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.     

Action of Uric Acid,Allopurinol and Oxypurinol on the Myeloperoxidase-Derived Oxidant Hypochlorous Acid

Both oxypurinol and uric acid react with the myeloperoxidase-derived oxidant hypochlorous acid at physiological pH, and they can protect the elastase-inhibitory capacity of human α₁ -antiprotease against inactivation by hypochlorous acid. Allopurinol does not protect α₁-antiprotease, possibly because the redox potential of allopurinol at physiological pH is too positive to permit oxidation by hypochlorous acid.     

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.     

A Reinvestigation of the Reaction of Desferrioxamine with Superoxide Radicals. A Pulse Radiolysis Study

The reaction of desferrioxamine with superoxide has been studied using the pulse radiolysis technique. The decay of O₂^- was not accelerated in the presence of up to 4 × 10^-4 M desferrioxamine at physiological pH. The rate constant was found to be lower than 2 × 10⁴ M^-1 S^-1. In acid solutions the rate constant of the reaction between desferrioxamine and HO₂ was found to be lower than 10₅ M^-1 S^-1. The reaction was not studied in alkaline solutions due to the high absorbance of desferrioxamine in the U. V. region. The pK of desferrioxamine was determined to be 9.2 ± 0.05.     

A Reappraisal of Xanthine Dehydrogenase and Oxidase in Hypoxic Reperfusion Injury: the Role of NADH as an Electron Donor

Xanthine oxidase (XO) is conventionally known as a generator of reactive oxygen species (ROS) which contribute to hypoxic-reperfusion injury in tissues. However, this role for human XO is disputed due to its distinctive lack of activity towards xanthine, and the failure of allopurinol to suppress reperfusion injury. In this paper, we have employed native gel electrophore-sis together with activity staining to investigate the role human xanthine dehydrogenase (XD) and XO in hypoxic reperfusion injury. This approach has provided information which cannot be obtained by conventional spectrophotometric assays. We found that both XD and XO of human umbilical vein endothelial cells (HUVECs) and lymphoblastic leukaemic cells (CEMs) catalysed ROS generation by oxidising NADH, but not hypoxanthine. The conversion of XD to XO was observed in both HUVECs and CEMs in response to hypoxia, although the level of conversion varied. Purified human milk XD generated ROS more efficiently in the presence of NADH than in the presence of hypoxanthine. This NADH oxidising activity was blocked by the FAD site inhibitor, diphenyleneiodo-nium (DPI), but was not suppressible by the molybdenum site inhibitor, allopurinol. However, in the presence of both DPI and allopwinol the activities of XD/XO were completely blocked with either NADH or hypoxanthine as substrates. We conclude that both human XD and XO can oxidise NADH to generate ROS. Therefore, the conversion of XD to XO is not necessary for post-ischaemic ROS generation. The hypoxic-reperfusion injury hypothesis should be reappraised to take into account the important role played by XD and XO in oxidising NADH to yield ROS.     

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.     

Linear and Non Linear Competition Plots in the Deoxyribose Assay for Determination of Rate Constants for Reaction of non Steroidal Antiinflammatory Drugs with Hydroxyl Radicals

Performing the deoxyribose (DR) assay for determination of the rate constants for reaction of non steroidal antiinflammatory drugs with hydroxyl radicals led to some unusual competition plots. The molecules from the arylpropionic family of drugs: ibuprofen, flurbiprofen, ketoprofen and naproxen produced the linear relationship. However, acemetacin, diclofenac Na, flufenamic acid, indometacin, niflumic acid, tolmetin Na and sulindac presented non linear competition plots manifesting at relatively low drug concentrations. This effect was corrected by increasing DR concentrations from 2.8 mM to 15 mM. The modification did not affect rate constants values for those derivatives which already presented a linear plot at 2.8 mM, but allowed to calculate rate constants for other compounds. It is suggested that the experimental conditions have to be adapted particularly for those derivatives with a relatively high. rate constant for reaction with the radical species. The oxicam derivatives (tenoxicam and piroxicam) presented another kind of deviation that revealed a prooxidant effect in this system: non linear plots were also observed at relatively low drug concentrations, but in the opposite direction than for the other molecules. This last effect was independent of DR concentration but could be corrected by increasing ascorbate concentration in the system.     

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.     

An E.S.R. Investigation of the Reactive Intermediate Generated in the Reaction Between FeII and H2O2 in Aqueous Solution. Direct Evidence for the Formation of the Hydroxyl Radical

The technique of E.S.R. spectroscopy, when employed in conjunction with a continuous flow system, provides direct evidence for the nature of free radicals formed from organic substrates in the presence of Fe^II and H₂O₂ in aqueous solution. It is shown, both via the identification of hydroxyl-radical adducts to alkenes and via the observed site-selectivity of radical attack, that the hydroxyl radical is formed as the reactive intermediate in the presence of various chelators (e.g. EDTA, DTPA). This approach also allows the rate constants for the Fe^II-H₂O₂ reaction in the presence of the different chelates to be determined; values obtained are in reasonable agreement with most of those measured by other methods. Examples of radical oxidation (by Fe^III) and reduction (by Fe^II) are revealed.