1. Inactive renin was isolated from human amniotic fluid by chromatography with DEAE-cellulose, pepstatin-aminobutyl-agarose, octylagarose and Cibacron Blue F3GA columns.

2. Before and after isolation from amniotic fluid, inactive renin could be activated by incubation with pepsin and by dialysis to pH 3.3, and the acid activation could be reversed by subsequent incubation at pH 7.4 and 37°C.

3. Inactive renin was not activated by procedures expected to dissociate renin from an inhibitor.

4. The results suggest a pH-dependent conformational change as the mechanism of reversible acid activation of inactive renin in amniotic fluid.

5. There are chromatographic and activation similarities of inactive renin from human plasma, kidney and amniotic fluid.

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