Biosynthesis of Heparin

The synthesis of the glucuronic acid residue, which in several glycosaminoglycans occurs as a link between a serine-bound neutral trisaccharide and the polysaccharide chain proper, has been investigated in heparin-producing mastocytoma tissue. A membrane-bound enzyme was detected which catalyzed the transfer of glucuronic acid from uridine diphosphoglucuronic acid to 3-O-β-d-galactosyl-d-galactose, a structural unit occurring in the neutral trisaccharide. A partial separation of this glucuronosyltransferase from an enzyme catalyzing the synthesis of β-d-glucuronidic bonds to N-acetylglucosamine residues was achieved by treatment of the microsomal particles with ammonium hydroxide in the presence of detergent. This procedure brought 67% of the latter enzyme into solution, whereas only 18% of the total UDP-glucuronic acid-3-O-β-d-galactosyl-d-galactose glucuronosyltransferase activity was recovered in soluble form after ultracentrifugation. Subsequent gel chromatography of the soluble enzyme fraction afforded a 330-fold purification of the latter glucuronosyltransferase. In conjunction with earlier studies, the results of the present investigation permit the formulation of a complete mechanism for the biosynthesis of the heparin-polypeptide linkage region.