Effects of uridine nucleotides and nucleotide pyrophosphatase on glycolipid alpha and beta-N-acetylgalactosaminyltransferase activities in guinea pig microsomes.

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.