Influence of Oxygen Concentration on Redox Cycling of Alloxan and Dialuric Acid

Abstract

Alloxan, a chemical diabetogen, decays in the absence of reductants into alloxanic acid. In the presence of glutathione, it is reduced via the alloxan radical into dialuric acid, which autoxidizes back to alloxan. During this redox cycling process, reactive oxygen species are formed that destroy beta-cells in islets of Langerhans. Previous experiments were conducted with oxygen concentrations about ten times as high as within cells. The aim of our in vitro study was to evaluate the impact of different oxygen concentrations (0, 25, 250 micromol/l) at a given initial ratio of glutathione and alloxan on this redox cycling. Reduction of alloxan, oxidation of glutathione, and the formation of glutathiol (GSSG) were continuously recorded by HPLC for 90 minutes at 25 degrees C in air, calibration gas, or argon. In the absence of reductants, alloxan irreversibly decomposed into alloxanic acid regardless of oxygen presence. When the reaction system contained glutathione, decomposition was significantly retarded and therefore influenced by oxygen. In argon, decay could not be observed due to its reduction and the absence of oxygen. Increasing oxygen concentration enabled a redox cycling and therefore an ongoing decay. The highest decomposition along with the highest consumption of glutathione occurred at 250 micromol/l oxygen. The lower the oxygen, the more dialuric acid could be detected. After calculation, about 33 redox cycles per hour generates an amount of reactive oxygen species sufficient to damage pancreatic beta cells and induce insulin deficiency.

Alloxan-dialuric acid cycling: A complex redox mechanism

“…Alloxan and dialuric acid act as a redox couple driven by reduced glutathione (GSH) or L-cysteine, generating in the presence of oxygen, both O 2 + ( and HO 2 + [5,9]. The GSH-dependent formation of dialuric acid from alloxan is reported to be a decisive pre-condition for the subsequent ROS generation [9].…”

“…The GSH-dependent formation of dialuric acid from alloxan is reported to be a decisive pre-condition for the subsequent ROS generation [9]. Alloxan is suggested to oxidize GSH to GSSG [27] via the alloxan radical [5]. During this redox cycling process, two molecules of GSH are oxidized to one molecule of GSSG and during each cycle one molecule of oxygen is simultaneously reduced to one molecule of hydrogen peroxide [5].…”

“…It is in acid-base equilibrium with deprotonated species with pK values of 6.6, 8.4 and 13.1 [7]. In neutral or alkaline solutions, alloxan suffers a rearrangement described as a 'benzilic acid rearrangement' to form alloxanic acid [5,7].…”

“…Reduction of alloxan, oxidation of GSH and formation of oxidized glutathione (GSSG) were continuously recorded in experiments conducted with different oxygen concentrations to evaluate the impact of oxygen on the redox cycling initiated by a given initial ratio of GSH and alloxan [5].…”

Suppressive effects of electrolyzed reduced water on alloxan-induced apoptosis and type 1 diabetes mellitus

“…Dialuric acid is then readily autooxidized back to ALX establishing a redox-cycle reaction for the generation of superoxide radicals and H 2 O 2 Schulte im Walde et al 2002;Washburn and Wells 1997). Furthermore, recent studies have shown that enhanced ALX toxicity depends on the released iron from ferritin mediated by ascorbate (Sakurai et al 2006), and the involvement of oxygen on the redox cycling between ALX and dialuric acid (Brömme et al 2005). These experimental data indicate that various effectors are related to apoptotic b-cell death as a primary cause of T1DM.…”