STRUCTURAL DIFFERENCES BETWEEN 2 POPULATIONS OF ARTICULAR-CARTILAGE PROTEOGLYCAN AGGREGATES

To determine if articular cartilage contains structurally distinct pop ulations of proteoglycan aggregates, we extracted and purified proteog lycans from canine knee cartilage under associative conditions. Equili brium density gradient centrifugation separated three proteoglycan pop ulations, on the basis of differences in sedimentation velocity, into groups of 21, 106, and 270 S. Electron microscopic examination showed that the 21 S samples contained free aggrecan molecules and clusters o f aggrecan molecules, with a mean of five aggrecan molecules per clust er. The 106 and 270 S samples contained proteoglycan aggregates consis ting of central hyaluronan filaments with multiple attached aggrecan m olecules. The two populations of aggregates did not differ in mean agg recan length or in the spacing of aggrecan molecules along the hyaluro nan filaments, but the slower sedimenting aggregates (106 S) had signi ficantly shorter hyaluronan filaments as measured by electron microsco py (mean hyaluronan length, 400 compared with 1,162 nm) and one-third as many aggrecan molecules per aggregate (mean number of aggrecan mole cules per aggregate, 15 compared with 44). This study shows that artic ular cartilage contains aggrecan clusters and two structurally distinc t populations of proteoglycan aggregates. The differences between the two types of aggregate, in particular the number of aggrecan molecules per aggregate, may reflect differences in their assembly, stability, or turnover and give them different mechanical and biological properti es.