Summary

Plasma factor XIII (plasma protransglutaminase) circulates as anA₂B₂ tetramer bound to the γ’ variant chains of fibrinogen "2". Duringclotting the A subunits of fXIII are cleaved by thrombin to form fXIIIa(transglutaminase) and in the presence of calcium ions, activated A₂*subunits dissociate from the B subunits. When purified plasma fXIII orrecombinant cellular factor XIII (A₂) was incubated with fibrinogen inthe presence of calcium ions (≥ 50 µM) a non-synerizing gel formedconcomitant with formation of γ dimers, followed by Aγ polymers, andeventually γ trimers and γ tetramers. As is the case of fXIIIa, thefXIII-mediated crosslinking rate was enhanced in the presence ofthiols. After an initial lag period, fXIII catalyzed fibrinogen crosslinkingat ~75% of the rate of fXIIIa under typical crosslinking conditions(100 Loewy u/ml, 5 mM CaCl₂ ≥ 500 µM DTT). Fibrin wascrosslinked about 8 times more rapidly by fXIII than was fibrinogen,and after an initial lag period fXIII crosslinked fibrin at nearly the samerate as fXIIIa. Substituting plasma for purified fXIII as the source forfXIII resulted in robust fibrinogen crosslinking activity. In contrast tothe high level of fXIII-mediated crosslinking activity observed withfibrinogen or fibrin as substrates, when transglutamination was measuredusing cadaverine incorporation into casein, fXIII was 30-fold lessactive than fXIIIa. Thus, factor XIII displays constitutive enzymaticactivity with respect to fibrinogen and fibrin. The results further indicatethat uncleaved fXIII in plasma provides a potent source of readilyavailable crosslinking activity in clotting blood. Fibrinogen 2, whoseγ’chains bind fXIII B subunits, was crosslinked 3.5 times more slowly by fXIII than was fibrinogen 1 (lacking γ’ chains), suggesting thatcomplex formation between fibrinogen 2 and plasma fXIII plays asignificant role in down-regulating potential plasma fXIII-mediatedcrosslinking activity. Since fibrin is a considerably better substrate forfXIII than is fibrinogen, the rate at which crosslinking takes place in afibrinogen-containing plasma environment is much lower than it wouldbe if fibrin were present.