Electrochemical oxidation of N-methylated derivatives of uric acid

Electrochemical oxidation of 2-thiouracil at pyrolytic graphite electrode

2-Thiouracil has been studied in phosphate buffers of pH 1.95-11.08 using linear and cyclic sweep voltammetry, coulometry, controlled potential electrolysis and spectral studies. One well-defined oxidation peak I(a) in the pH range 1.95-11.08 was noticed. The number of electrons involved in peak I(a) was found to be four in a thin layer cell whereas under exhaustive electrolysis condition oxidation was found to involve six electrons. A reduction peak II(c) (2e, 2H(+)) is noticed in the reverse sweep. Spectral studies during oxidation were carried out at different pH. Kinetic studies indicated that the decay of the UV-absorbing intermediate is a first order reaction. The products of the electrooxidation have been characterized and a tentative EC mechanism has been suggested for the oxidation of 2-thiouracil.

Spermine Participates in Oxidative Damage of Guanosine and 8-Oxoguanosine Leading to Deoxyribosylurea Formation

Oxidation of single- or double-stranded DNA containing a 7,8-dihydro-8-oxoguanosine lesion with the one-electron oxidant Na2IrCl6 in the presence of spermine led to formation of a covalent adduct that was analyzed by gel electrophoresis, HPLC, ESI-MS, and UV-vis. The adduct was labile to heat, exhibiting a t1/2 of 12 h at 37 degrees C, and the ultimate hydrolysis product was characterized as a deoxyribosylurea lesion. Data from model studies with 1,3-diaminopropane vs 1,4-diaminobutane are consistent with initial formation of a C5 spermine adduct from a dehydro-8-oxoguanosine intermediate, followed by rearrangement to a spiroaminal subject to slow hydrolysis at C4 of the purine. Spermine adducts could also be formed from oxidation of the analogous G-containing oligomer from reaction with singlet oxygen, albeit in lower yield. These results are surprising in light of the traditional view that spermine is radioprotective against DNA oxidation.

Spectroscopic characterization of intermediates in the urate oxidase reaction

The oxidation of urate catalyzed by soybean urate oxidase was studied under single-turnover conditions using stopped-flow absorbance and fluorescence spectrophotometry. Two discrete enzyme-bound intermediates were observed; the first intermediate to form had an absorbance maximum at 295 nm and was assigned to a urate dianion species; the second intermediate had an absorbance maximum at 298 nm and is believed to be urate hydroperoxide. These data are consistent with a catalytic mechanism that involves formation of urate hydroperoxide from O2 and the urate dianion, collapse of the peroxide to form dehydrourate, and hydration of dehydrourate to form the observed product, 5-hydroxyisourate. The rate of formation of the first intermediate was too fast to measure accurately at 20 degreesC; the second intermediate formed with a rate constant of 32 s-1 and decayed with a rate constant of 6.6 s-1. The product of the reaction, 5-hydroxyisourate, is fluorescent, and its release from the active site occurred with a rate constant of 31 s-1.

On-line mass spectrometric investigation of the peroxidase-catalysed oxidation of uric acid

The enzymatic and electrochemical oxidation pathways of uric acid were determined on-line with thermospray-tandem mass spectrometry. Products and intermediates formed as a result of electrooxidation were monitored as the electrode potential was varied. Electrochemical results served as a model for the enzymatic studies. In fact, electrochemical studies were essential for elucidating the structures of intermediates because of the high conversion efficiencies in electrooxidation. Products and intermediates formed as a result of enzymatic oxidation of uric acid were monitored as the reaction time was varied. When the enzymatic oxidation of uric acid with peroxidase and H2O2 was studied, the same intermediates and products were observed as in the electrochemical oxidation. The tandem mass spectrometric results provide convincing evidence that the primary intermediate produced during both the enzymatic and electrochemical oxidation of uric acid has a quinonoid diimine structure. The primary intermediate can follow three distinct reaction pathways to produce the identified final products. The final enzymatic and electrochemical oxidation products observed in these studies were urea, CO2, alloxan, alloxan monohydrate, allantoin, 5-hydroxyhydantoin-5-carboxamide and parabanic acid.

Reaction of methylated urates with 1,1-diphenyl-2-picrylhydrazyl

The kinetics of the reaction between the stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH) and methylated urates was studied. Urates that had methyl groups on the 1,3,9, or on the 1 and 3 or 1 and 9 nitrogens reacted with DPPH 15 to 77% faster than uric acid. Urates substituted with methyl groups on the 7 nitrogen or on both the 3 and 9 nitrogens reacted with DPPH at rates that were less than 0.1 that of uric acid. 3,7,9-Trimethyluric acid and 1,3,7,9-tetramethyluric acid reacted with DPPH at barely detectable rates. DPPH reacted with uric acid, the monomethylated urates, and some of the dimethylated urates in a ratio of 2:1. DPPH reacted with other dimethylated and trimethylated urates in a ratio of 1:1. Semiempirical MNDO calculations indicate that the most stable radical of uric acid is formed by hydrogen abstraction from the 3, 7 or 9 position. The most stable species resulting from loss of a second hydrogen lack hydrogens at the 3 and 7 positions or the 7 and 9 positions. For maximum reactivity with DPPH, methylated uric acid derivatives must have a hydrogen at nitrogen 7 and one of the hydrogens at either the 3 or 9 position.