoreactivity was studied by radioimmunoassay in patients who received intrauterine infusion of hypertonic saline to terminate pregnancy. FPB immunoreactivity increased with thrombin treatment (TIFPB) suggesting the presence of a larger FPB-containing peptide, since purified FBP was not altered by thrombin, whereas thrombin increases the immunoreactivity of B.beta.1-42 (which includes FPB) 10-fold. TIFPB immunoreactivity in plasma, drawn 4 h after hypertonic saline infusion eluted from Sephadex G-50 similarly to isolated B.beta.1-42. Streptokinase, incubated with normal plasma progressively generated TIFPB immunoreactivity, which showed a major component which eluted from Sephadex G-50 similarly to B.beta.1-42. Streptokinase generated TIFBP much more rapidly in reptilase-treated plasma that contained fibrin I, (which still includes FPB), indicating that fibrin I was preferred over fibrinogen as a substrate for plasmin cleavage of arginine (B.beta.42)-aline (B.beta.43). Serial studies were then made in 10 patients receiving intrauterine hypertonic saline. Fibrinopeptide A (FPA) levels rose immediately, reached a peak between 1 and 2 h, were declining at 4 h and were normal at 24 and 48 h. TIFPB levels rose slightly in the 1st h, reached a peak at 4 h and had returned to base-line values at 24 h. Serum fibrinogen degradation product levels were unchanged at 1 h, reached their highest level at 4 h and were still markedly elevated at 24 and 48 h. Fibrinogen levels dropped slightly being lowest at 4 and 24 h. Platelet counts declined in parallel with the fibrinogen levels over the first 4 h, but continued to decrease through 48 h. .beta. Thromboglobulin (.beta.TG) levels generally paralleled FPA levels whereas platelet factor 4 (PF4) levels showed only slight changes. Immediately after intrauterine hypertonic saline infusion thrombin was apparently formed that cleaves FPA from fibrinogen to produce fibrin I and release .beta.TG and PF4 from platelets. Later plasmin cleaves B.beta.1-42 from fibrin I to produce fragment X, which was further degraded to form serum fibrinogen degradation products. This sequence of proteolysis indicated that plasmin action on fibrin I served as a mechanism that regulates fibrin II formation by removing the B.beta. chain cleavage site, which was required for thrombin action in converting fibrin I to fibrin II.
