1. Normal human plasma contains a pro-activator of inactive renin. The pro-activator is activated at physiological pH in plasma that has been pretreated with acid. This activation in vitro leads to the conversion of inactive renin into the active form with simultaneous generation of kallikrein activity.

2. The endogenous activator of inactive renin has the same pH profile and inhibitor spectrum as plasma kallikrein.

3. Inactive renin can also be activated by exposure of plasma to exogenous trypsin, and in normal plasma the quantities of inactive renin that are activated after acidification and with trypsin are identical. Prekallikrein (Fletcher factor)-deficient plasma, however, has much lower renin activity after acidification than with trypsin. Thus acid activation of inactive renin depends on plasma prekallikrein, whereas the action of trypsin is independent of prekallikrein.

4. Highly purified tissue (pancreatic) kallikrein, in a concentration of less than 2 × 10−8 mol/l, activates inactive renin that has been isolated from plasma by ion-exchange chromatography. In this respect it is at least 100 times more potent than trypsin.

5. It is therefore possible that plasma and/or tissue (renal) kallikreins are also involved in the activation of inactive renin in vivo.

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