1. A soluble fraction of renal cortical homogenate (cytosol) and renin extracted from isolated renin granules of the dog kidney were kept at 0°C.

2. Although the molecular weight of the renin in the cytosol on day 1 was estimated to be about 40 000 by gel filtration, the renin was completely converted into a higher—molecular—weight form (60 000) by day 7. The renin in the granules kept its molecular size of 40 000 during cold storage.

3. This type of molecular—weight conversion could be performed without protease inhibitors.

4. Dithiothreitol neither inhibited the conversion into the higher—molecular—weight form of renin during cold storage nor led to a reduction in the molecular weight of renin, although the oxidation of thiol groups has been proposed as the mechanism for the molecular—weight conversion of renin.

5. Keeping a mixture of renin from the granules and crude renin—binding substance at 0°C for 7 days resulted in the conversion of the renin into the higher—molecular—weight form, indicating that the renin—binding substance we have previously described is required for the conversion during cold storage.

6. Acidification caused the higher—molecular—weight form of renin formed in the cytosol to change to the lower—molecular—weight form, with a concomitant increase in activity of about 50%.

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