Role of inter-α-inhibitor and its related proteins in urolithiasis.: Purification of an inter-α-inhibitor related protein from the bovine kidney

Urine contains several macromolecules that inhibit calcium oxalate (CaOx) crystallization. Among them is bikunin, the light chain of most of the inter-alpha-inhibitor (I alpha I) family of glycoproteins. This study aimed to verify whether bikunin and other members of the I alpha I family are synthesized in the kidneys or derived exclusively from the plasma. Proteins extracted from homogenized bovine kidney were applied successively to three chromatographic steps on DEAE-Sephacel, Sephacryl S-300, and Mono Q column. The inhibitory activity was assayed using a CaOx crystallization system. The presence of I alpha I-related proteins was determined by electrophoresis and Western blotting. The results showed that kidney extract contained a 125-kDa protein that cross-reacted with anti-I alpha I antibodies. This protein inhibited CaOx crystallization efficiently. According to its molecular weight and immunoreaction with anti-I alpha I antibody, the 125-kDa protein could be pre-alpha-inhibitor. The latter is known to encompass a heavy chain and bikunin, which may explain its inhibitory activity against CaOx crystallization. Consequently, we hypothesize that kidneys may produce some I alpha I-related proteins that are involved in the inhibition of stone formation.