KININS
BIOSYNTHESIS OF KININS
Kinins
are potent vasodilator peptides formed enzymatically by the action of enzymes
known as kallikreins or kininogenases acting on protein substrates called
kininogens. The kallikrein-kinin system has several features in common with the
renin-angiotensin system.
Kallikreins
are present in plasma and in several organs and tissues, including the kidneys,
pancreas, intestine, sweat glands, and salivary glands. Plasma prekallikrein
can be activated to kallikrein by trypsin, Hageman factor, and possibly
kallikrein itself. In general, the biochemical properties of tissue kallikreins
are different from those of plasma kallikreins. Kallikreins can convert
prorenin to active renin, but the physiologic significance of this action has
not been established.
Kininogens—the
precursors of kinins and substrates of kallikreins—are present in plasma, lymph,
and interstitial fluid. Two kininogens are known to be present in plasma: a
low-molecular-weight form (LMW kininogen) and a high-molecular-weight form (HMW
kininogen). About 15–20% of the total plasma kininogen is in the HMW form. It
is thought that LMW kininogen crosses capillary walls and serves as the
substrate for tissue kallikreins, whereas HMW kininogen is confined to the
bloodstream and serves as the substrate for plasma kallikrein.
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