Effect of experimental hyperoxaluria on renal calcium oxalate monohydrate binding proteins in the rat

To determine the functional role of calcium oxalate binding proteins in the nucleation, aggregation and retention of calcium oxalate crystals under physiological and hyperoxaluric conditions. Materials and methods Hyperoxaluria was induced in rats using 0.75% of ethylene glycol in drinking water. Calcium oxalate binding proteins were isolated and fractionated by cellulose column chromatography. Three major protein peak fractions were obtained (73 kDa in Tris-HCl buffer, 20 kDa in 0.05 mol/L NaCl buffer and 23 kDa in 0.3 mol/L buffer). Oxalate binding and the inhibition of crystal nucleation and aggregation by these fractions were determined.

The adsorption of calcium oxalate monohydrate (COM) was ubiquitous in rat tissues and subcellular organelles, but the percentage adsorption varied; maximum absorption occurred in kidneys and pancreas, with microsomes showing maximal adsorption in the kidney. Hyperoxaluric rat tissues showed a greater percentage adsorption. Microsomes were enriched with the 20 kDa protein, while nuclei contained the 23 kDa protein in higher concentrations. COM-binding proteins derived from hyperoxaluric rat kidney had a greater content of 74 kDa and 23 kDa proteins with increased oxalate-binding activities. In the crystal-growth studies, the 74 kDa protein was a promoter, while the other protein fractions inhibited crystallization. In hyperoxaluria, the crystal-growth promoting activity of the 74 kDa protein was further increased, while the inhibition by the 20 and 23 kDa proteins was decreased. The 74 kDa protein derived from control rats formed single COM crystals in a crystal growth system, while the hyperoxaluric rat fraction induced the aggregation of COM crystals.

COM-binding proteins (the 74 and 23 kDa fractions) were expressed more in hyperoxaluric rats. In hyperoxaluria the 74 kDa protein tended to promote crystal nucleation and aggregation, and the 20 and 23 kDa proteins were less inhibitory, which increases the risk of stone formation.