Identification and Characterization of a Calcium Oxalate Crystal Growth Protein Inhibitor from Human Renal Stone Matrix

Abstract

Kidney stone formation is a complex process involving multiple factors. Although saturation product for CaOx is frequently exceeded in normal urine, most humans do not form stones. It has been assumed that inhibitors of urolithiasis have protective effect, although their precise role has not been defined. Kidney stones invariably comprise a combination of inorganic crystals and organic macromolecules consisting principally of proteins. Many proteins occur in stones, but their role in urolithiasis remains unknown. Calculi contain some proteins normally present in urine, in addition to others arising from injury inflicted by the stones themselves, making it impossible to discriminate between those that bind to the stone as it grows, but play no role in its development, and those that may be involved in regulating the formation of stone crystals. The inhibition is generally understood to arise mainly from the non-dialyzable molecules of urine, particularly acidic glycoproteins and glycosaminoglycans. Some inhibitor molecules have been identified, including Tamm Horsfall protein, uropontin , calgranulin, bikunin, and prothrombin F1 fragment. There are several hypotheses regarding kidney stone formation. According to one hypothesis, it is related to intratubular crystal nucleation, growth, and aggregation. While other hypothesis explains that the locale of crystal deposition is at a renal interstitium near or at the tip of renal papillae. newlineInsoluble nature of kidney stone poses many challenges in extraction of proteins present in its organic matrix; therefore, a comparison of different methods of extraction from human calcium oxalate containing stones was done to examine the soluble matrix proteins involved in their biomineralization process. It was observed that EGTA extract exhibited highest inhibitory activity (98%) towards CaOx crystal growth followed by acetic acid (6.47%) and SDS extract (2.64%). Therefore, EGTA extraction method was selected for the renal stone extraction to carry out the further studies.