Aerobic oxidation of aminoacetone, a threonine catabolite: Iron catalysis and coupled iron release from ferritin

Aminoacetone (AA) is a threonine and glycine catabolite long known to accum ulate in cri-du-chat and threoninemia syndromes and, more recently, implica ted as a contributing source of methylglyoxal (MG) in diabetes mellitus. Ox idation of AA to MG, NH4+, and H2O2 has been reported to be catalyzed by a copper-dependent semicarbazide sensitive amine oxidase (SSAO) as well as by Cu(II) ions. We here study the mechanism of AA aerobic oxidation, in the p resence and absence of iron ions, and coupled to iron release from ferritin . Aminoacetone (1-7 mM) autoxidizes in Chelex-treated phosphate buffer (pH 7.4) to yield stoichiometric amounts of MG and NH4+, Superoxide radical was shown to propagate this reaction as indicated by strong inhibition of oxyg en uptake by superoxide dismutase (SOD) (1-50 units/mL; up to 90%) or semic arbazide (0.5-5 mM; up to 80%) and by EPR spin trapping studies with 5,5-di methyl-1-pyrroline-N-oxide (DMPO), which detected the formation of the DMPO -(OH)-O-. adduct as a decomposition product from the DMPO-O-2(.-) adduct. A ccordingly, oxygen uptake by AA is accelerated upon addition of xanthine/xa nthine oxidase, a well-known enzymatic source of O-2(.-) radicals. Under Fe (II)EDTA catalysis, SOD (< 50 units/mL) had little effect on the oxygen upt ake curve or on the EPR spectrum of AA/DMPO, which shows intense signals of the DMPO-(OH)-O-. adduct and of a secondary carbon-centered DMPO adduct, a ttributable to the AA(.) enoyl radical. In the presence of iron, simultaneo us (two) electron transfer from both Fe(II) and AA to O-2, leading directly to H2O2 generation followed by the Fenton reaction is thought to take plac e. Aminoacetone was also found to induce dose-dependent Fe(II) release from horse spleen ferritin, putatively mediated by both O-2(.-) and AA(.) encyl radicals, and the co-oxidation of added hemoglobin and myoglobin, which ma y be viewed as the initial step for potential further iron release. It is t hus tempting to propose that AA, accumulated in the blood and other tissues of diabetics, besides being metabolized by SSAO, may release iron and unde rgo spontaneous and iron-catalyzed oxidation with production of reactive H2 O2 and O-2(.-), triggering pathological responses. It is noteworthy that no ninsulin-dependent diabetes has been frequently associated with iron overlo ad and oxidative stress.