Nucleotide phosphatase

Cyclic nucleotide phosphatases are inhibited by caffeine. What effect would drinking a strong cup of coffee have on glycogen metabolism when epinephrine levels are dropping in the blood ... 

At very low values of EC, when AMP is elevated it is deaminated via AMP deaminase to inosine monophosphate (IMP). This further displaces the adenylate kinase reaction in the direction of ATP synthesis. The IMP is dephosphorylated by nucleotide phosphatase, and the inosine is phosphorylyzed via purine nucleotide phosphorylase, releasing hypoxanthine and ribose 1-phosphate. The latter is metabolized via the pentose phosphate pathway, and most of the carbon atoms enter glycolysis. Because this course of events depletes the overall adenine nucleotide pool, and hence the scope for ATP production in the longer term, it represents a metabolic last ditch stand by the cell to extract energy even from the energy currency itself ... 

Inosine is the only potential precursor of hypoxanthine, and it could be formed in some cells from inosinate by phosphatases. However, such a blind cycle is teleologically unsatisfactory, and furthermore, the nucleotide phosphatases are membrane bound (S13) and inhibited by the normal intracellular concentrations of ATP (M14). Thus, it must be assumed that inosine is formed from adenosine. 

P. Ekker. Activities of thymidine kinase and deoxyribo-nucleotide phosphatase during growth of cells in tissue culture. J. Biol. Chem. 241 659, 1966. 

Inhibition of enzyme activity may be due to substrate, product or other effector molecules, and the concentrations of these components in the cell may lead to regulation of the pathway. Competitive product inhibition has been ascribed a significant role in sialyltransferase reactions (Bernacki 1975, Klohs et al. 1979, Eppler et al. 1980 b) with CMP yielding apparent Kj values of 50 (xM (Klohs et al. 1979). This inhibition may be a self-regulatory effect in situ in view of the high nucleotide phosphatase activity of Golgi apparatus. A non-competitive inhibition of sialyltransferase activity was also observed with other nucleotides (Bernacki et al. 1978, Klohs et al. 1979). 

At various times the chloroplasts were removed and fractionated into crude envelopes, purified envelopes or OEM, lEM and washed thylakoids membranes (WT) and thelabelling in the MGDG and DGDG determined in each, (b) As in (a) except that after 10 secs exposure to UDP-l Cgal, the label was removed by the addition of nucleotide phosphatase, (c) Determination of Km and Vm using isolated OEM, IEM and WT. 

A subfamily of Rho proteins, the Rnd family of small GTPases, are always GTP-bound and seem to be regulated by expression and localization rather than by nucleotide exchange and hydrolysis. Many Rho GTPase effectors have been identified, including protein and lipid kinases, phospholipase D and numerous adaptor proteins. One of the best characterized effector of RhoA is Rho kinase, which phosphorylates and inactivates myosin phosphatase thereby RhoA causes activation of actomyosin complexes. Rho proteins are preferred targets of bacterial protein toxins ( bacterial toxins). 

Amylase, invertase, peroxida,se, phenolase, phosphatases, polygalacturonase, protea.se Flavonones and nucleotides... 

The isomerism existing between the pairs of nucleotides was attributed to the different locations of the phosphoryl residues in the carbohydrate part of the parent nucleoside,49 63 since, for instance, the isomeric adenylic acids are both hydrolyzed by acids to adenine, and by alkalis or kidney phosphatase to adenosine. Neither is identical with adenosine 5-phosphate since they are not deaminated by adenylic-acid deaminase,68 60 and are both more labile to acids than is muscle adenylic acid. An alternative explanation of the isomerism was put forward by Doherty.61 He was able, by a process of transglycosidation, to convert adenylic acids a" and 6 to benzyl D-riboside phosphates which were then hydrogenated to optically inactive ribitol phosphates. He concluded from this that both isomers are 3-phosphates and that the isomerism is due to different configurations at the anomeric position. This evidence is, however, open to the same criticism detailed above in connection with the work of Levene and coworkers. Further work has amply justified the original conclusion regarding the nature of the isomerism, since it has been found that, in all four cases, a and 6 isomers give rise to the same nucleoside on enzymic hydrolysis.62 62 63 It was therefore evident that the isomeric nucleotides are 2- and 3-phosphates, since they are demonstrably different from the known 5-phosphates. The decision as to which of the pair is the 2- and which the 3-phosphate proved to be a difficult one. The problem is complicated by the fact that the a and b" nucleotides are readily interconvertible.64,64... 

Cambella and Antia [385] determined phosphonates in seawater by fractionation of the total phosphorus. The seawater sample was divided into two aliquots. The first was analysed for total phosphorus by the nitrate oxidation method capable of breaking down phosphonates, phosphate esters, nucleotides, and polyphosphates. The second aliquot was added to a suspension of bacterial (Escherichia coli) alkaline phosphatase enzyme, incubated for 2h at 37 °C and subjected to hot acid hydrolysis for 1 h. The resultant hot acid-enzyme sample was assayed for molybdate reactive phosphate which was estimated as the sum of enzyme hydrolysable phosphate and acid hydrolysable... 

The answer is c. (Katzung, p 933.) Resistance to thioguanine occurs because of an increase in alkaline phosphatase and a decrease in hypoxanthine-guanine phosphoribosyl transferase. These enzymes are responsible, respectively, for the increase in dephosphorylation of thiopurine nucleotide and the conversion of thioguanine to its active form, 6-thioinosinic acid. 

Sharma, R. and Wang, J. H. Differential regulation of bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase isozzymesdby cyclic AMP-dependent protein kinase and calmodulin-dependent protein phosphatase. Proc. Natl Acad. Sci. U.S.A. 82 2603-2607,1986. 

Hepatotoxins include microcystins, which are cyclic heptapeptides (Fig. 5.1a) and cylindrospermopsin, a sulfated guanidinium alkaloid (Fig. 5. lb). Microcystins bind to certain protein phosphatases responsible for regulating the distribution of cytoskeletal proteins (Zurawell et al. 2005 Leflaive and Ten-Hage 2007). Hepatocytes exposed to microcystins eventually undergo cellular deformation, resulting in intra-hepatic bleeding and, ultimately, death (Carmichael 2001 Batista et al. 2003). In contrast, cylindrospermopsin appears to have a different mode of activity, possibly involving inhibition of protein or nucleotide synthesis (Codd et al. 1999 Froscio et al. 2003 Reisner et al. 2004). Nevertheless, microcystins are the most common cyanotoxins isolated from cyanobacterial blooms (Sivonen and Jones 1999). 

In addition to the enzymes that catalyse the formation of nucleotides and polynucleotides, a large number of catabolic systems exist which operate at all levels of the internucleotide pathways. The ribonucleases and deoxyribonucleases that degrade polynucleotides are probably not significantly involved in purine analogue metabolism, but the enzymes which dephosphorylate nucleoside 5 -monophosphates are known to attack analogue nucleotides and may be of some importance to their in vivo activity. Phosphatases of low specificity are abundant in many tissues [38], particularly the intestine [29]. Purified mammalian 5-nucleotidases hydrolyse only the nucleoside 5 monophosphates [28] and... 

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