title: Effects of uridine nucleotides and nucleotide pyrophosphatase on glycolipid alpha and beta-N-acetylgalactosaminyltransferase activities in guinea pig microsomes.
abstract: Membrane-bound alpha and beta-N-acetylgalactosaminyltransferases (EC 2.4.1.0) which catalyze formation of non-reducing terminal linkages of Forssman hapten and globoside, respectively, could be differentiated with respect to the different effects of UDP on the two enzyme activities. UDP markedly inhibited the alpha-transferase activity, in contrast to its stimulatory action on the beta-transferase. These effects of UDP were similar to those of UDPglucose, which was demonstrated to be a competitive inhibitor (Ki, 3.3 - 10(-5) M for UDP-N-acetylgalactosamine) for the alpha-transferase reaction. Other uridine derivatives tested suppressed both the transferase activities, being more inhibitory for the alpha-transferase than for the beta-transferase. Under the synthetic conditions of these aminoglycolipids, UDP-N-acetylgalactosamine as a donor was simultaneously degraded into N-acetylgalactosamine-1-phosphate and finally into N-acetylgalactosamine by UDP-N-acetylgalactosamine pyrophosphatase, which is part of the membrane system. UDPglucose was confirmed as being able to prevent the enzymatic hydrolysis of UDP-N-acetylgalactosamine. UDPglucose, therefore, acts to suppress both the alpha-N-acetylgalactosaminyltransferase (but not the beta-transferase) and the pyrophosphatase activities. The inhibitory effect of UDPglucose on the alpha-transferase activity was most probably due to its direct action on the transferase rather than its function in protecting UDP-N-acetylgalactosamine donor from pyrophosphatase action.
mesh_terms: D000818:Animals; D005646:Fucosyl Galactose alpha-N-Acetylgalactosaminyltransferase / Q000378:metabolism*; D006168:Guinea Pigs; D006602:Hexosyltransferases / Q000378:metabolism*; D007668:Kidney / Q000201:enzymology; D008861:Microsomes / Q000201:enzymology*; D011755:Pyrophosphatases / Q000378:metabolism*; D014500:Uracil Nucleotides / Q000494:pharmacology*; D014529:Uridine / Q000494:pharmacology; D014531:Uridine Diphosphate Galactose / Q000494:pharmacology; D014532:Uridine Diphosphate Glucose / Q000494:pharmacology; D014537:Uridine Diphosphate N-Acetylglucosamine / Q000494:pharmacology
chemical_list: D014500:Uracil Nucleotides; D014531:Uridine Diphosphate Galactose; D014537:Uridine Diphosphate N-Acetylglucosamine; D006602:Hexosyltransferases; D005646:Fucosyl Galactose alpha-N-Acetylgalactosaminyltransferase; D011755:Pyrophosphatases; D014532:Uridine Diphosphate Glucose; D014529:Uridine