The peptide beta 43-47 which increases microvascular permeability is released by plasmin during cleavage of fragment Y of fibrinogen.

A synthetic pentapeptide corresponding to sequence 43-47 of human fibrinogen B beta chain elicited, in rabbits, antibodies that during immunoblotting recognized intact fibrinogen, fragments X and Y as well as the B beta chain. Since fragment Y is the last peptide product which reacts with anti-beta 43-47 antibodies, splitting of fragment Y into fragment D and fragment E must be accompanied by plasmin cleavage of the peptide bond beta Lys-47-Ala-48.