Metabolism of plasminogen in healthy subjects: effect of tranexamic acid

The metabolism of human plasminogen labeled with radioactive iodine was studied in 12 healthy men. The labeled plasminogen had a high specific activity and the same elution on Sephadex G-100 as the plasminogen activity in plasma. Immunoelectrophoresis revealed a single precipitin line. Polyacrylamide gel electrophoresis revealed six main bands, all with plasminogen properties and radioactivity. The purified plasminogen behaved as a homogeneous protein in the turnover experiments. The plasma radioactivity data were adequately approximated by a sum of two exponential terms. The metabolism of plasminogen was therefore represented by a two-compartment mammillary model. Results in the 12 normal subjects were as follows: plasma plasminogen concentration 20.8+/-1.9 mg/100 ml; intravascular plasminogen pool 0.66+/-0.14 g; intravascular fraction 0.59+/-0.06; fractional catabolic rate 0.55+/-0.09 of the plasma pool per day; half-life of the plasma radioactivity 2.21+/-0.29 days. Circulating large-molecular-weight degradation products of labeled plasminogen could not be detected by Sephadex G-100 gel filtration. The plasminogen turnover rate was normal in a patient with Behçet's syndrome and low circulating plasminogen activator activity. This finding supports the concept that under normal conditions the primary pathway of plasminogen catabolism is not via the formation of plasmin. The in vivo effect of tranexamic acid, a potent inhibitor of plasminogen activation, on the turnover of labeled plasminogen was studied in five normal subjects. When 1 g was administered perorally t.i.d. to three of them, one showed an increased plasminogen turnover. A 2 g dose administered t.i.d. to the other two caused markedly increased catabolism in both. This increase may be attributable to a direct reversible effect of tranexamic acid on the plasminogen molecule.