Effects of proteoglycan on hydroxyapatite formation under non-steady-state and pseudo-steady-state conditions

Addition of chondroitin sulfate (CS) or cartilage proteoglycan to metastable calcium phosphate solutions inhibits the formation of hydroxyapatite (HA). However, pre-equilibration of CS or proteoglycan with calcium prior to the addition of phosphate results in higher levels of HA precipitation compared to control solutions of identical calcium and phosphate activity. These findings indicate that the inhibition of HA formation by proteoglycans and CS is largely due to calcium binding. Further, its ability to bind calcium ions reversibly suggests that proteoglycan may act as a promoter, not an inhibitor, of calcification in cartilage.