Ion-exchange reactions between acid mucopolysaccharides and various cations

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Abstract
The interactions of cations with acid mucopolysaccharides in solution have been treated as ion-exchange reactions. The relative affinities of the cations Na+, K+, Mg2+, Sr2+ and Ba2+ for chondroitin sulphate A, chondroitin sulphate B, heparin and heparin monosulphate have been measured and recorded in the form of exchange parameters calculated from a Rothmund and Kornfeld equation. The orders of increasing cation affinity are: K+, Na+, Mg2+, Ca2+, Sr2+, Ba2+ for chondroitin sulphate A; K+, Na+, Mg2+[image]Sr2+[image]Ba2+for chondroitin sulphate B; Na+, K+, Mg2+, Sr2+, Ba2+for heparin; Na+, Mg2+, Ca2+[image]Sr2+, Ba2+ for heparin monosulphate. The differences in the orders of affinity have been attributed to differences in the structures of the various acid mucopolysaccharides. Exchange parameters have been found to vary with ionic strength, and a theoretical evaluation of the origins of this variation has been made. Under the experimental conditions used, the association of cations with acid mucopolysaccharides in solution is thought to be mainly of an electrostatic type. Mucopolysaccharides extracted from cartilage by mild procedures bind cations more strongly than the materials extracted by relatively harsh procedures. The cause of the stronger binding appears to be associated with the state of polymerization of the mucopolysaccharide, but the effect of the protein which is extracted with the mucopolysaccharide has not been completely evaluated. A comparison of the affinity of cations for acid mucopolysaccharides in solution and for cartilage-bound acid mucopolysaccharide has shown that the order of affinity for cations of the cartilage-bound acid mucopolysaccharide under the experimental conditions described by Dunstone is similar to that for chondroitin sulphate A in solution. However, when ion-exchange methods with tracer cations are used to study reactions with cartilage, the exchange parameters vary with the cation composition of the cartilage phase and cause the order of affinity for cations to alter. This type of variation does not occur when the acid mucopolysaccharide is present in solution. The exchange parameters for reactions between cartilage and cations vary with ionic strength but the variation is considerably less than that which occurs when the acid mucopolysaccharide is present in solution. The significance of these results with respect to the function of acid mucopolysaccharides in calcification processes is briefly discussed.

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