RENAL CALCIUM-OXALATE BINDING-PROTEIN - STUDIES ON ITS PROPERTIES

To understand the mechanism of calcium oxalate (CaOx) crystal retentio n within the kidneys, calcium oxalate binding protein was isolated and characterized. The specific activity of calcium oxalate binding prote in in the homogenate was 4.54 nmol/mg protein. The renal medulla showe d higher CaOx binding activity than that of papilla or cortex, and amo ng the cellular fractions the nucleus exhibited highest specific activ ity. Several tissues showed CaOx binding activity suggesting its ubiqu itous 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 mol ecular weight of 45 kDa. The CaOx binding protein had the kinetic prop erties of concentration and time dependency, optimum temperature and s ubstrate saturability. Scatchard plot analysis showed a single affinit y site with a kDa of 41 nM and B-max of 6.7 nmol/mg protein. The bindi ng activity was inhibited by the anion transport inhibitor DIDS and su bstrate analogs like succinate and oxamide, while EGTA or ruthenium re d had no effect on binding, suggesting that the protein binding was ox alate 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 huma n kidneys, as well as other rat tissues.