An N‐terminal peptide from link protein is rapidly degraded by chondrocytes, monocytes and B cells

A peptide cleaved from the link‐protein component of human and pig proteoglycan aggregates by trypsin and stromelysin was taken up and degraded further by human monocytes, B cells, chondrocytes and by mouse peritoneal macrophages. Monocytes were able to process the peptide twice as rapidly as peritoneal macrophages and some 16 times more rapidly than articular chondrocytes. The B cell line Priess, which unlike the monocytes and macrophages could not take up or degrade whole proteoglycan aggregates, was able to degrade the peptide at a rapid rate. Synthetic, unglycosylated peptides consisting of the first 16 and 13 N‐terminal amino acids of human link protein, corresponding to its stromelysin‐cleavage and trypsin‐cleavage products, were also taken up and degraded in a similar manner to the natural products and, in addition, were able to block uptake of the 125 I‐labelled natural peptides. The isoelectric points of the re‐secreted breakdown fragment from both the synthetic and natural peptides were identical and each peptide was processed by the cells to produce a single radiolabelled fragment. Each of these fragments was eluted with the same retention time during HPLC, indicating that the natural peptides were derived from the N‐terminal region of the link. Since a proportion of the link protein extracted from human and pig cartilage has already undergone proteolysis to remove peptides from its N‐terminal region, these peptides may be produced in articular cartilage during the normal process of turnover and ageing. Although a physiological function for this protein has not been established, it may have a homeostatic role in the regulation of proteoglycan synthesis.