Distribution and Fate of I 131 -Labeled Components of the Fibrinolysin System

Abstract

The distribution, metabolism, and excretion of intravenously administered I131-labeled components of the fibrinolysin system were studied in dogs. lodination of plasmin tended to decrease slightly its in vitro fibrinolytie activity. Inaetivation appeared to depend on degree of iodination. Todination of strepto-kinase or urokinase did not decrease their plasminogen activator potential. All plasmins and plasminogen, regardless of the position of the iodine tag, adsorbed to preformed clots in the dog. Spontaneously activated human and chloroform-activated bovine plasmin adsorbed to a lesser extent than streptokinase-or urokinase-activated human plasmin. Clots removed from piasmin-treated dogs lysed in vitro within 24 hours. However, streptokinase or urokinase alone, on the basis of radioactivity measurements, did not exhibit any affinity for clots, nor did they cause any clot lysis. Plasma clearance curves revealed the presence of at least two processes in all preparations: a fast one (average half-life four minutes) responsible for the removal of over 60 per cent of the injected radioactivity and a slower process with a biological half-life greater than four hours. Pour hours after the injection of labeled compounds, the thyroid and stomach were the only organs which contained significant percentages of the injected radioactivity. Major excretory routes for the radioactivity were the urinary and biliary systems. Persistence of circulating radioactivity at a time when no free fibrinolytie activity could be determined in the blood was interpreted as complex formation between plasmin and antiplasmin.