Bikunin prevents adhesion of calcium oxalate crystal to renal tubular cells in human urine.

Crystal-renal tubular cell interactions are important factors in crystal retention and development of kidney stones. It has been reported that human urine, especially its macromolecular fraction, distinctively prevented calcium oxalate monohydrate (COM) crystal adhesion to tubular cells. This study was designed to find and isolate a specific substance in human urine with a strong inhibitory effect against crystal adhesion. A protein from the urine was purified by two anion exchange chromatography columns and one gel filtration column. The inhibition activity for COM crystal adhesion to Madin-Darby canine kidney cells was determined quantitatively. Amino acid sequence of the protein was analyzed and then subjected to homology search in the GenBank protein database. A specific human urine protein that inhibited the COM crystal adhesion to the cells was isolated and identified. Molecular mass of the protein was approximately 35 kD. The first 20-amino acid sequence from the N-terminal of the purified protein was structurally homologous with the light chain of inter-alpha-trypsin inhibitor, also called bikunin. The isolated bikunin inhibited crystal adhesion at a minimum concentration of 10 ng/ml, and blocked completely at 200 ng/ml. It is concluded that bikunin may contribute to the regulation of crystal adhesion and retention within tubules during kidney stone formation.