[Uronic-acid-rich protein: a new glycoprotein inhibiting the crystallization of calcium oxalate in vitro].

During about ten years, nephrocalcin was considered the main calcium oxalate crystal growth inhibitor. Today, it appears only a urinary protein inhibitor among other ones such as non polymerized Tamm-Horsfall protein, uropontin or crystal matrix protein (CMP), a protein derived from prothrombin. All these molecules are able to inhibit either crystal growth or aggregation of calcium oxalate in urine. Another protein, named renal lithostathine, was also reported to be a potent inhibitor of secondary nucleation and growth of calcium carbonate crystals. A new urinary inhibitor of calcium oxalate formation was isolated from the urine of healthy subjects using chromatographic procedures. It is a macromolecule with a molecular weight (MW) of approximately 35 kDa as estimated by polyacrylamide gel electrophoresis. Its carbohydrate content represents an average of 8.5% of its MW. Glutamic and aspartic acids represent 24% of total amino acids. This protein is called Uronic-Acid-rich Protein (UAP) because of its uronic acid content. The same protein isolated from the urine of stone formers showed less inhibitory activity than that purified from the urine of healthy subjects. Structural modifications may explain this diminution of its inhibitory activity. This protein was also purified from rat urine using same procedures. Human and rat UAP exhibit similar biochemical characteristics and strongly inhibit calcium oxalate crystallization in vitro. Partial amino acid sequence analysis showed a homology with inter-alpha-trypsin inhibitor (ITI), confirmed by the immunological results on Western blot. Nevertheless, various chemical and enzymic treatments revealed that UAP and ITI are not identical molecules. Consequently, urine contains several macromolecular substances belonging to ITI superfamily which are involved in the inhibition of calcium oxalate crystallization. UAP takes place among the most efficient macromolecular substances known as inhibitors in calcium oxalate nephrolithiasis.