Methionyl-kallidin

Incubation of a protein fraction from blood plasma with trypsin gives rise to peptides with conspicuous biological effects. Pain, dilation of peripheral blood vessels, increased coronary flow and enhanced capillary permeability were observed on administration of these protein fragments [8]. In the early sixties the nonapeptide bradykinin and its precursors, kallidin and methionyl-kalUdin were isolated in pure form and their amino acid sequences determined soon after. 

There are 3 known plasma kinins, namely the nonapeptide bradykinin or kallidin-9 [154, 157, 158, 363, 364, 473, 523], lysyl-bradykinin or kaUidin-10 [499,665] and methionyl-lysyl-bradykinin [155,159,556]. These pharmacologically active polypeptides behave simlarily, although quantitative differences exist between them [141, 156, 412]. Bradykinin increases vascular permeability in the rat after intradermal injection [82,107,167,633] and also after sub-plantar injection into the paw [110, 210, 389, 601], and as such plasma kinins are potential mediators of the anaphylactoid oedema. 

The name kinin in this text always refers to three distinct biologically active peptides nonapeptide bradykinin (nine amino acids), decapeptide kallidin (ten amino adds) and undecapepdde methionyl-lysyl-bradykinin (eleven amino adds). All other peptides will be characterized specifically. 

The chemical structures of bradykinin, kallidin and methionyl-lysyl-brady-kinin ( = methionyl-kallidin) are shown in Figure 6. 

The half-life of kinins in the circulation is measured in seconds. By the action of plasma kininase they are rapidly converted to biologically inactive compounds. The main kininase in the plasma is the carboxypeptidase N (kininase I) that splits off the C-terminal residue of bradykinin— the Phe -Arg bond. In addition, another kininase was discovered in human plasma— kininase II. which splits the Pro -Phe bond of bradykinin. Moreover, kallidin and methionyl-kallidin can be converted to bradykinin by the action of plasma aminopeptidase. 

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