Kinins

(1) Cytokinins, substances in plants and microorganisms, derivatives of 6-aminopurine. Many kinins are found in seeds and fruits; fewer are found in roots, stems, and leaves. For example, 1 g of a kinin called zeatin is obtained from every 70 kg of corn kernels (of milky ripeness). Kinins stimulate cell division, participate in the formation of roots and stems and in the differentiation of new organs, and help to prolong the life of cut leaves. Thus, the application of kinins to yellowed cut leaves causes them to turn green. These and other important functions of kinins are associated with their participation both in the formation of RNA, DNA, and protein and in the redistribution of metabolites in the plant; the application of kinins to one-half of a yellowed leaf intensifies the flow of metabolites from the other, untreated, half. One of the especially active kinins is kinetin, which has been isolated in yeasts.

(2) In animals, substances of a polypeptide nature with a broad spectrum of biological activity. Kinins relax the smooth musculature of the blood vessels, lower blood pressure, increase the permeability of the capillaries, produce sensations of pain, and contract or relax the smooth musculature of isolated organs.

Three kinins have been found in mammals and man: bradykinin, a linear nonapeptide (from nine amino acids); lysylbradykinin (or kallidin), a decapeptide (from ten amino acids); and methionyllysylbradykinin, an undecapeptide (from 11 amino acids). All kinins are formed in the blood plasma or in the intercellular spaces, splitting off (under the influence of kallikrein) from kininogen, an inactive proteinic precursor. A milliliter of human blood plasma normally contains 0–0.002 micrograms of kinins. Because of rapid enzymic inactivation in the blood and tissues (under the influence of kininase), kinins exert a predominantly local effect. The physiological role of kinins is associated with the regulation of local blood flow and capillary permeability.