Summary
Factor XIII (FXIII) is of high importance in the regulation of fibrinolysis. It crosslinks
α2-antiplasmin (α2AP) and fibrin and by this way protects fibrin from the prompt elimination by plasmin.
Although FXIII of platelets has been implicated in this protective mechanism, the
role of platelets and platelet FXIII in the crosslinking process is far from being
elucidated. As demonstrated by SDS PAGE and by immunoblotting for α2AP, intact normal platelets resuspended in FXIII-free plasma or FXIII-free fibrinogen
solution catalyzed the crosslinking of fibrin chains and also the crosslinking of
α2AP to fibrin α-chains. With FXIII-deficient platelets no crosslinking reaction could
be observed indicating that the crosslinking with normal platelets was, indeed, due
to platelet FXIII and not to another, putative platelet transglutaminase. However,
the crosslinking of α2AP to fibrin induced by the FXIII of intact platelets resuspended in FXIII-free plasma
was considerably less extensive than the crosslinking carried out by the FXIII of
normal plasma in the presence of FXIII-free platelets. Furthermore, the replacement
of FXIII-free platelets by normal platelets in normal FXIII-containing plasma resulted
in little, if any, difference in the crosslinking process. When crosslinking was induced
by highly purified plasma FXIII the presence of intact FXIII-free platelets significantly
accelerated the formation of α-chain polymers as well as the incorporation of α2AP-fibrin α-chain hetero-dimer into these polymers. The results indicate that, in
physiological conditions, platelet FXIII plays only a minor role in the crosslinking
of α2AP and fibrin; however, platelets, independently of their FXIII content, promote the
crosslinking reaction by providing a catalytic surface on which the formation of highly
crosslinked fibrin polymers is accelerated.
