Heterologous expression in yeast has previously shown that the tomato cDNA LE-ACO1 encodes a functional 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACO) protein [Hamilton, A. J., Bouzayen, M. & Grierson, D. (1991) Proc. Natl Acad. Sci. USA 88, 7434-7437]. In the present work, full-length cDNAs encoding the two other members of the tomato ACO family (LE-ACO2 and LE-ACO3) were isolated and expressed in Saccharomyces cerevisiae. Analysis of the predicted amino acid sequences showed that the ACO1 and ACO3 proteins are highly similar (95%) while ACO2 is more divergent (89%). Yeast strains transformed with each of the three cDNAs were able to convert exogenous ACC to ethylene, the ACO1 strain exhibiting the highest activity in vivo and the ACO3 and ACO2 strains reaching 65% and 45% of ACO1 maximum activity, respectively. None of the ACO activities expressed in yeast required addition of ascorbate in vivo. ACO activities assayed in vitro revealed no significant differences between the three isoforms with regards to optimum temperature (29 degrees C), optimum pH (6.8-7.2), absolute dependence for ascorbate, Fe2+ and carbon dioxide, and inhibition by iron-chelating agents (1,10-phenanthroline and EDTA), Co2+ and free-radical scavengers (n-propyl gallate). However, differences were detected in the apparent Km values for ACC, the pI and the specific activity. The biochemical features that might explain the differences between the isoenzyme activities are discussed.