Identification of Ser‐Leu‐Met‐Lys‐bradykinin isolated from chemically modified high‐molecular‐weight bovine kininogen

Bovine blood plasma contains at least two kininogens, viz., high-molecular weight (HMW) and low-molecular weight (LMW) kininogen; each consists of a single polypeptide chain with a molecular weight of 76 000 or 50 000, respectively [I] . Both of these include the bradykinin moiety in their inner portions surrounded by a disulfide loop [2]. Upon incubation with plasma kallikrein or snake venom kininogenase, they yield the corresponding kinin-free proteins which consist of a heavy and a light chain [2] derived, respectively, from the NHzand COOH-terminal portions of the parent molecule [3] . The heavy chains have the COOH-terminal sequence -Leu-MetLysOH [4], giving rise to the speculation that the heavy chain is contiguous with the kinin moiety in both kininogens, inasmuch as Met-Lys-bradykinin has been isolated from bovine plasma [5]. However, this sequence does not agree with those of the two kinin-containing peptides, Ser-Arg-Met-Lysbradykinin and Gly-Arg-Met-Lys-bradykinin, which were isolated by Hochstrasser and Werle [6] from the peptic digest of bovine plasma Cohn fraction IV-6. To resolve this contradiction, we have re-examined bovine kininogen and established the sequence of residues preceding the kinin moiety. The strategy used was to prepare a chemically modified HMW kininogen, digest it with pancreatic kallikrein, isolate the kinincontaining peptide from the digest, and determine its amino acid sequence. The results indicate that the sequence is Ser-Lcu-Met-Lys-bradykinin, showing that the COOH-terminus of the heavy chain is indeed juxtaposed to the kinin moiety. 2. Materials and methods