Nonrandom structural features in the heparin polymer

Abstract

Computer simulation studies were used to prepare an ensemble of heparin number chains. The polydispersity of these chains was simulated by introducing a specific "fraction of terminators", and it closely resembled the experimentally observed polydispersity of a porcine mucosal, glycosaminoglycan heparin. The same percentage of simulated chains contained antithrombin III (ATIII) binding site sequences as are typically found to contain ATIII binding sites using affinity chromatography. Heparin lyase action was then simulated by using Michaelis-Menten kinetics. In one model, heparin chains were constructed from the random assembly of monosaccharide units using the observed mole percentage of each. After simulated depolymerization, the final oligosaccharides formed were compared to the observed mole percentage of each. After simulated depolymerization, the final oligosaccharides formed were compared to the observed oligosaccharide products. The simulation which assumed a random distribution of monosaccharide units in heparin did not agree with experimental observations. In particular, no ATIII binding site sequences were found in the simulated number chains. The results of this simulation indicate that heparin is not simply a random assembly of monosaccharide units. These results are constructed from randomly assembled oligosaccharides at the mole percentage in which each is found in the final product mixture. The action of heparin lyase was then simulated, and the distribution of the oligosaccharide products was measured throughout the simulated time course of the depolymerization reaction. The simulated rate of formation and final concentration of a particular oligosaccharide which contains a portion of heparin''s ATIII binding site were similar to those experimentally. These results are consistent with the random distribution of ATIII binding sites within glycosaminoglycan heparin. These results are in contrast to the reported ordered distribution of ATIII bindings sites in proteoglycan heparin and may be the result of a loss of order following the processing of proteoglycan heparin by endo-.beta.-glucuronidase.