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PubMed1972

Enzymic transfer of glucose and xylose from uridine diphosphate glucose and uridine diphosphate xylose to bilirubin by untreated and digitonin-activated preparations from rat liver.

1. Digitonin-treated and untreated homogenates, cell extracts and washed microsomal preparations from liver of Wistar R rats are capable of transferring sugar from UDP-glucose or UDP-xylose to bilirubin. No formation of bilirubin glycosides occurred with UDP-galactose or d-glucose, d-xylose or d-glucuronic acid as the sources of sugar. 2. Procedures to assay digitonin-activated and unactivated bilirubin UDP-glucosyltransferase and bilirubin UDP-xylosyltransferase were developed. 3. In digitonin-activated microsomal preparations the transferring enzymes had the following properties. Both enzyme activities were increased 2.5-fold by pretreatment with digitonin. They were optimum at pH6.6-7.2. Michaelis-Menten kinetics were followed with respect to UDP-glucose. In contrast, double-reciprocal plots of enzyme activity against the concentration of UDP-xylose showed two intersecting straight-line sections corresponding to concentration ranges where either bilirubin monoxyloside was formed (at low UDP-xylose concentrations) or where mixtures of both the mono- and di-xyloside were synthesized (at high UDP-xylose concentrations). Both enzyme activities were stimulated by Mg(2+); Ca(2+) was slightly less, and Mn(2+) slightly more, stimulatory than Mg(2+). Of the activities found in standard assay systems containing Mg(2+), 58-78% (substrate UDP-glucose) and 0-38% (substrate UDP-xylose) were independent of added bivalent metal ion. Double-reciprocal plots of the Mg(2+)-dependent activities against the concentration of added Mg(2+) were linear. 4. In comparative experiments the relative activities of liver homogenates obtained with UDP-glucuronic acid, UDP-glucose and UDP-xylose were 1:1.5:2.7 for untreated preparations and 1:0.29:0.44 after activation with digitonin. 5. Bilirubin UDP-glucuronyltransferase was protected against denaturation by human serum albumin, whereas bilirubin UDP-xylosyltransferase was not. 6. Digitonin-treated and untreated liver homogenates from Gunn rats were inactive in transferring sugar to bilirubin from UDP-glucuronic acid (in agreement with the work of others), UDP-glucose or UDP-xylose.

Animals↗
PubMed1973

Competitive inhibition and substrate activity of uridine diphosphate 6-deoxygalactose for Escherichia coli uridine diphosphate galactose 4-epimerase.

UDP-6-deoxygalactose inhibits the UDP-galactose 4-epimerase (EC 5.1.3.2) from Escherichia coli in a competitive manner with respect to the substrate UDP-galactose, giving K(i) 1.3x10(-3)m. As a substrate for the enzyme, it is transformed into UDP-6-deoxyglucose, although the reaction stops before equilibrium is attained. Possible causes of this behaviour are discussed.

Binding, Competitive↗
PubMed1978

Mechanism of the in vitro breakdown of guanosine 5'-diphosphate 3'-diphosphate in Escherichia coli.

Degradation of guanosine tetraphosphate (ppGpp) involves an enzyme associated with the ribosomal fraction from spoT+ strains of Escherichia coli. Double-label experiments with pp[3h]gpp, pp[3H]Gpp, or pp[3H]Gpp as substrate strongly suggest that ppG is the degradation product and that the enzyme releases two phosphates coordinately from the 3' position of ppGpp. In the absence of pppA this reaction proceeds in an uncoupled fashion, yielding ppG and PPi, but in the presence of pppA the decay is considerably enhanced and a pppA-ppi exchange reaction occurs in which the 3'-pyrophosphoryl group of ppGpp displaces the gamma and beta phosphates of pppA. Sodium PPi at 4 m7 inhibits decay of ppGpp regardless of whether or not pppA is present.

Diphosphates↗