Dependence of in Vivo Ethylene Production Rate on 1-Aminocyclopropane-1-Carboxylic Acid Content and Oxygen Concentrations

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Abstract
1-Aminocyclopropane-1-carboxylic acid (ACC) is aerobically oxidized in plant tissues to form ethylene by ethylene-forming enzyme (EFE). The effect of substrate (ACC and oxygen) concentrations on ethylene production rate by plant tissues was investigated. The Km value for O2 in ethylene production varied greatly depending on the internal ACC content. When ACC levels in the tissue were low (below its Km value), the concentration of O2 giving half-maximal ethylene production rate ([S]0.5) ranged between 5 and 7%, and was similar among different tissues. As the concentration of ACC was increased (greater than its Km value), [S]0.5 for O2 decreased markedly. In contrast, the Km value for ACC was not much dependent on O2 concentration, but varied greatly among different plant tissues, ranging from 8 micromolar in apple (Malus sylvestris Mill.) tissue to 120 micromolar in etiolated wheat (Triticum aestivum) leaf. Such a great variation was thought to be due to the different compartmentation of ACC within the cells in different tissues. These kinetic data are consistent with the view that EFE follows an ordered binding mechanism in which EFE binds first to O2 and then to ACC.