The activation of aconitase by ferrous ions and reducing agents

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Abstract
It was shown that the highly purified aconitase preparation possessed little activity in the absence of cofactors. It was not activated by cysteine alone; the addition of Fe2+ increased the activity 15-fold, while the addition of Fe2+ and cysteine gave a 70-fold increase in activity. The optimum conditions required both for activating aconitase and for the estimation of enzymic activity were determined. Aconitase was not activated by Mn2+, and Cu2+ was inhibitory. A Michaelis-Menten relationship was shown to exist between the concentration of Fe2+ and the enzyme activity. It was concluded that a Fe2+-enzyme complex was partly responsible for aconitase activity. A Michaelis-Menten relationship was also found to exist between the enzyme activity and the concentration of the reducing agents: cysteine, thioglycollate, and ascorbic acid. It was concluded that these compounds form an enzyme-reducing agent complex which is partly responsible for enzyme activity. Only higher concentrations of glutathione appreciably activated aconitase; there was no simple Michaelis-Menten relationship between the concentration of glutathione and the enzyme activity. The conclusion was drawn that the active form of aconitase is either an enzyme-Fe2+ - reducing agent or an enzyme-Fe2+ -activator complex.
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