The effect of calcium on the macromolecular properties of heparan sulfate

Heparan sulfate from human aorta has been subjected to a physico-chemical analysis in buffers of physiological ionic strength containing either sodium chloride or calcium chloride. A molecular weight of 50,000 was obtained both in sodium and calcium solutions by sedimentation equilibrium and from sedimentation and diffusion coefficients. The values obtained for So20,w in sodium and calcium chloride solutions were 2.28 X 10(-13) and 2.70 X 10(-13) sec, respectively, and the corresponding values for Dl20,w were 2.7 X 10(-7) and 3.1 X 10(-7) cm2/sec, respectively. Diffusion coefficients calculated from data obtained by gel chromatography were in excellent agreement with those determined by conventional techniques. The results indicated that the molecule contracts in the presence of calcium, presumably due to an increased binding of counter-ions with a concomitant decrease in charge density. Circular dichroism spectra above 200 nm, where the substituted amino group contributes to the absorption, gave no indication of a conformational change in the polysaccharide upon transformation from the sodium to the calcium salt. When the polysaccharide was dissolved in a salt solution, physiological both in ionic strength and in sodium to calcium ratio, it sedimented as the sodium salt. The sedimentation coefficient, the diffusion coefficient and the apparent molecular weight all displayed a concentration dependence. This dependence was much less in calcium chloride than in sodium chloride in determinations of molecular weight. Therefore there are advantages in performing molecular weight determinations of glycosaminoglycans in calcium solutions.