1. The properties of inactive and active renin in human plasma and amniotic fluid were studied chromatographically. Activation was achieved at pH 3.3 with and without added pepsin.

2. Acid activation of renin was time- and temperature-dependent but was inhibited by dilution of the sample. The dilution effect was corrected by adding pepsin. Such characteristics indicate that activation at low pH is catalysed by intrinsic enzymes.

3. Separation and/or dilution of the activating enzyme during ion-exchange chromatography concealed the eluted position of inactive renin and reduced the amount recovered. Only after full activation of the eluted renin was achieved with added pepsin was a distinct peak of inactive renin exposed.

4. At pH 7.5 inactive renin carried a lower negative charge than the active enzyme. This charge difference was lost after activation.

5. No molecular-weight differences between active, inactive renin or the International Renin Standard were detected by gel filtration. No renin of larger molecular weight was present.

6. These findings will be helpful in purification studies of human inactive renin.

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