Nucleotides oxidation rate constants

There has been continued interest in the radiation chemistry of the purines since early reports on oriented DNA by Graslund et al. [35] which suggest that the main trapping site of one-electron oxidation in DNA is the guanine base. It is remarkable that in aqueous solution, the electron adducts of the purine nucleosides and nucleotides undergo irreversible protonation at carbon with a rate constant 2 orders of magnitude higher than that for carbon protonation of the electron adduct in thymidine [36]. It is therefore important to know the properties of the various purine reduction products and to ask why they have not been observed in irradiated DNA. 

The reaction of hydrated electrons formed by radiolysis with peroxydisulfate yields the sulfate radical anion SO4 which is a strong chemical oxidant (Eqx = 2.4 V/NHE) [50, 58]. The oxidation of both purine and pyrimidine nucleotides by S04 occurs with rate constants near the diffusion-controlled limit (2.1-4.1 x 10 M s ). Candeias and Steenken [58a] employed absorption spectroscopy to investigate acid-base properties of the guanosine cation radical formed by this technique. The cation radical has a pKa of 3.9, and is rapidly deprotonated at neutral pH to yield the neutral G(-H) . Both G+ and G(-H) have broad featureless absorption spectra with extinction coefffcients <2000 at wavelengths longer than 350 nm. This has hampered the use of transient absorption spectra to study their formation and decay. Candeias and Steenken [58b] have also studied the oxidation of di(deoxy)nucleoside phosphates which contain guanine and one of the other three nucleobases by SO4 , and observe only the formation of G+ under acidic conditions and G(-H) under neutral conditions. 

The oxidized form of superoxide reductase formed in this reaction is reduced back by rubredoxin, dependent ultimately on reduced pyridine nucleotides via intermediate electron carriers [65]. The reaction of SOR with superoxide is also very fast, the reaction rate constant being of an order of 10 M s. It has been demonstrated that CuZnSOD can also function as superoxide reductase reducing superoxide at the expense of oxidation of ferrocyanide, or as superoxide oxidase, oxidizing superoxide at the expense of reducing ferricyanide [66]. Both ferri- and ferrocyanide are unphysiological substrates but the enzyme can also act as superoxide reductase with nitroxyl anion oxidizing it to nitric oxide [67]. 

Rate Constants for Oxidation of Sugars and Nucleotides by Oxoruthenium(IV) and Diplatinum(II)... 

Further support for this idea comes from the similarity of the quenching rate constants for the various mononucleotides. There appears to be no preference for nucleotides of any one base, and if base oxidation occurred, a significantly faster rate constant would be expected for more reactive bases, such as guanine (75). 

The kinetic studies also argue strongly for 1 oxidation. First, all of the nucleotides are more reactive than deoxyribose and ribose. This result can be ascribed to more effective activation of the 1 position by the nucleic acid base compared to hydroxyl, which is likely to be less electron-donating. This trend is evident even after correction of rate constants for the electrostatic binding preequilibrium. In fact, this same trend is evident in the Pt2(pop)4 rate constants. Since Pt2(pop)4 is a tetraanion, the reactions of nucleotides are actually discouraged electrostatically relative to those of the neutral sugars—yet nucleotides are more reactive by about an order of magnitude in rate constant. 

Direct observation of enol ether type radical cations such as are expected to be important in the fragmentation of nucleotide C4 radicals is not possible by the time-resolved laser flash photolysis technique owing to the lack of a suitable chro-mophore. However, it has recently been demonstrated that if such an LFP experiment is conducted in the presence of a triarylamine then any diffusively free enol ether radical cations oxidize the amine to the corresponding highly colored ami-nium radical cation. In this manner the overall rate constant for fragmentation and cage escape may be determined (Scheme 4) [1 Ij. 

Two major messengers feed information on the rate of ATP utilization back to the TCA cycle (a) the phosphorylation state of ATP, as reflected in ATP and ADP levels, and (b) the reduction state of NAD, as reflected in the ratio of NADH/NAD. Within the cell, even within the mitochondrion, the total adenine nucleotide pool (AMP, ADP, plus ATP) and the total NAD pool (NAD plus NADH) are relatively constant. Thus, an increased rate of ATP utilization results in a small decrease of ATP concentration and an increase of ADP. Likewise, increased NADH oxidation to NAD by the electron transport chain increases the rate of pathways producing NADH. Under normal physiological conditions, the TCA cycle and other... 

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