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  • Title: Molecular-size-dependent variations in the proportions of chains with high binding affinities for antithrombin in rat skin heparin proteoglycans.
    Author: Horner AA.
    Journal: Biochem J; 1989 Sep 15; 262(3):953-8. PubMed ID: 2590178.
    Abstract:
    Approximately half of all rat skin heparin proteoglycans have polysaccharide chains that have no sites with high binding affinity for antithrombin. The rest have chains with high-affinity antithrombin-binding-site densities ranging from zero to five sites per chain, with a high degree of variation. Proteoglycans vary in size because of diversity in the number of chains per molecule; the relationship between proteoglycan size and high-affinity antithrombin-binding-site density has not been studied previously. Polydisperse heparin proteoglycans from rat skin, labelled biosynthetically with 35S, were fractionated by gel filtration on Bio-Gel A-150m and arbitrarily divided into five fractions of decreasing average molecular size. Fractionation of these products on antithrombin-agarose showed that the proportion of proteoglycans with high affinity for antithrombin decreased from 39% to 25% as molecular size decreased. However, as the molecular size of high-affinity proteoglycans decreased, the proportion of their chains that had high affinity increased from 29% to 59%. Therefore molecular size is a significant factor in determining the proportion of high-affinity chains in heparin proteoglycans. A model of heparin biosynthesis is proposed in which areas of specific enzyme activity that control the synthesis of the antithrombin-binding-site sequence are sparsely and nonrandomly distributed on mast-cell Golgi membranes. It is postulated that the likelihood of a developing proteoglycan encountering one of these hypothetical areas is molecular-size-dependent.
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