Initiation of plasma prorenin activation by Hageman factor-dependent conversion of plasma prekallikrein to kallikrein.

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Abstract
Plasma prorenin is an inactive form of renin (EC 3.4.99.19) that can be converted to active renin in acid-treated plasma by an endogenous serine protease that is active at alkaline pH (alkaline phase activation). To identify this enzyme the ability of Hageman factor fragments, plasma kallikrein (EC 3.4.21.8), and plasmin (EC 3.4.21.7) to activate prorenin in acid-treated [human] plasma was tested. All 3 enzymes initiated prorenin activation; 50% activation was achieved with Hageman factor fragments at 1 .mu.g/ml, plasma kallikrein at 2-4 .mu.g/ml, or plasmin at 5-10 .mu.g/ml. The alkaline phase of acid activation occurred normally in plasminogen-free plasma but was almost completely absent in plasmas deficient in either Hageman factor or prekallikrein; alkaline phase activation was restored to these latter plasmas when equal parts were mixed together. Therefore, both Hageman factor and prekallikrein were required for alkaline phase activation to occur. Although plasma kallikrein could activate prorenin in plasma deficient in either Hageman factor or prekallikrein, Hageman factor fragments were unable to activate prorenin in prekallikrein-deficient plasma. Alkaline phase prorenin activation is initiated by Hageman factor-dependent conversion of prekallikrein to kallikrein which, in turn, leads to activation of prorenin. A possible link may exist between the coagulation-kinin pathway and the renin-angiotensin system.