Biosynthesis of heparin. III. Formation of iduronic acid residues

Incubation of a microsomal fraction from mouse mastocytoma with UDP [¹⁴C] glucuronic acid and unlabeled UDP N acetylglucosamine resulted in the formation of a nonsulfated polysaccharide containing [¹⁴C] glucuronic acid as the only labeled component present in significant amounts. Similar incubations, carried out in the presence of 3' phosphoadenylylsulfate yielded a sulfated polymer, with [¹⁴C] iduronic acid comprising 1/3 of the total labeled uronic acid. Products obtained by sulfation of preformed ¹⁴C labeled glycosaminoglycan showed the same proportion of [¹⁴C] iduronic acid. Since the preformed polysaccharide contained radioactive glucuronic acid only, it is concluded that the [¹⁴C] iduronic acid residues of the sulfated product had been formed by C 5 epimerization of labeled glucuronic acid units, on the polymer level. Oligosaccharides, isolated after deaminative cleavage of ¹⁴C labeled microsomal polysaccharide, were characterized with regard to the contents and location of iduronic acid and sulfate residues. The results showed that iduronic acid units and ester sulfate groups were accumulated within the same sections of the polysaccharide chain, and suggest therefore that the epimerization of glucuronic acid residues is somehow linked to the sulfation of hydroxyl groups.