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 125I-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.