Effect of link protein and free hyaluronic acid binding region on spacing of proteoglycans in aggregates.

Aging of articular cartilage results in accumulation of aggrecan fragments of various sizes that retain their ability to aggregate even though they may have relatively few glycosaminoglycan chains. Residual binding of partially degraded aggrecan may prevent binding of newly synthesized aggrecan subunits that have greater numbers of glycosaminoglycan chains. This study was undertaken to determine the effects of various relative molar ratios of intact aggrecan, link proteins, and hyaluronic acid binding region fragments on the structure of reconstituted aggregates. High molar ratios of link proteins relative to aggrecan decreased the spacing between adjacent aggrecan subunits; low molar ratios of hyaluronic acid binding region relative to aggrecan (4:1 or less) had no significant effect on spacing, and high molar ratios resulted in an increase in the spacing and a decrease in the percentage of aggrecan subunits found in aggregates. These data suggest that the density of aggrecan subunits on the aggregate is determined primarily by steric hindrance of the glycosaminoglycan chains of the aggrecan subunits and that, to a limited extent, partial degradation of aggrecan in an aggregate allows attachment of more aggrecan subunits.