An inhibitor of activated factor XIII, inhibiting fibrin cross-linking but not incorporation of amine into casein.

An inhibitor of fibrin cross-linking was studied. The inhibitor was quantitatively separated together with the IgG fraction of plasma, but attempts to neutralize the purified inhibitor by commercial anti-IgG sera gave inconstant results, possibly due to anti-factor XIII activity demonstrable in these sera. The inhibitor prevented cross-linking by activated factor XIII, even when tested on preformed fibrin, but was inactive when assayed by the use of a synthetic substrate (monodansylcadaverine). Since, the inhibitor remained in the supernatant after defibrination, and was thus not associated with clottable proteins, the present findings suggest inhibition of activated f. XIII and not blockade of cross-linking sites on fibrinogen.