STUDIES ON PLASMA KININS

SUMMARY Release of vasodilator peptides (collectively known as “plasma kinin”) from human plasma was measured using synthetic bradykinin as a reference standard. Under optimal conditions an equivalent of 3–4 μg, of bradykinin/ml. of plasma can be recovered. As the kinin is liberated its precursor (kininogen) is quantitatively exhausted. Trypsin and glandular kallikreins (as in saliva or urine) release the maximal amount of kinin and consume all of the precursor. In this they differ from plasma kallikrein whose activation is initiated by contact with foreign surfaces, organic solvents, and acidification, These “intrinsic” mechanisms are dependent on the presence of Hageman factor (also required for blood clotting) and “component A”, which can be identified with plasma kallikreinogen. Activated component A (i.e. plasma kallikrein) releases 1–1.5 μg. of kinin/ml. in a rapid reaction with approximately one‐third of kininogen. It is suggested that this is the precursor of nonapeptide only and corresponds to “component B”. In the other 2/3 of kininogen the kinin is bound as a decapeptide which can either be released as such (by glandular kallikreins) or serve as another source of the nonapeptide (with trypsin). The affinity of plasma kallikrein for this substrate is very low.