Trypsin‐Induced Activation of Renin Precursor in Plasma of Normal and Anephric Man

A two-stage incubation protocol has been employed to ascertain whether substantial trypsin-induced elevation of human plasma renin activity (PRA) ("activation") is mediated by the renin system. In stage 1, plasma, or serum, was exposed at 4°C, or 22°C, to exogenous trypsin. This was followed by stage 2, a standard incubation protocol at 37°C for determination of PRA by radioimmunoassay of the generated angiotensin 1. The following evidence supports the view that the action of trypsin is mediated by the renin system(1) Activation is generally greater in stage 2 at a pH of 5.8 (near optimum for renin), than at 7.4 (near optimum for trypsin). (2) Activation in stage 2 occurs even in the presence of the trypsin inhibitors diisopropylfluorophosphate (DFP) and lima bean trypsin inhibitor (LBTI), indicating that it is independent of continued tryptic activity. (3) Activation in stage 2 occurs in plasma from which a trypsin-gel complex has been physically removed by centrifugation and filtration at the end of stage 1. (4) Exposure of plasma to trypsin for 1, 10, 40, or 70 minutes in stage 1, followed by identical incubations in stage 2, results in similar activation throughout, suggesting that the activating effect of added trypsin is essentially complete within 1 minute of addition. (5) Trypsin does not appear to generate tetradecapeptide in plasma, from which angiotensin is produced by the action of pseudorenin. These observations suggest that trypsin rapidly initiates changes in the renin system, then is neutralized by endogenous trypsin inhibitors. The greatly enhanced angiotensin generation observed thereafter probably reflects an increase in renin, due to activation of an undefined renin precursor. Unexpectedly, precursor was found also in long-term postnephrectomy plasma of men and women. A range of higher molecular weight, acid-activatable renins was isolated from fresh, normal, human kidney cortex, suggesting this as a possible source of precursor in normal plasma. The precise nature and source of precursor in normal and postnephrectomy plasma remain to be elucidated.