Urinary trypsin inhibitor (UTI), a Kunitz-type protease inhibitor, directly
binds to some types of cells via cell-associated UTI-binding proteins (UTI
-BPs). Here we report that the 40-kDa protein (UTI-BP40) was purified from
the cultured human chondrosarcoma cell line HCS-2/8 by UTI affinity chromat
ography. Purified UTI-BP40 was digested with trypsin, and the amino acid se
quences of the peptide fragments were determined. The sequences of six tryp
tic fragments of UTI-BP40 were identical to subsequences present in human l
ink protein (LP). Authentic bovine LP and UTI-BP40 displayed identical elec
trophoretic and chromatographic behavior. The UTI-binding properties of UTI
-BP40 and LP were indistinguishable. Direct binding and competition studies
strongly demonstrated that the NH2-terminal fragment is the UTI-binding pa
rt of the LP molecule, that the COOH-terminal UTI fragment (HI-8) failed to
bind the NH2-terminal subdomain of the LP molecule, and that LP and UTI-BP
40 exhibited significant hyaluronic acid binding. These results demonstrate
that UTI-BP40 is identical to LP and that the NH2-terminal domain of UTI i
s involved in the interaction with the NH2-terminal fragment of LP, which i
s bound to hyaluronic acid in the extracellular matrix.