In vivo turnover of canine plasminogen-Fraction I (plg) and the plasminmodified-plasminogen (m-plg) labeled with radioiodine and in vitro binding of either plg or m-plg to fibrin were studied. The plg prepared by the method of Deutsch and the m-plg prepared from the purified plg by the method of Sodetz and Castellino were labeled with 125I and 131I, respectively, by the iodine monochloride method of McFarlane.
1) The plasma clearance of radioiodine in 6 dogs injected with 125I m-plg was described by a biexponential curve within a observation period of 72 hours. The half-life of the slower component was 0.96±0.06 days and the catabolic rate of m-plg calculated from the analysis of Atencio's two compartment motabolic model was 1.216/day, respectively, whereas those for 131I-plg were 1.636±0.150 days and 0.602/day, respectively.
2) About 2.5μ Ci of both 125I-m-plg and 131I-plg solution were added to one ml of canine plasma or canine fibrinogen solution, then the mixture was clotted by the addition of 0.1ml of CaCl2 solution (0.25M) or of thrombin solution (100u/ml). The mixture containing fibrin clot were incubated for 10min., 30min., 1 hour or 2 hours. After completion of the incubation, the clot were squeezed by a glass stick and the unadsorbed radioactivity was washed out by 0.05M phosphate buffer (pH 8.0). Then the radioactivities of 125I and 131I in fibrin were measured. When the plasma was clotted by thrombin, the binding of both 125I-m-plg and 131I-plg to fibrin was about 20.1% and 12.4%, respectively. Whereas the plasma was clotted by CaCl2, the binding of these to fibrin were 25.5% and 16.5%, respectively. (p<0.05). In case of the canine fibrinogen solution, the binding of the m-plg and the plg to fibrin after addition of thrombin was increased to 27% and 19.3%, respectively.
3) One day after I. V. injection with 131I-plg to each of 3 dogs, 20×103 units of urokinase (UK) was injected intravenously. The plasma samples were obtained and were clotted by an addition of CaCl2, then the radioactivity of fibrin was measured. It was increased to 2.7 times as compared to initial value.
From these data, it may be concluded that the plasma clearance of the m-plg was faster and the m-plg has higher affinity for fibrin than the plg. The I. V. administration of UK resulted in the activation of a part of plg into plasmin, then the plasmin modified 131I-plg to 131I-m-plg which can be easily bound to fibrin.