Sugar nucleotide inhibition of UDP-glucose pyrophosphorylase from calf liver

Abstract

UDPglucose pyrophosphorylase (UTP:α- d-glucose-1-phosphate uridylyltransferase, EC 2.7.7.9) was partially purified from calf liver. Kinetic properties and inhibition of the enzyme by UDP sugars were determined with an isotopic assay in which the product, UDP- d-[ 14C]glucose, was collected and measured on DEAE-cellulose discs. The Michaelis-Menten constants for the substrates UTP and glucose-1-P were 0.1 mM and 0.03 mM, respectively. With glucose-1-P at 0.04 mM and UTP at 0.2 mM several UDP sugars inhibited the enzyme. The compounds and percent inhibitions at the concentrations indicated were 0.08 mM UDP- d-glucose (87%), 0.45 mM UDP- d-glucuronic acid (56%), 0.43 mM UDP- d-galacturonic acid (48%), 0.36 mM UDP- d-xylose (32%), 0.45 mM UDP- d-mannose (23%), and 0.35 mM UDP- d-galactose (14%). UDP- d-glucose gave the lowest K t value (0.005 mM), followed by UDP- d-glucuronic acid ( K t = 0.15 mM ), UDP- d-galacturonic acid ( K t = 0.21 mM ), and UDP- d-xylose ( K t = 0.24 mM ). These four compounds exhibited competitive inhibition with respect to UTP and noncompetitive inhibition with respect to glucose-1-P . One or more of these sugar nucleotides may regulate the UDPglucose pyrophosphorylase reaction in vivo.