Studies on a Proteolytic Enzyme in Human Plasma

Abstract

Much confusion exists concerning the relationship between the proteolytic activity of plasma and the coagulation of blood. Evidence has been offered both to support and deny the hypothesis that the activated proteolytic enzyme of plasma accelerates blood clotting. The dab relating the proteolytic activity of plasma to clotting were reviewed by Nolf (1), by Ferguson (2), and by Macfarlane and Biggs (3). Delezenne and Pozerski (4) in 1903 reported that serum became proteolytic following the addition of chloroform. Nolf (5) observed that chloroform-treated serum was capable of clotting solutions of fibrinogen, and he suggested that proteolysis was a necessary step in the clotting process (1). The chloroform-activated proteolytic property of plasma was shown by Tagnon (6) to be present in its acid-insoluble globulin fraction. This fraction, too, was reported to clot fibrinogen solutions as well as oxalated plasma (7).The proteolytic properties of plasma globulin can be activated not only by chloroform, but also by bacteria-free filtrates of cultures of beta hemolytic streptococci (8-11). The active principle present in these filtrates has been called streptococcal fibrinolysin (12) or streptokinase (13). Ferguson (14) observed that a fraction of plasma globulin, activated by streptococcal fibrinolysin, was thromboplastic, and concluded that the proteolytic enzyme of plasma plays a r61e in blood clotting.Further evidence suggesting a relationship between proteolysis and coagulation has been provided by a study of trypsin inhibitors. The inhibitors of trypsin derived from pancreas and soy bean were observed to inhibit the activity of the proteolytic enzyme of plasma (15, 16). These substances also inhibited blood coagulation (17)(18)(19), lending support to the belief that the proteolytic enzyme of plasma participates in the clotting mechanism.On the other hand, certain data cast doubt that proteolysis plays any physiologic r61e in dotting. In particular, it was reported that purified preparations of the proteolytic enzyme of plasma did not convert prothrombin to thrombin, nor clot fibrinogen (20, 21). Furthermore, preparations of plasma globulin were said to be separable into two fractions. One of these fractions promoted clotting, while the other exhibited proteolytic activity (22).

Fractionation of Plasma Globulin for Prothrombin, Thrombokinase, and Accessory Thromboplastin

“…Travis and Ferguson (40)). It is also probable that thrombokinase is distinct from that particular fibrinolytic enzyme called plasmin (41). This does not mean that thrombokinase is not an enzyme, nor does it even exclude the possibility that the kinase, under proper conditions, might show some proteolytic or fibrinolytic activity.…”