Purification and characterization of UDP-galactose-4-epimerase from bovine tissues

Overexpression and purification of the galactose operon enzymes from Escherichia coli

A convenient new procedure for the purification of galactokinase, galactose-1-phosphate uridyltransferase, and UDP-galactose 4-epimerase overexpressed in Escherichia coli is presented. The procedure is shorter than any other described in the literature and facilitates the purification of the three recombinant enzymes in considerable amounts and at high purity and specific activity. The purified gal operon enzymes were biochemically characterized by gel-filtration column chromatography and isoelectric focusing, and the Km values for their substrates were determined.

Substrate properties of 5-fluorouridine diphospho sugars detected in hepatoma cells

Nucleotide sugars derived from 5-fluorouridine were studied in cultured AS-30D hepatoma cells as well as in kinetic enzyme assays in vitro in comparison with the physiologic uridine diphospho sugars. Hepatoma cells converted 5-fluoro [14C]uridine to 5-fluorouridine diphospho (FUDP) glucose, FUDP-galactose, FUDP-N-acetylglucosamine, FUDP-N-acetylgalactosamine, and trace amounts of FUDP-glucuronate, as analyzed by different systems of high-performance liquid chromatography. 5-Fluoro[14C]uridine and [14C]uridine, at concentrations of 5 microM in the culture medium, were phosphorylated by the cells during 60 min to similar amounts of FUTP and UTP, respectively, while the synthesis of [14]FUDP-sugars was reduced to 14% as compared to that of [14C]UDP-sugars. FUDP-sugars, synthesized by chemical and enzymatic procedures, were assayed in vitro as substrates for enzymes of UDP-sugar metabolism. Km and V values in a range comparable to that of the respective UDP-sugars were determined for FUDP-sugars in the reactions catalyzed by UDP-glucose pyrophosphorylase, galactose-1-phosphate uridylyltransferase, UDP-glucose 4-epimerase, UDP-N-acetylglucosamine 2-epimerase, glycogen synthase, and UDP-glucose dehydrogenase. Our experiments in hepatoma cells and with enzymes in vitro have revealed additional reactions of FUDP-sugar metabolism demonstrating a metabolite pattern analogous to that of UDP-sugars. The amounts of FUDP-sugars formed relative to UDP-sugars in intact cells were smaller than suggested on the basis of their kinetic comparison in vitro.