1. Activation of inactive renin in rat plasma has been studied with different trypsin concentrations and incubation times at pH 6.2 and 4°C.

2. Trypsin concentrations below 2 mg/ml, lower than endogenous rat plasma anti-trypsin activity, do not activate inactive renin, whereas maximal activation is obtained with trypsin at 6 mg/ml for 1 min at 4°C, pH 6.2.

3. Under these conditions trypsin can cleave dialysable fragments from renin substrate. ANG I can be generated at 37°C with a pH optimum of 5.3. Nevertheless, the ANG I formation at pH 6.2 was totally unaffected.

4. Incubations longer than 2 min with trypsin at 6 mg/ml can induce a direct cleavage of dialysable ANG I-containing fragments strongly interfering with the measurements of renin activity at pH 6.2.

5. On average 40% of the total renin measured in plasma of normotensive WK rats is in the inactive form, although a wide range of variation is observed.

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