The effect of plasmin on the subunit structure of human fibrinogen.

The changes in the subunit structure of human fibrinogen during plasmin digestion have been followed sequentially by examining the degraded products electrophoretically in sodium dodecyl sulfate on polyacrylamide gels in the absence or presence of mercaptoethanol. On the basis of the molecular weights of the products, their carbohydrate content, and the relative amounts of each product present during digestion, it has been possible to deduce the sequence of structural changes in fibrinogen during digestion and the polypeptide chain composition of the major transient and terminal digestion products. The CY chains, which contain no detectable carbohydrate, were the first subunits in fibrinogen to be degraded by plasmin. The j3 chains, which contain carbohydrate, were degraded more slowly than CK chains. The initial, but transient, degradation product, Fragment X, was structurally heterogeneous. Forms appearing early in the digestion contained extensively degraded LY chains and intact /3 and y chains. The next transient fragment formed contained extensively degraded a chains and partially degraded /? chains. The y chains of fibrinogen, which also contain carbohydrate, were more resistant than 01 and p chains, but on prolonged digestion, were cleaved by plasmin. As y chains begin to be degraded, and the LY and /? chains are further degraded, other forms of Fragment X appear along with Fragments D and Y. On the basis of this sequence of structural changes it has been possible to deduce the general subunit structure of the major terminal digestion products (Fragments D and E). Fragment D contains partially degraded fl and y chains and extensively degraded CY chains combined by disulfide bonds. Fragment E contains extensively degraded CY, p, and y chains which are also combined through disulfide bonds. A scheme describing these structural changes has been proposed. The ability of partially degraded fibrinogen to form visible clots on treatment with thrombinand fibrin-stabilizing factor