Rat Renal and Plasma Prorenin Are Activated In Vitro by Different Mechanisms

Abstract

Abstract —The aim of the present study was to purify and identify a plasma protein fraction (PreR-Co) involved in renal prorenin activation and to explore its capacity to process plasma prorenin. PreR-Co was obtained from plasma as a single electrophoretic band by (NH 4 ) 2 SO 4 precipitation, Sephacryl S-200 HR gel filtration, anti-rat albumin immunoaffinity, and ion-exchange chromatography. The amidase, esterase, and kallikrein activities of PreR-Co were studied, as was its N-terminal amino acid sequence. Rat kidney extract or plasma (normal or previously treated with acid to pH 2.8) were incubated with PreR-Co for 15 minutes at 37°C. Renin concentration was measured by incubation with homologous angiotensinogen. The same protocol was repeated with samples activated by trypsin. The N-terminal amino acid sequence was IIGGSMDAKGSFP, which had a homology of 90% with the β-chain of haptoglobin, 69% with serine-proteases, and 65% with kallikreins. The renin concentration in rat kidney extract was 34±4 ng of angiotensin I (Ang I) · mg of tissue −1 · h −1 . After PreR-Co or trypsin treatments, renin concentrations were 211±7 and 110±11 ng of Ang I · mg of tissue −1 · h −1 , respectively. The plasma renin concentration in normal plasma was 67.6±13.3 ng of Ang I · mL −1 · h −1 , and no significant difference was observed after PreR-Co treatment. However, a significant increase (202.8±7.8 ng of Ang I · mL −1 · h −1 ; P <0.01) was found after trypsin treatment. The isolated PreR-Co acts on renal prorenin but not on plasma prorenin. These results suggest that active renin is processed in the kidney by a circulating enzyme that may have a role in the regulation of circulating renin.