Activation and Characterization of Inactive Big Renin in Plasma of Patients with Diabetic Nephropathy and Unusual Active Renin*

Human plasma contains inactive renin, which is converted to an active enzyme closely resembling renin upon dialysis to pH 3.3 and then to pH 7.5 or by exposure to trypsin or pepsin. Characterization of inactive renin has been hindered by incomplete activation in diluted plasma. We have compared methods of renin activation in whole and diluted plasma and in eluates from gel filtration. Plasma of diabetic patients with nephropathy and nonuremic hyperkalemia (DN) was used because it contains large amounts of inactive renin and less than normal amounts of active renin. In whole plasma, the same amount of inactive renin was activated by dialysis (pH 3.3 → 7.5), with or without added pepsin (50 μg/ml) at pH 3.3 or by incubation with trypsin (0.2– 1.0 mg/ml) at pH 7.5. When plasma was diluted, dialysis (pH 3.3 → 7.5) activated only 15%, but incubation with pepsin (50 μg/ml) at pH 3.3 and 32 C converted all inactive renin to active enzyme; slightly less activation occurred when diluted plasma was incubated with trypsin (0.05–0.2 mg/ml) at pH 7.5. The concentration of trypsin was critical, as excess trypsin destroyed active renin. After gel filtration, only 10% of the inactive renin of whole plasma was activated in eluates dialyzed to pH 3.3 → 7.5. Incubation of eluates with trypsin (0.1 mg/ml) yielded 40-60% of the inactive renin of plasma. Optimal trypsin concentration varied in different eluates. Pepsin activation in eluates was 80–100%; pepsin did not destroy active renin. Inactive renin, demonstrated by pepsin activation, eluted in a large peak with an apparent molecular weight of 58,000 (Sephadex G-100) or 53,000 (Sephacryl S-200). The small amounts of active renin in DN plasma eluted in a broad zone near the peak of inactive renin. No peak of active renin eluted with a molecular weight of 40,000, characteristic of the active renin in normal plasma. A defect in the intrarenal conversion of inactive renin of large molecular weight to a smaller active form may explain the functional hyporeninemia in DN patients.