Hydroxyl Radical-Induced Reactions in Polyadenylic Acid as Studied by Pulse Radiolysis: Part I. Transformation Reactions of Two Isomeric OH-Adducts

The absorption spectra of polyadenylic acid (polyA) radicals in N₂0 saturated aqueous solution have been measured as a function of time (up to 15 s) following an 0.4μS electron pulse. The spectra and their changes were analysed by comparison with those from monomeric adenine derivatives (nucleosides and nucleotides) which had been studied by Steenken.¹

The reaction of OH^· radicals with the adenine moiety in poly A results in the formation of two hvdroxvl adducts at the positions C-4 [polyA40H^·] and C-8 [polyA80H^·]. Each OH-adduct undergoes a unimol-ecular transformation reaction before any bimolecular or other unimolecular decay occurs. These reactions are characterized by different rate constants and _pH dependencies. The polyA40H^· adduct undergoes a dehydration reaction to yield a neutral N⁶ centered radical (rate constant K_deh= 1.4 × 10⁴s^-1 at ^pH7.3). This reaction is strongly inhibited by H⁺. In comparison with the analogous reactions in adenosine phosphates, the kinetic _pK value for its inhibition is two ^pH units higher. This shift is the result of the counter ion condensation or double-strand formation. The polyA80H^· adduct undergoes an imidazole ring opening reaction to yield an enol type of formamidopyrimidine radical with the resulting base damage (k^r.o. = 3.5 × 10⁴ s ^-1 at pH7.3). This reaction in contrast is strongly catalysed by H⁺and OH^-, similar as for adenosine but different compared to the nucleotides.     

On the Specificity of Allopurinol and Oxypurinol as Inhibitors of Xanthine Oxidase. A Pulse Radiolysis Determination of Rate Constants for Reaction of Allopurinol and Oxypurinol with Hydroxyl Radicals

Allopurinol has been employed as a “specific” inhihitor of xanthine oxidase in studies of hypoxic/ reoxygenation injury. Pulse radiolysis was used to establish rate constants for the reactions of allopurinol and its major metabolite oxypurinol with hydroxyl radicals: values were (1.45 ± 0.241 × 10⁹ M^-1 s^-1 for allopurinol and (4.95 ± 0.84) × 10⁹ M^-1 s^-1 for oxypurinol. These rate constants show that, in view of the amounts of allopurinol that have been used in animal studies. hydroxyl radical scavenging by this molecule could contribute to its biological actions. especially if animals are pre-treated with allopurinol. so allowing oxypurinol to form. The ability of allopurinol to protect tissues not containing xanthine oxidase against reoxygenation injury may be related to radical scavenging by allopurinol and oxypurinol.     

On the Specificity of Allopurinol and Oxypurinol as Inhibitors of Xanthine Oxidase. A Pulse Radiolysis Determination of Rate Constants for Reaction of Allopurinol and Oxypurinol with Hydroxyl Radicals

Allopurinol has been employed as a “specific” inhihitor of xanthine oxidase in studies of hypoxic/ reoxygenation injury. Pulse radiolysis was used to establish rate constants for the reactions of allopurinol and its major metabolite oxypurinol with hydroxyl radicals: values were (1.45 ± 0.241 × 10⁹ M^-1 s^-1 for allopurinol and (4.95 ± 0.84) × 10⁹ M^-1 s^-1 for oxypurinol. These rate constants show that, in view of the amounts of allopurinol that have been used in animal studies. hydroxyl radical scavenging by this molecule could contribute to its biological actions. especially if animals are pre-treated with allopurinol. so allowing oxypurinol to form. The ability of allopurinol to protect tissues not containing xanthine oxidase against reoxygenation injury may be related to radical scavenging by allopurinol and oxypurinol.     

Reversible H-Atom Abstraction from Alcohols by Thiyl Radicals: Determination of Absolute Rate Constants by Pulse Radiolysis

Hahn-Meitner-Institut Berlin, Bereich Strahlenchemie, Glienicker Str. 100, 1000 Berlin 39, Fed. Rep. Germany

Penicillamine thiyl radicals, PenS', are shown to abstract hydrogen atoms from 2-propanol and to establish an equilibrium

PenS' + (CH₁)₂ CHOH ? PenSH + (CH₃)₂ COH.

The rate constants for the forward and back reaction have been determined to (1.4 ± 0.3) × 10⁴ and (1.2 ± 0.3) × 10⁸ M^-1 s^-1, respectively, by means of pulse radiolysis. The data have been obtained from various independent methods which include direct measurements and competitive schemes involving irreversible interception of the alcohol radical by electron acceptors (e.g. CCl₄. PNAP) and/or the thiyl radical by antioxidants (e.g. α-tocopherol). The results demonstrate that the reaction of carbon-centered radicals with thiols, in radiation biology commonly known as “repair” reaction, may be reversed and thus imply the possibility of thiyl radical induced biological damage.