Contact activation of blood coagulation is inhibited by plasma factor XIII b-chain.

The effect of the purified bovine plasma factor XIII b-chain on contact activation of blood coagulation was studied in human and bovine plasma using either an ellagic acid-phospholipid suspension (Cephotest) or dextran sulfate as activating surface. Contact activation was monitored by the generation of amidolytic activity towards a synthetic chromogenic substrate (S-2302) for factor XIIa and plasma kallikrein. The factor XIII b-chain, which is released from tetrameric factor XIII (a2b2) in the late stages of blood coagulation, inhibits contact activation induced by both activation surfaces mentioned. It was shown that a 5 min preincubation of the factor XIII b-chain with the activation surface increases its inhibitory effect. Light scattering measurements indicated a concurrent binding of the factor XIII b-chain to the Cephotest material. Because factor XIII (a2b2) itself had no such inhibitory activity, the present finding that the factor XIII b-chain inhibits contact activation may point to a novel feed-back inhibition mechanism of blood coagulation.