1. A high-molecular-weight renin (Mr 60 000) was formed by the reaction of a low-molecular-weight renin (Mr 40 000) with a renin-binding substance in canine renal cortical extract in the presence of the sulphydryl (SH) group oxidizing agent potassium tetrathionate; thus the reaction required SH oxidation.

2. Renin extracted from isolated renin granules was adsorbed on to thiopropyl Sepharose 6B, and then liberated with dithiothreitol (50 mmol/l), indicating that it possessed on SH moiety(s).

3. However, the renin was capable of reaction with the renin-binding substance even after its SH moiety (or moieties) was protected with 5,5′-dithiobis-(2-nitrobenzoic acid).

4. The high-molecular-weight renin was converted into the low-molecular-weight renin by incubation (37°C, 15 min) with cytosol (soluble fraction) of renal cortex and liver. Such converting ability was diminished after the cytosol was treated with perchloric acid or potassium tetrathionate.

5. These results suggest that the reaction of renin with the renin-binding substance does not require disulphide bond(s) and that an enzymelike substance which is sensitive to SH oxidation is involved in the conversion from the high-molecular-weight renin into the low-molecular weight renin.

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