Interaction of proteoglycans and chondroitin sulfates with calcium or phosphate ions.

Abstract

Equilibrium dialysis was used to study the distribution of calcium or phosphate ions at equilibrium in dialysis cells containing proteoglycan or chondroitin sulfate in one compartment. A higher concentration of calcium or lower concentration of phosphate was found in the cell compartment containing the polymer than in the compartment separated from it by a semipermeable membrane. The difference in calcium concentration across the boundary represents bound calcium. The formation constant (K) for the complex of bound calcium with chondroitin sulfate was investigated and found to be affected by two main factors: ionic strength and calcium/glucuronate ratio. K decreased rapidly with increasing ionic strength up to 0.15. At low ionic strength and high Ca2+/glucuronate ratios, evidence has been obtained that more calcium is bound by the polymers than can be accounted for by the simple equilibrium system, involving the combination of one calcium per disaccharide unit period, whose operation is consistent with the K values found at low Ca/glucuronate ratios over a wide range of ionic strengths. Infrared spectra obtained at high and low Ca2+/glucuronate ratios also indicate the existence of two calcium forms of proteoglycans. Viscosity and ultracentrifugal data show that differences exist between proteoglycans in calcium and sodium solutions. The data for disaggregated proteoglycan preparations indicate that their calcium-binding properties are very similar to those of chondroitin 4-sulfate and are determined by the same factors. One aggregated puppy rib proteoglycan, however, showed significantly greater K values than chondroitin 4-sulfate; these decreased after disaggregation.