Pyridoxalphosphate-dependent ethylene production from methionine by isolated chloroplasts

The role of O2.- in the production of HO.: in vitro and in vivo

In vitro O2.- reduces Fe(III) to Fe(II), which, in turn, reduces the H2O2, yielding Fe(II)O or HO.. In vivo O2.- increases the supply of free iron by oxidatively attacking the [4Fe-4S] clusters of dehydratases such that they release Fe(II), which can then reduce H2O2. In vivo, O2.- also increases the production of H2O2 by acting as an oxidant toward the dehydratases and toward other cellular reductants.

The toxicology of molecular oxygen

Molecular oxygen, itself not very reactive, can be converted by photosensitization to electronically excited singlet states, and by partial reduction to the superoxide and hydroxyl free radicals and to hydrogen peroxide. The very considerable toxicity of oxygen, which is due primarily to the properties of these derivatives, is ordinarily overlooked because aerobes have evolved an elaborate system of defenses which is reasonably adequate under ambient conditions. This toxicity becomes all too apparent when these defenses are overwhelmed at elevated pO2 or through the action of compounds which increase the conversion of oxygen to its more reactive derivatives. We will here consider the threat posed by oxygen and the defenses which make aerobic life possible.

Mechanisms of oxygen activation by nitrofurantoin and relevance to its toxicity

Purified ferredoxin-(cytochrome c)-NADP+ oxidoreductase and xanthine oxidase were found to catalyse the reduction of nitrofurantoin to the free radical. Under aerobic conditions, the nitrofurantoin radical underwent autoxidation to regenerate the parent compound with the concomitant production of superoxide and eventually hydrogen peroxide. The nitrofurantoin radical was also shown to react with hydrogen peroxide to generate a highly reactive species which was capable of oxidising methionine to ethylene. This active oxygen radical appeared to be identical with the crypto-OH . radical, previously proposed as being formed from the analogous reaction of the methyl viologen radical with hydrogen peroxide [R.J. Youngman and E.F. Elstner, FEBS Lett. 129, 265 (1981)]. Catalase inhibited nitrofurantoin-dependent ethylene formation in both enzyme systems, whereas superoxide dismutase was only inhibitory in the xanthine oxidase mediated reaction. Although the primary function of the respective enzyme systems is to generate the nitrofurantoin radical, the xanthine oxidase reaction is markedly more complex than that of ferredoxin-(cytochrome c)-NADP+ oxidoreductase. The differences between the two enzyme reactions appear to be due to the endogenous autoxidation of xanthine oxidase. The aerobic activation of nitrofurantoin by xanthine oxidase involved the superoxide anion as an intermediate, whereas the nitrofuran was directly reduced by ferredoxin-(cytochrome c)-NADP+ oxidoreductase without a requirement for active oxygen species.

Rapidly induced ethylene formation after wounding is controlled by the regulation of 1-aminocyclopropane-1-carboxylic acid synthesis

Bean leaves from Phaseolus vulgaris L. var. Pinto 111 react to mechanical wounding with the formation of ethylene. The substrate for wound ethylene is 1-aminocyclopropane-1-carboxylic acid (ACC). It is not set free by decompartmentation but is newly synthesized. ACC synthesis starts 8 to 10 min after wounding at 28°C, and 15 to 20 min after wounding at 20°C. Aminoethoxyvinylglycine (AVG), a potent inhibitor of ethylene formation from methionine via ACC, inhibits wound ethylene synthesis by about 95% when applied directly after wounding (incubations at 20°C). AVG also inhibits the accumulation of ACC in wounded tissue. AVG does not inhibit conversion of ACC to ethylene. Wound ethylene production is also inhibited by cycloheximide, n-propyl gallate, and ethylenediaminetetraacetic acid.

Formation of ethylene from methionine. Reactivity of radiolytically produced oxygen radicals and effect of substrate activation

Ethylene was determined by gas chromatography after reaction of radiolytically produced OH and O2- radicals with methionine, methionine + pyridoxal phosphate and S-adenosyl-methionine (SAM). Both oxygen radicals, alone or in combination, liberate ethylene from methionine and methionine/pyridoxal phosphate. From SAM ethylene was primarily produced by the combined attack of OH nad H2O2 or O2-.

Oxygen activation in isolated chloroplasts. Mechanism of ferredoxin-dependent ethylene formation from methionine

Low-potential electron acceptors of photosystem I of chloroplast lamellae produce superoxide anions (0-2) and hydrogen peroxide by autoxidation, but have no effect on ethylene formation from methionine; equimolar amounts of ferredoxin are less active in photosynthetic O-2 and H2O2 production but strongly stimulate ethylene production from methionine. 2. Ten to fifty units of superoxide dismutase inhibit fifty to two hundred units of superoxide dismutase stimulate ethylene formation from methionine by chloroplast lamellae in the presence of ferredoxin. This stimulation is stronger at pH 7.0 than at pH 7.8. Catalase inhibits ethylene formation from methionine. 3. Pulse-radiolytic production of nitrite (NO-2) from hydroxylamine, initiated by hydroxyl radicals (.OH) or O-2, shows no difference in the presence or absence of ferredoxin, nor do the decay kinetics of O2. 4. From the above observations and from model reactions (xanthine/xanthine oxidase; iron salts in the presence of H2O2), it is concluded that reduced ferredoxin in the presence of H2O2 forms a Fenton-type oxidizing species for methionine, generating ethylene in the presence of pyridoxal phosphate. 5. Inhibitory effects of both superoxide dismutase and catalase in oxygen-dependent reactions need not necessarily indicate the participation of the 'Haber-Weiss' reaction.