Ion-exchange reactions between acid mucopolysaccharides and various cations.

In studies of cation-binding by acid mucopolysaccharides and the possible significance of these reactions in calcification processes, it has been shown, in cartilage, that the chondroitin sulphate component is solely responsible for the cation-binding capacity, that the cation-binding reactions are of the ion-exchange type and that different cations exhibit varying degrees of affinity for the cartilage (Boyd & Neuman, 1951; Dunstone, 1959, 1960). Cations have been shown to inactivate, in vitro, the calcification of rachitic-bone sections (Sobel, 1955). The order of inactivating power of these cations differs from their order of affinity for cartilage (Dunstone, 1960). These differences may be due either to the chondroitin sulphate of the cartilage differing structurally or in its combination with protein from that in the calcifying tissue used by Sobel (1955), or it may be that the process of inhibition of calcification does not involve simple cation-exchange competition. The cartilage preparations so far used represent particular cases. Samples taken from different sources or subjected to different treatments might yield a different order of cation affinity. Consequently, a study, under a variety of experimental conditions, of the association of these cationic inhibitors of calcification with a number of structurally different sulphated acid mucopolysaccharides (chondroitin sulphate A, chondroitin sulphate B, heparin, heparin monosulphate) in solution and with cartilage-bound chondroitin sulphate has been undertaken. The factors that influence the interaction between cations and acid mucopolysaccharides have been investigated and an attempt has been made to give a quantitative interpretation to the association phenomena. The affinities of the cations for the acid mucopolysaccharides have been measured by the ion-exchange methods outlined by Schubert (1952) and by Djurfeldt & Samuelson (1957a, b), and have been recorded in the form of exchange parameters calculated from Rothmund & Kornfeld (1918) equations.