Ribose lability

Two points thus argue against the participation of ribose in nucleic acid formation the lability of the molecule and the problems with its synthesis (the concentrations of the starting materials are too high). Other, newer and more effective syntheses seem necessary, whereby prebiotic conditions (although these are not known precisely) strongly limit the possibilities. 

The heat-labile enzyme which catalyzes this aldol condensation was purified eightfold from the extracts of E. coli. By combining this phos-phodesoxyriboaldolase with purified phosphoriboaldolase from yeast, Rackerl 7(b)-108 was able to demonstrate the long sought conversion of D-ribose into desoxy-D-ribose, e. g.,... 

Ma Q. Induction and superinduction of 2,3,7,8-tetrachlorodibenzo-rho-dioxininducible poly(ADP-ribose) polymerase Role of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator transcription activation domains and a labile transcription repressor. Arch Biochem Biophys 2002 404(2) 309-16. 

Kawaichi et al. 110), working with poly(ADP-ribose) synthetase purified from rat liver, demonstrated that the extent of automodification was directly proportional to the concentration of NAD. During automodification branching of the polymer was observed to occur (1 branch for every 50 ADP-ribose residues). In addition, the au-tomodified enzyme was less active than the unmodified enzyme theiiCm for NAD gradually increased and Vmax decreased as the modification proceeded. The base lability indicated an ester linkage between the polymer and synthetase 110). 

Koide and co-workers (ISO) demonstrated that when purified rat liver nuclei were incubated with [ C]NAD and the various nuclear protein fractions isolated, 40% of the total label was associated with histones with 50% of this with Hi. Under the conditions of the experiment, the poly(ADP-ribose) existed as trimers. The extent of modification reaction was dependent on the NAD concentration with 60% of the maximal at 1 mM NAD. The heterogeneity of the modified histones was observed on acid-urea gel electrophoresis where they migrated some 10- 20% slower than unmodified histone. A shift of the histone absorbance maximum from 275 to 259 nm was observed due to the presence of large amounts of ADP-ribose. The ADP-ribose to histone linkage was labile in dilute base and neutral hydroxylamine, as described earlier by Hayaishi 84). When HI was cleaved with chymo-trypsin at the central phenylalanine (residue 106), both halves of the molecule were found to be modified poly(ADP-ribose) was linked to glutamate 2 and glutamate 116. 

The mono(ADP-ribose) residue modifying the protein turns over rapidly. This was indicated by two lines of evidence [13]. First, when SMP were incubated with [ H]-NAD and, after attaining a steady state level of modification as judged by incorporation of tritium, were then supplied with [ C]-NAD, there was loss of tritium and simultaneously incorporation of [ C] into the protein. Secondly, when ATP was added to SMP, mono(ADP-ribose) was lost from the protein within seconds. Whether the removal of mono(ADP-ribose) from the protein is enzyme-catalyzed remains to be seen. The bond between mono(ADP-ribose) and the 32,000 mol.wt. protein in SMP is acid stable, alkali sensitive, and labile in neutral hydroxyl amine. Formation of it is inhibited by arginine blocking reagents [13]. 

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