Ethylene Production by Isolated Chloroplasts

Isolated chloroplast lamellae from spinach or sugar-beet leaves produce ethylene from methyl-mercaptopropanal (MMP) or 2-keto-4-methylmercaptobutyrate (KMB) in the light. The pH-optimum for ethylene production from MMP is 6.3; ethylene production from KMB has an apparent pH-optimum at about pH 5. Ethylene formation in red light (90 kerg-cm⁻². sec⁻¹) from the above substrates is stimulated by ferredoxin and inhibited by either DCMU (10⁻⁵M),ferredoxin together with NADP, catalase, superoxide dismutase or under anaerobic conditions. From the inhibition by either DCMU, NADP or anaerobic conditions it is concluded, that an intact electron transport system from water as electron donor to oxygen as electron acceptor is necessary for ethylene formation. H2 O2 alone does not stimulate ethylene formation from MMP or KMB. Inhibition by both catalase and superoxide dismutase support the view, that ethylene formation from MMP or KMB is driven by the OH-radical, which is formed from H₂O₂ and the superoxide free radical ion. The presented data suggest that in addition to ferredoxin another membrane-bound factor is involved in photosynthetic oxygen reduction and ethylene formation. This factor (ORF = oxygen reducing factor) stimulates photosynthetic oxygen reduction in the presence of ferredoxin yielding H2O2 in addition to O₂^˙-, which is the product of the autooxidation of reduced ferredoxin. During photosynthetic ethylene formation from MMP or KMB, the production of the OH-radical from H₂O₂, and O₂^˙- according to H₂O₂+O₂^˙-→OH˙+OH^-+O₂, (Haber and Weiss, Proc. Roy. Soc. ‘Ser. A 147, 332 [1934], Beauchamp and Fridovich, J. Biol. Chem. 245, 4641 [1970]) seems to be the rate-limiting step.