The discovery of kinins and many of their subsequent investigations were made independently by Werle1 and Rocha e Silva and his associates.2 The latter team came upon the discovery through studying the venom of a Brazilian snake in 1949; they noted that the venom's proteolytic enzymes liberated from the serum a factor which caused contraction of isolated guinea pig ileum and produced hypotension on injection into the vein of a rabbit. They named this vasodilating, smooth-muscle contracting hormone—subsequently identified as a nonapeptide—"bradykinin," and its inactive precursor, "bradykininogen."
Werle arrived at similar results by an entirely different route. He extracted from the pancreas and the salivary glands a proteolytic enzyme which splits off a smooth-muscle contracting factor from an inactive precursor in the serum. The enzyme was subsequently named "kallikrein" and the hormone, "kallidin." The latter, a decapeptide, has the same mode of action as bradykinin but a different