Monosaccharide-H2O2 reactions as a source of glycolate and their stimulation by hydroxyl radicals

An analysis of the H2O2-induced breakdown and transformation of different keto-monosaccharides at physiological concentrations reveals that glycolate and other short-chained carbohydrates and organic acids are produced. Depletion of monosaccharides and glycolate synthesis occurs at increased rates as the length of the carbohydrate chain is decreased, and is significantly increased in the presence of trace amounts of Fe2+ ions (10 μM). Rates of monosaccharide depletion (initial concentration of 3 mM) observed were up to 1.55 mmol h−1 in the case of fructose, and 2.59 mmol h−1 in the case of dihydroxyacetone, depending upon pH, H2O2 concentration, temperature and the presence or absence of catalytic amounts of Fe2+. Glycolate was produced by dihydroxyacetone cleavage at rates up to 0.45 mmol h−1 in the absence, and up to 1.88 mmol h−1 in the presence of Fe2+ ions (pH 8). Besides glycolate, other sugars (ribose, glyceraldehyde, glucose), glucitol (sorbitol) and organic acids (formic and 2-oxogluconic acid) were produced in such H2O2-induced reactions with fructose or dihydroxyacetone. EPR measurements demonstrated the participation of the OH radical, especially at higher pH. Presence of metal ions at higher pH values, resulting in increased glycolate synthesis, was accompanied by enhanced hydroxyl radical generation. Observed changes in intensity of DEPMPO-OH signals recorded from dihydroxyacetone and fructose reactions demonstrate a strong correlation with changes in glycolate yield, suggesting that OH radical formation enhances glycolate synthesis. The results presented suggest that different mechanisms are responsible for the cleavage or other reactions (isomerisation, auto- or free-radical-mediated oxidation) of keto-monosaccharides depending of experimental conditions.