ANTI-ENZYME IMMUNITY 281 



nature of the chemical bonds which Hnk fibrinogen units together; 

 and, (b) the geometrical arrangement of the fibrinogen units in the 

 fibrin structure. 



In very dilute solution, the mode of junction of fibrinogen units 

 in forming fibrin clot was found to be primarily an end-to-end tridimen- 

 sional polymerization, indicated by the fact that a volume fraction of 

 as little as 0.02 to 0.04 g/liter of fibrinogen solution sufficed for 

 gelation, which meant that the ratio of length to diameter of the net- 

 work strand is very great. They characterized two extreme types of 

 fibrin clot, "coarse" and "fine." In "fine" clot, the structural unit was 

 small, transparent, elastic, friable and non-synerizing, and was assumed 

 to be a single chain of fibrinogen molecules, joined end-to-end with 

 fairly strong crosslinks. The fact that fibrin was found to be insoluble 

 in reagents such as urea and potassium thiocyanate, which often dis- 

 sociate protein structures, was offered as evidence that at least some 

 of the crosslinks are primary chemical bonds. Secondary bonding was 

 not expected, for the conditions used for the formation of "fine" clot 

 were not conducive. They assumed that the properties of the clot are 

 similar to those of a swollen gel of vulcanized rubber. They also con- 

 sidered the opening of a single fibrinogen molecule to form an extended 

 polypeptide chain as observed in the denaturation of corpuscular pro- 

 teins as another mechanism for providing a strand with an axial ratio of 

 several hundred. It was held unlikely, however, for the reason that the 

 "fine" clot structure is stable. Denatured corpuscular proteins are 

 generally so hydrophobic that they aggregate and form compact precipi- 

 tates unless held in solution by urea, strong acid or alkali or detergents. 

 By contrast, a fine structure of fibrin clot was stated to be well-dispersed 

 at pH 7; it was rigid, but the strands showed no tendency to roll up and 

 the opacity studies showed no great tendency to lateral aggregation. 



The permeability of the film to small protein molecules was studied 

 by Ferry and Morrison (1947b). Among these, the permeability of the 

 serum proteolytic enzyme plasmin (fibrinolysin) was determined by 

 measuring the rate of fibrin digestion. The weight of fibrin digested 

 was proportional to the time, and the rate of digestion increased 

 with increasing concentration of enzyme. Moreover, the rate of fibrin 

 destruction was roughly proportional to the thickness of the film, which 

 indicated to them that the enzyme penetrates the structure and that 

 digestion proceeds throughout the volume of the fibrin film. If the film 



