The Role of NADH:Fe(III)EDTA Oxidoreductase in Ethylene Formation from 2-Keto-4-Methylthiobutyrate

Abstract

A mechanism for the formation of ethylene from 2-keto-4-methylthiobutyric acid (KMBA), a deaminated derivative of l-methionine, was studied with NADH:Fe(III)EDTA oxidoreductase purified from Cryptococcus albidus IFO 0939. The characteristics of the reactions catalyzing the NADH:Fe(III)EDTA oxidoreduction and the formation of ethylene were compared and found to be almost identical. A chemical ethylene-forming system, composed of Fe(II)EDTA, KMBA and oxygen, was constructed and the characteristics of the formation of ethylene were compared with those of the enzymatic reaction. Both the enzymatic and the chemical ethylene-forming reactions were strongly inhibited by scavengers of free radicals, such as benzoic acid, hydroquinone and catalase, and both were activated by H 2O 2. From these results, we propose the following mechanism for the formation of ethylene from KMBA. The first step involves the NADH:Fe(III)EDTA oxidoreduction that is catalyzed by NADH:Fe(III)EDTA oxidoreductase, with Fe(II)EDTA always being supplied as the result. Oxidation of Fe(II)EDTA by molecular oxygen yields the superoxide radical anion (O 2 . ̄ ) which can undergo the dismutation reaction to form hydrogen peroxide. Hydrogen peroxide in turn can react with Fe 2+ via the Fenton reaction, generating the hydroxyl radical (OH •), which can serve as an oxidizing agent in the oxidation of KMBA to ethylene. The role of the activity of NADH:Fe(III)EDTA oxidoreductase in the formation of ethylene from KMBA in vivo is discussed.