[Action of proteolytic enzymes on fibrinogen]

The principal substrate of protealytic enzymes is fibrinogen. Thrombin severs four ARG-GLY bonds in the alpha A and beta B chains of its molecule, on the side of the terminal-N. It thus liberates two fibrinopeptides A and B, and leads to the formation of fibrin. Plasmin, by contrast, acts upon the fibrinogen molecule first by hydrolysis of the alpha and beta chains liberating the X fragment and three peptides A, B and C. It continues on the alpha, beta and gamma chains of fragment X, leading to the appearance of fragments Y and D. Fragment Y is in turn hydrolysed into a second fragment D and fragment E. The initial (X and Y) or terminal (D and E) fibrinogen breakdown products each possess their own anticoagulant properties together with immunological properties which may be used in their estimation.