Uronic acids in heparin

Examination of early -n.m.r. spectra of heparin and of chemically modified heparins121 prompted a reinvestigation of N,0-desulfated, carboxyl-reduced heparin, leading to the isolation of substantial amounts of L-iditol pentaacetate.121,122 In addition, improved conditions for the acid hydrolysis of heparin and carboxyl-reduced heparin gave increased recoveries of L-iduronic acid and 1,6-anhydro-L-idose, respectively.123 These findings confirmed the L-enantiomeric designation of the iduronic acid, and established that it is the main uronic acid in heparin. 

Meyer and Wolfrom and their coworkers detected the uronic acid in the barium acid salt of heparin both qualitatively and quantitatively, and considered it to be the source of the acidity in the acid salt. Amino sugar was also estimated. These results, which cannot be said to be as precise as the elemental data, indicate a ratio of 1 1 for amino sugar and hexuronic acid. 

It appears that the main chemical structure of heparin as determined by degradation studies by the research groups of Wolfrom, Meyer and Stacey is an a-D-(l— 4) linked backbone structure of alternating hexosamine and uronic acid in which the hexosamine is 6-SO4, jV -S04-glucosamine and the uronic acid, 2- or 3-S04-glycuronic acid. The ratio of 2-5-2-6 rather than... 

The identification of L-iduronic acid as the major glycuronic acid constituent of heparin proved to be a much slower process than the identification of the amino sugar residue. Although this compound was detected in acid hydrolyzates of heparin116117 and heparin oligosaccharides,118 its yield was usually poor, because of the drastic conditions used for the acid hydrolysis (which are known to lead to extensive destruction of uronic acid).119120 Also, L-iduronic acid escaped detection as L-idose in the hydrolyzates of carboxyl-reduced heparin, probably because L-idose is readily converted into 1,6-anhydro-L-idose under the usual hydrolytic conditions. 

Although the glycosidic bonds of uronic acid residues are usually more resistant to acid hydrolysis than those of neutral polysaccharides,218 these linkages in heparin are more readily cleavable then those between the hexosamine and the uronic acid residues. Disaccharides obtained by exhaustive hydrolysis of heparin with 0.5 MHC1 at 80° contained hexu-... 

Periodate oxidation was used in early work for obtaining information on the substitution pattern of heparin.1,100 102 In fully N-substituted (N-sulfated or N-acetylated, or both) glycosaminoglycans, the only bond susceptible to splitting by periodate is C-2-C-3 of unsubstituted uronic acid residues. As illustrated in Scheme 4 for heparin, only nonsulfated D-glucuronic acid and L-iduronic acid residues are split by periodate. 

Fig. 11. —Proposed Model for aTemary Complex between Heparin (HEP), Antithrombin (AT), and Thrombin (T), Involving an Octadecasaccharide Sequence of the Heparin Chain.419 [Within the large brace, full circles denote uronic acid residues, the small brace encompassing the minimal binding-site for AT. The thrombin-binding region is presumed to be situated towards the nonreducing end of the AT-binding sequence,933 which is arbitrarily assumed to be in the middle of the heparin chain.]...

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