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At least 955 records · Page 53Linked to original sources
PubMed1969

Inhibition of pyrophosphatase activity of mouse duodenal lkaline phosphatase by magnesium ions.

Duodenal alkaline phosphatase of juvenile (11-day-old) mice, like other non-specific alkaline phosphatases, has the ability to hydrolyse PP(i). When a constant Mg(2+)/PP(i) concentration ratio is maintained, plots of velocity as a function of PP(i) concentration are consistent with Michaelis-Menten kinetics. Mg(2+) activates pyrophosphate hydrolysis and maximal activity is obtained at a constant Mg(2+)/PP(i) concentration ratio of 0.66. At higher ratios there is strong inhibition. At constant concentrations of Mg(2+) and increasing concentrations of PP(i), the velocity-substrate (PP(i)) concentration plots show sigmoidal dependence. By assuming that the true substrate is MgP(2)O(7) (2-) complex, and using complexity constants, the concentrations of free Mg(2+), Mg(2)P(2)O(7) and MgP(2)O(7) (2-) were calculated in assay mixtures ranging in PP(i) concentration from 0.1 to 2.5mm and in total Mg(2+) concentration from 0.6 to 2.6mm. From these data, the concentrations of added Mg(2+) and PP(i) in the assay mixtures were selected so that the velocity could be measured (1) at three fixed concentrations of free Mg(2+) ions with varied concentrations of MgP(2)O(7) (2-) and (2) at four fixed concentrations of Mg(2)P(2)O(7) with varied concentrations of MgP(2)O(7) (2-). Lineweaver-Burk and Hill plots from these data showed that the inhibition is caused by free Mg(2+) ions, of a mixed type and consistent with Michaelis-Menten kinetics. The sigmoidal dependence observed between velocity and PP(i) concentration at constant concentration of total Mg(2+) is therefore not due to allosteric inhibition. It is due to a combined effect of (1) inhibition by free Mg(2+) ions, (2) depletion of the true substrate, MgP(2)O(7) (2-), owing to the formation of Mg(2)P(2)O(7) and (3) the manner in which the concentrations of these three molecular or ionic species change when PP(i) concentration is increased maintaining the total Mg(2+) concentration constant.

Alkaline Phosphatase↗
PubMed1989

2-Aminoethylarsonic acid as an analogue of ethanolamine phosphate. Endowment of ethanolamine-phosphate cytidylyltransferase with CTP pyrophosphatase activity.

2-Aminoethylarsonic acid was tested for its ability to act as a substrate for ethanolamine-phosphate cytidylytransferase as a cytidylyl acceptor in place of ethanolamine phosphate. The expected product, like all mixed anhydrides of arsonic acids, should hydrolyse spontaneously with regeneration of the substrate analogue and CMP formation; such CMP production was observed. The limiting velocity with aminoethylarsonic acid is about 90% that with ethanolamine phosphate, and the Michaelis constant is below 20 mM.

Arsenicals↗
PubMed1995

A UDP-sugar pyrophosphatase is developmentally regulated in the rat retina.

Rat retina tissue contains relatively high amounts of GD3 in relation to ganglio-series gangliosides even in the adult stages. This was attributed in part to an activity ratio between the enzyme that converts GM3 to GD3 [sialytransferase II (ST-II)] and the enzyme that converts GM3 to GM2 [N-acetylgalactosaminyltransferase (GalNAc-T)] favorable to ST-II. Here we report the presence in the rat retina tissue of an activity that hydrolyzes one of the substrates of GalNAc-T, the donor sugar nucleotide UDP-GalNAc. Chromatographic analyses of the products of degradation indicate that the activity corresponds to a UDP-sugar pyrophosphatase/phosphodiesterase I. The activity is developmentally regulated, increasing after day 4 of postnatal development to reach values approximately 10-fold higher in the adult tissue. The activity sediments with the microsomal membranes, also hydrolyzes UDP-Gal, does not hydrolyze CMP-NeuNAc, requires Mn2+, and does not require detergent. Kinetic data showed that the same activity hydrolyzes UDP-GalNAc and UDP-Gal, each one acting as competitive inhibitor for the hydrolysis of the other (Km and Ki for UDP-GalNAc, 48 and 33 microM, respectively; Km and Ki for UDP-Gal, 5 and 12 microM, respectively). In another set of experiments, it was found that the activities of the GalNAc-T and the enzyme that converts GM2 to GM1 [galactosyltransferase II (Gal T-II)] increased about threefold from birth to day 4 and then decreased to stabilize by day 6 in values that were similar to those at birth and about one-half those of ST-II.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1998

Identification and characterization of the Nudix hydrolase from the Archaeon, Methanococcus jannaschii, as a highly specific ADP-ribose pyrophosphatase.

The MJ1149 gene from the Archaeon, Methanococcus jannaschii, has been cloned and expressed in Escherichia coli. The 19-kDa protein containing the Nudix box, GX5EX7REUXEEXGU, has been purified and identified as a highly specific enzyme catalyzing the Mg2+-dependent hydrolysis of ADP-ribose according to the equation: ADP-ribose + H2O --> AMP + ribose-5-phosphate. The enzyme retains full activity when heated to 80 degreesC, and the rate of hydrolysis is 15-fold higher at 75 degreesC than at 37 degreesC in keeping with the thermophilicity of the organism. This is the first Nudix hydrolase identified from the Archaea, indicating that the family of enzymes containing the Nudix signature sequence is represented in all three kingdoms.

Adenosine Diphosphate Ribose↗