Bradykinin and kallidin are potent vasodilators and hypotensive agents that have different peptide structures: bradykinin is a nonapeptide, whereas kallidin is a decapeptide. Kallidin is lysyl-bradykinin; that is, it has an additional lysine at the NH2 terminus of the chain. These two compounds are made available from kininogen, a blood globulin, on hydrolysis. Trypsin, plasmin, or the proteases of certain snake venoms can catalyze the hydrolysis of kininogen.

Bradykinin is one of the most powerful vasodilators known; 0.05 to 0.5 f g/kg intravenously can decrease blood pressure in all mammals investigated so far.

Although the kinins per se are not used as medicinals, kallikrein enzyme preparations that release bradykinin from the inactive precursor have been used in the treatment of Raynaud disease, claudication, and circulatory diseases of the eyegrounds. (Kallikreins is the term used to designate the group of proteolytic enzymes that catalyze the hydrolysis of kininogen, forming bradykinin.)

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