Biological implications of the structural, antithrombin affinity and anticoagulant activity relationships among vertebrate heparins and heparan sulphates.

We analysed the distribution, structural characteristics, antithrombin-III-binding properties and anticoagulant activities of heparins and heparan sulphates isolated from the tissues of a wide range of vertebrates. Heparin has a curiously limited distribution, since it was absent from lower aquatic vertebrate species, present in only certain organs such as intestine in many higher vertebrates, and completely absent from the rabbit among mammals examined. The heparins were structurally diverse, and they exhibited a broad range of anticoagulant activities, from approx. 50% to 150% of average commercial heparins. Although there was a rough correlation between the anticoagulant potency of the starting isolate and the proportional content of material exhibiting high-affinity binding to the proteinase inhibitor antithrombin III, activities of high-affinity fractions from heparins low in activity overlapped those of low-affinity fractions from highly active heparins. Heparan sulphates, which in contrast were isolated from nearly all vertebrate organs, contained high-affinity subfractions constituting up to 5% of the starting material and possessing anticoagulant potencies of 2-30 units/mg. In consideration of the heparin data, we infer that its biological function is either species-specific or may be served by other molecular elements, and that there exists considerable diversity in the antithrombin-III-binding sequence of heparin. The more-generally distributed glycosaminoglycan heparan sulphate possesses within its variable structure a small high-affinity subfraction with low anticoagulant potency, whether isolated from aorta or other tissues. Although heparan sulphate appears to have an essential function at the cellular level, we suggest that this is probably not that of providing heparin-like antithrombotic effects on vascular surfaces.