Biosynthesis of Chondroitin Sulfate: I. Galactosyl Transfer in The Formation of the Carbohydrate-Protein Linkage Region

Abstract

Abstract The biosynthesis of the first glucuronic acid residue in the polysaccharide-protein linkage region of chondroitin sulfate has been studied in a cell-free system from embryonic chick cartilage. Transfer of glucuronic acid from uridine diphosphoglucuronic acid occurred to the galactose moiety present at the nonreducing terminal of the disaccharide, 3-O-β-d-galactosyl-d-galactose, or larger, similar fragments from the linkage region. By comparison, free d-galactose was practically inactive as a substrate. Mixed substrate experiments indicated that transfer of glucuronic acid to oligosaccharide acceptors containing N-acetylgalactosamine residues at their nonreducing ends proceeded equally well in the presence of 3-O-β-d-galactosyl-d-galactose. Under similar conditions, substrates for the transfer of the first glucuronic acid residue, such as 3-O-β-d-galactosyl-d-galactose and 3-O-β-d-galactosyl-4-O-β-d-galactosyl-d-xylose, mutually impeded product formation. An analogous effect was observed for a tri- and a pentasaccharide from chondroitin 6-sulfate containing N-acetylgalactosamine moieties as acceptors for glucuronic acid. These findings suggest that the transfer of glucuronic acid to the neutral trisaccharide of the linkage region is mediated by an enzyme distinct from that involved in the formation of the remainder of the polysaccharide chain, or at least by an independent active site on the same enzyme.