Two N-acetylgalactosaminyltransferase are involved in the biosynthesis of chondroitin sulfate.
Two N-acetylgalactosaminyltransferases, designated I and II, have been purified from the microsomal fraction of calf arterial tissue and separated on Bio-Gel A. N-Acetylgalactosaminyltransferase I was purified 450-fold. It requires Mn2+ for maximal activity and transfers N-acetylgalactosamine residues from UDP-[1-3H]GalNAc in beta-glycosidic configuration to the non-reducing terminus of the acceptor substrates GlcA(beta 1-3)Gal(beta 1-3)Gal, GlcA(beta 1-3)Gal(beta 1-4)Glc and GlcA(beta 1-3)Gal. Even-numbered chondroitin oligosaccharides serve as acceptors for N-acetylgalactosaminyltransferase II, which transfers N-acetylgalactosamine from UDP-[1-3H]GalNAc to the non-reducing glucuronic acid residues of oligosaccharide acceptor substrates. Maximum transfer rates were obtained with a decasaccharide derived from chondroitin. Longer or shorter-chain chondroitin oligosaccharides are less effective acceptor substrates. All reaction products formed by N-acetylgalactosaminyltransferases I and II are substrates of beta-N-acetylhexosaminidase, which splits off the transferred [1-3H]GalNAc completely. In the microsomal fraction N-acetylgalactosaminyltransferase II had a 300-fold higher specific activity than N-acetylgalactosaminyltransferase I. In contrast to enzyme I, enzyme II loses much of its activity during the purification procedure and undergoes rapid thermodenaturation. GlcA-Gal-Gal is a characteristic sequence of the carbohydrate-protein linkage region of proteochondrioitin sulfate. The acceptor capacity of this trisaccharide suggests that N-acetylgalactosaminyltransferase I is involved in the synthesis of the carbohydrate-protein linkage region. Since N-acetylgalactosaminyltransferase II is highly specific for chondroitin oligosaccharides, we conclude that it participates in chain elongation during chondroitin sulfate synthesis.
D000818:Animals; D001011:Aorta / Q000201:enzymology / Q000378:metabolism; D002384:Catalysis; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D002807:Chondroitin / Q000031:analogs & derivatives*; D002809:Chondroitin Sulfates / Q000096:biosynthesis*; D002850:Chromatography, Gel; D004355:Drug Stability; D005700:Galactosyltransferases / Q000378:metabolism*; D006358:Hot Temperature; D008861:Microsomes / Q000201:enzymology; D017350:N-Acetylgalactosaminyltransferases; D010441:Peptide Chain Elongation, Translational; D012995:Solubility; D013379:Substrate Specificity
D002807:Chondroitin; D002809:Chondroitin Sulfates; D005700:Galactosyltransferases; D017350:N-Acetylgalactosaminyltransferases; C003742:UDPgalactosamine-galactose acetylgalactosaminyltransferase