Polypeptide proteinase inhibitor from human articular cartilage

Abstract

A polypeptide proteinase inhibitor from human articular cartilage has been purified to homogeneity by stepwise Sephadex G-75, heparin-Sepharose and octyl-Sepharose affinity chromatography. The inhibitor is strongly cationic (p I ⩾ 10.5) and consists of two non-identical polypeptides associated by means of electrostatic and/or hydrophobic interactions. Amino acid analysis of the aggregate confirmed that the polypeptide was rich in basic, and hydrophobic amino acids and contained only one disulphide bridge. Sedimentation equilibrium studies showed that the aggregate had M w = 7000 which could be dissociated into two polypeptides each of M w = 3500. While the subunits were primarily serine proteinase inhibitors the aggregate form could also inhibit bacterial collagenase and pepsin but not thermolysin nor the cystein proteinases, ficin or bromelain. Binding of 125I-labelled human cartilage inhibitor to heparin, keratan sulphate and proteoglycan subunit was demonstrated using gel exclusion chromatography but no interaction was detected with chondroitin 6-sulphate or hyaluronic acid. Binding of cartilage inhibitor subunits to link proteins was also shown by polyacrylamide electrophoresis. These data suggest that the human cartilage inhibitor may be localised at specific sites on the proteoglycan complex where it would be ideally placed to attenuate degradation by matrix proteinases or constitute part of an enzyme-inhibitor complex.