Characterization and kinetic parameters of ethylene-forming enzyme from avocado fruit.

Biosynthesis of the phytohormone ethylene in higher plants proceeds via the following pathway: S-adenosylmethionine----1-aminocyclopropane-1-carboxylic acid (ACC)----ethylene. Ethylene-forming enzyme (EFE), the enzyme responsible for the oxidation of ACC to ethylene, has been only partially characterized in vitro. We have obtained authentic EFE activity in vitro from extracts of avocado fruit (Persea americana Mill. cv Hass). Ammonium sulfate fractionation revealed the presence of two EFE activities, which we designate as EFE1 and EFE2. EFE1 activity utilizes ACC and O2 as substrates and requires Fe(II) and ascorbate as cofactors. The enzyme has a relatively low Km (32 microM) for ACC, discriminates diastereomers of 1-amino-2-ethyl-cyclopropane-1-carboxylic acid, and is inhibited competitively by 2-aminoisobutyric acid, thus confirming its identity with authentic EFE. Activity is retained in a 100,000 x g supernatant and has a pH optimum of 7.5-8.0, suggesting a cytosolic localization.