Bicarbonate/CO2-Facilitated Conversion of 1-Amino-cyclopropane-1-carboxylic Acid to Ethylene in Model Systems and Intact Tissues

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Abstract
Bicarbonate markedly enhances ethylene production from 1-aminocyclopropane-1-carboxylic acid (ACC) in model chemical systems where the conversion is free radical-mediated, in thylakoid membrane suspensions of Phaseolus vulgaris L. cv. Kinghorn where the reaction is light-dependent, and in microsomal membrane suspensions and intact tissues where the reaction is enzymically mediated. In 2 model systems generating free radicals, the Fenton reaction and a reaction mixture containing xanthine/xanthine oxidase, NaHCO3 (200 mM) increased the formation of ethylene from ACC by 84-fold and 54-fold, respectively. Isolated thylakoid membranes also proved capable of ACC-dependent ethylene production, but only upon illumination, and this too was enhanced by added NaHCO3. As well, light-induced inhibition of ACC-dependent ethylene production by leaf discs was relieved by adding 200 mM NaHCO3. Finally, NaHCO3 (200 mM) augmented ACC-dependent ethylene production from young carnation flowers by about 4-fold, and the conversions of ACC to ethylene by microsomes isolated from carnation flowers and etiolated pea epicotyls were higher by 1900 and 62%, respectively, in the presence of 200 mM NaHCO3. This increased production of ethylene appears not to be due to bicarbonate or CO2-induced release of the gas from putative receptor sites, since the addition of NaHCO3 to sealed reaction mixtures after the ACC to ethylene conversion had been terminated had no effect. Spin-trapping studies have confirmed that bicarbonate does not facilitate the formation of free radicals thought to be involved in the conversion of ACC to ethylene. Nor did bicarbonate alter the physical properties of the membrane bilayer, which might indirectly modulate the activity of the membrane-associated enzyme capable of converting ACC to ethylene. Rather, bicarbonate appears to directly facilitate the conversion of ACC to ethylene, and the data are consistent with the view that CO2 derived from bicarbonate is the active molecular species.