Antiplasmin Activity of Rat Serum Slow α‐Globulins

S ummary . The antiplasmin activity of serum slow α 1 ‐ and α 2 ‐globulins has been studied in rats injected with streptokinase/human serum and human or rat plasmin. Highly significant falls in serum slow α 1 ‐globulin were noted soon after administration of streptokinase and human or rat plasmin; this suggested a reaction between plasmin and slow α 1 ‐globulin with rapid removal of the resultant complexes. Rats with immune complex nephritis (ICN) given streptokinase showed a highly significant increase in slow α 2 ‐globulin, but no change in slow α 2 ‐globulin was noted following injection of human plasmin. Using fibrin agar plates additional evidence was obtained that rat slow α 1 ‐globulin inhibited digestion of fibrin by plasmin. We conclude that of the two globulins in the rat only slow α 1 ‐globulin shows antiplasmin activity and we believe that this represents the functional analogue of α 1 ‐macroglobulin in the human.