Takashi Takagi and Russell F. Doolittle* abstract: The amino acid sequence of the 36-residue peptide released during the activation of human plasma factor XIII by thrombin has been determined. The corresponding peptide from bovine material is made up of 37 amino acids; besides the additional residue, 5 amino acid interchanges are evident upon comparisonwith the human peptide. The amino-terminal sequences of the two nonidentical polypeptide chains com-prising plasma factor XIII were also studied, before and after activation. In both the human and bovine, arginylglycine bonds in one of the chains (a chain) are cleaved by thrombin during the activation process. Before activation, the amino p-L actor XIII is the precursor of a transamidase-type enzyme which stabilizes vertebrate fibrin by the introduction of e-(y-glutamyl) lysine cross-links.* 1 The calcium-dependent enzyme, which is activated by thrombin (Buluk et al., 1961; Lorand and Konishi, 1964), exists in two forms, one found in plasmaand another in blood platelets and certain other tissues (Bohn, 1970). The plasma factor is composed of two nonidentical chains designated a and b (Schwartz et al., 1971; Takagi and Konishi, 1972) and has a molecular weight of about 320,000 (Loewy et al., 1961a; Schwartz et al., 1973; Takagi and Konishi, 1972). The molecular weights of the a and b chains as determined on sodium dodecylsulfate gels vary somewhat with species, but are usually similar and between 75,000 and 85,000, corresponding to a structural arrangement of a2b2. The subunits are not held together by interchain disulfide bonds. The platelet factor is made up only of dimerized a chains, a native molecular weight of about 180,000 having been re-ported (Bohn, 1970). Onsodium dodecyl sulfate gels the a chains of human platelets and plasma are reportedly indistinguishable (Schwartz et al., 1971). In the case of both the plasma and platelet factors, treatment with thrombin results in a diminution of the a-chain molecular weight (Schwartz et al., 1971), indicating that peptide material has been removed. Calcium ions do not have tobe present for the thrombin-mediated event, but the transamidase activity does require their presence (Lorand and Konishi, 1964; Schwartz et al., 1973; Takagi and Doolittle, 1973). This report deals with an investigation of the thrombin-catalyzed activation of factor XIII from human and bovine plasma and human platelets. We have established that in all three cases thrombin treatment results in the removal of a single polypeptidefrom the amino terminusof the a chain. The nature of the linkage split has been determined, as well t From the Departmentof Chemistry, University of California, San Diego, La Jolla, California 92037. Received October 16, 1973. This work was supported by the U. S. Public Health Service Grant No.