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The possible role of enzymes in plasma and blood cells for the conversion of inactive renin was investigated in mice of the Balb C strain. The concentrations of both inactive and active renin were unchanged in unfractionated plasma and whole blood during incubation at 37°C for 5 h, indicating, that inactive renin had not been activated. Purified platelets, mononuclear cells (monocytes and lymphocytes), and polymorphonuclear leukocytes were incubated separately with partially purified inactive renin at 37°C for 1 h. Inactive renin was neither activated nor degraded by whole blood cells. Polymorphonuclear neutrophils were also obtained in mice after i.p. injection of fluid thioglycollate medium. Extracts of the polymorphonuclear neutrophils were incubated with partially purified, inactive renin. During incubation at pH 7.5 for 1 h at 37°C, inactive renin was stable and no activation was registered, whereas degradation, without a concomitant increase in active renin, occurred at pH 4.5.Extracts of polymorphonuclear neutrophils were found to contain active, but no inactive renin. The mean amount of renin per cell was 0.5 x 10-11 Goldblatt units (GU) (range 0.3 x 10-11 GU – 0.9 x 10-11 GU; n = 6).These findings argue against a role for the blood cells and plasma in a physiological activation of inactive renin, and do not support the concept that activation of inactive renin in the circulation may participate in homeostasis.