Hyaluronan interactions with hydroxyapatite do not alter in vitro hydroxyapatite crystal proliferation and growth.

The interaction of hyaluronan (Mr range 80-120 x 10(4)) with poorly crystalline hydroxyapatite, such as is found in calcified cartilage and bone, was studied to challenge the hypothesis that free hyaluronan found in proteoglycan aggregate preparations could affect in vitro mineralization. Using a Langmuir adsorption isotherm, based on uronic acid content, hyaluronan was found to bind to hydroxyapatite with an affinity K of 0.12 ml/microgram uronate and N = 6.8 micrograms uronate/m2 hydroxyapatite binding sites. This is contrasted with K = .047 ml/microgram uronate and N = 9.0 micrograms uronate/m2 for a bovine nasal proteoglycan monomer preparation. Although the proteoglycan monomer and aggregate preparations have been reported to inhibit hydroxyapatite growth at concentrations of 1 mg/ml, using solution concentrations of 0, 0.01, 0.1 and 1 mg/ml hyaluronan there were no detectable alterations in the rate of seeded hydroxyapatite growth and proliferation. These data indicate that although in vitro hyaluronan may bind with weak affinity to hydroxyapatite, this interaction does not affect mineral growth, and the presence of hyaluronan would not contribute to the increased inhibitory potential of cartilage proteoglycan aggregate relative to monomer preparations.

The effect of phosphatidylserine on in vitro hydroxyapatite growth and proliferation.

The acidic phospholipid phosphatidylserine (PS) has been reported to have variable effects on in vitro hydroxyapatite proliferation. PS promotes in vitro mineralization in systems in which calcium-PS-phosphate complexes are allowed to form, and inhibits in vitro mineralization when incorporated into liposomes. To investigate these diverse effects, a Langmuir adsorption isotherm was used to determine the affinity of PS for hydroxyapatite crystals, based on binding of 14C-PS to synthetic hydroxyapatite crystals of specific surface 54 m2/g. Using this model, PS was found to bind to hydroxyapatite crystals with an affinity comparable to that of the amino acid phosphoserine (K = 3.33 ml/mumol). Coating the surface of hydroxyapatite seed crystals with PS reduced their rate of proliferation in a metastable calcium phosphate solution in which calcium-PS-phosphate complexes were previously shown to promote hydroxyapatite formation. The extent of inhibition of hydroxyapatite seeded growth was directly related to the proportion of the hydroxyapatite surface covered with PS. These data suggest that PS may have multiple effects on hydroxyapatite formation in situ, and that mineral-PS interactions can retard crystal proliferation.