Inhibition of the hydrolytic activity of thrombin by chitin heparinoids
Heparin, an anticoagulant polysaccharide, inhibits the thrombin activity that plays the most important role in blood clotting. Inhibition of thrombin activity is accelerated in the presence of antithrombin III (AT-III). The heparin-AT-III complex also inhibits Factor Xa, which transforms prothrombin into thrombin. As Chitin has a similar skeletal structure to heparin, the biomedical properties of chitin have received considerable attention. Chitosan (N-deacetylated chitin) has been used to prepare heparin-like derivatives, because it can be chemically modified under homogeneous conditions in few reaction steps. Although there have been many attempts prepare chitosan heparinoids, the role of each functional group has not been clarified satisfactorily. Use of chitin as a precursor for heparinoids allowa the role of ether-linked functional groups to be demonstrated independently of influence by N-sulfate groups. Carboxymethylation of chitin was employed, as carboxymethyl groups have been reported to augment the anticoagulant activity of sulfated dextran. Carboxymethylation of chitin to d. s. 0.8 under mild conditions causes mainly 6-o-substitution O-(Carboxymethyl)-chitin (CM-chitin) has been reported to adsorb bovine blood proteins, with a positive contribution by Ca2+, and the extent of adsorption is regulated by the degree of substitution. The present report describes the preparation the preparation of various chitin derivatives by the sulfation of chitin and CM-chitin under mild conditions, and studies of inhibition of thrombin activity, using bovine fibrinogen. Inhibition was observed to be increased upon introduction of carboxymethyl groups in the sulfated chitin =, as also observed for sulfated dextran. The sulfated CM-chitin showed activity comparable to that of heparin.