1. Plasma samples from 31 normal subjects were treated (at 4°C, pH 7.0, for 2 min) with different concentrations of trypsin (500, 1000, 2000, 3000 and 4000 μg/ml) in order to assess which concentration yielded the maximum activation of inactive renin.

2. Endogenous antitryptic activity was also measured in all samples; the mean value ± sd (in μg of trypsin inhibited by 1 ml of plasma) was 953 ± 550 μg/ml (range 34–1800 μg/ml).

3. In the entire group of subjects the values of trypsin-activated renin measured with trypsin at 2000 μg/ml were significantly higher than those obtained with lower or higher trypsin concentrations.

4. With subjects divided into subgroups according to their endogenous anti-tryptic activity, the maximum yield of activation was reached with trypsin at 2000 μg/ml.

5. No significant correlations were found between single values of active, inactive or trypsin-activated renin and the corresponding levels of endogenous anti-tryptic activity. However, a weak but significant correlation (r = 0.39, P < 0.05) was found between single values of anti-tryptic activity and the corresponding percentage of activation of inactive renin.

6. Thus the maximum activation of inactive renin at 4°C for 2 min is obtained with trypsin at 2000 μg/ml independently of the corresponding endogenous anti-tryptic activity. It is not excluded that the content of protease inhibitors in human plasma might affect the proportion in vivo of circulating active and inactive renin.

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