A NOVEL CARTILAGE PROTEIN (CILP) PRESENT IN THE MID-ZONE OF HUMAN ARTICULAR-CARTILAGE INCREASES WITH AGE

A novel, somewhat basic noncollagenous protein was purified from guani dine hydrochloride extracts of human articular cartilage using cesium chloride density gradient centrifugation, followed by ion-exchange chr omatography at pH 5, and gel filtration on two serially coupled column s of Superose 6 and Superdex 200. The protein of 91.5 kDa contains a s ingle polypeptide chain substituted with N-linked oligosaccharides. It appeared unique to cartilage as studied by enzyme-linked immunosorben t assay and immunoblots of various tissue extracts. Its concentration in articular cartilages showed some variability with age being lower i n young individuals. It represents a chondrocyte product, since it is synthesized by articular chondrocytes in explant cultures. Interesting ly, the distribution of the protein in the articular cartilage provide s important information on the nature of chondrocytes at different com partments in the tissue. Thus, chondrocytes in the middle/ deeper laye rs of the tissue in particular, appeared to have produced the protein and deposited it in the interterritorial matrix. The protein was neith er seen in the superficial nor in the deepest regions of the articular cartilage. Based on its immunolocalization we have named this protein CILP (cartilage intermediate layer protein).