URONIC-ACID-RICH PROTEIN - A NEW GLYCOPRO TEIN INHIBITING CRYSTALLIZATION OF CALCIUM-OXALATE IN-VITRO

During about ten years, nephrocalcin was considered the main calcium o xalate crystal growth inhibitor. Today, it appears only a urinary prot ein inhibitor among other ones such as non polymerized Tamm-Horsfall p rotein, uropontin or crystal matrix protein (CMP), a protein derived f rom prothrombin. All these molecules are able to inhibit either crysta l 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 ne w urinary inhibitor of calcium oxalate formation was isolated from the urine of healthy subjects using chromatographic procedures. II is a m acromolecule with a molecular weight (MW) of approximately 35 kDa as e stimated by polyacrylamide gel electrophoresis. Its carbohydrate conte nt represents an average of 8.5% of its MW. Glutamic and aspartic acid s represent 24% of total amino acids. This protein is called Uronic-Ac id-rich Protein (UAP) because of its uronic acid content. The same pro tein isolated from the urine of stone formers showed less inhibitory a ctivity than that purified from the urine of healthy subjects. Structu ral modifications may explain this diminution of its inhibitory activi ty. This protein was also purified from rat urine using same procedure s. Human and rat UAP exhibit similar biochemical characteristics and s trongly inhibit calcium oxalate crystallization in vitro. Partial amin oacid sequence analysis showed a homology with inter-alpha-trypsin inh ibitor (ITI), confirmed by the immunological results on Western blot. Nevertheless, various chemical and enzymic treatments revealed that UA P and ITI are not identical molecules. Consequently, urine contains se veral macromolecular substances belonging to ITI superfamily which are involved in the inhibition of calcium oxalate cristallization. UAP la kes place among the most efficient macromolecular substances known as inhibitors in calcium oxalate nephrolithiasis.