A 56,000-dalton renin-binding protein in hog kidney is an endogenous renin inhibitor.

Renin-binding protein (RBP) in hog kidney was characterized and identified as a renin inhibitor. The molecular weight (Mr) of rBP is 56,000 when estimated by gel filtration. Its isoelectric point is 4.9. Upon binding to renin, it forms a renin.RBP complex with Mr = 113,000 as revealed by sedimentation equilibrium using an air-driven ultracentrifuge (Airfuge). The renin.RBP complex, formed by incubating hog kidney extract at 37 degrees C, is inactive when assayed with little dilution. On extensive dilution, however, this complex dissociates and acquires renin activity. The equilibrium between renin and renin.RBP complex and the equilibration rate were also studied by high performance liquid chromatographic gel filtration. The equilibrium was shifted to the complex, suggesting that RBP is present in an excess of renin in the kidney. The complex was shown to dissociate during the gel filtration due to rapid equilibration. Therefore, the elution position of the complex varied depending on flow rates, thereby making it difficult to determine the Mr of the complex by gel filtration. The renin.RBP complex is unlikely to occur in the normal kidney because renin and RBP are localized in the granule and cytosol fractions, respectively. RBP may provide security against accidental rupture of renin granules under abnormal circumstances.