Inter-alpha-inhibitor: a protein family involved in the inhibition of calcium oxalate crystallization.

Inter-alpha-inhibitor (I alpha I) is a serine protease inhibitor present in human plasma. It has a molecular weight of about 220 kDa which encompasses 3 chains including two heavy chains and one light chain. The light chain, known as bikunin, is responsible for the antitryptic activity of I alpha I in the inhibition of various enzymes, such as trypsin and chymotrypsin. Under physiologic or certain pathologic circumstances, several macromolecules related to I alpha I appear in plasma and urine. However, the physiologic role of I alpha I remains unclear. As far as urolithiasis is concerned, two urinary macromolecules related to I alpha I have been isolated and shown to be potent inhibitors of calcium oxalate formation. One of these inhibitors, uronic-acid-rich protein (UAP), has been identified and well characterized. The sequence of the first 18 amino acid residues of UAP is identical with that of bikunin. Furthermore, the immunoreaction between UAP and I alpha I antibody using immunoblot analysis was positive. UAP isolated from the urine of stone formers exhibited less inhibitory activity towards calcium oxalate crystallization than that derived from the urine of healthy subjects. This suggests a structural abnormality of the inhibitor obtained from stone patients. The organic matrix extracted from kidney stones contained a protein antigenically related to I alpha I. We conclude that UAP is a member of I alpha I family taking part in inhibiting calcium oxalate crystallization, and modulating the formation of stones in the urinary tract.