Renal calcium oxalate binding protein: Studies on its properties

Abstract

To understand the mechanism of calcium oxalate (CaOx) crystal retention within the kidneys, calcium oxalate binding protein was isolated and characterized. The specific activity of calcium oxalate binding protein in the homogenate was 4.54 nmol/mg protein. The renal medulla showed higher CaOx binding activity than that of papilla or cortex, and among the cellular fractions the nucleus exhibited highest specific activity. Several tissues showed CaOx binding activity suggesting its ubiquitous nature. After being subjected to acetone precipitation, ethanol precipitation and HPLC chromatography, the renal protein revealed a 57-fold purity with a specific activity of 260 nmol/mg protein and a molecular weight of 45 kDa. The CaOx binding protein had the kinetic properties of concentration and time dependency, optimum temperature and substrate saturability. Scatchard plot analysis showed a single affinity site with a kDa of 41 nM and Bmax of 6.7 nmol/mg protein. The binding activity was inhibited by the anion transport inhibitor DIDS and substrate analogs like succinate and oxamide, while EGTA or ruthenium red had no effect on binding, suggesting that the protein binding was oxalate site specific. The molecular weight of the CaOx binding protein of different tissues was similar to that of renal cells. In conclusion, the presence of CaOx binding protein is demonstrated in rat and human kidneys, as well as other rat tissues.