Keishi Shimokawa

Evidence for the involvement of superoxide anion in the ethylene-induced chlorophyll a catabolism of Raphanus sativus cotyledons

Abstract We have studied the involvement of superoxide anion in the reactions catalysed by chlorophyll a bleaching enzyme purified from cotyledons of ethylene-treated radish ( Raphanus sativus ) seedlings. The enzyme catalyses the bleaching of chlorophyll a in the presence of H 2 O 2 and 2,4-DCP. SOD and scavengers of superoxide anions of a lower molecular weight, such as copper salicylate, NBT, Tiron, hydroquinone, and Mn 2+ , inhibited the enzymatic bleaching of chlorophyll a . The reaction was also inhibited by radical scavengers, such as n -propyl gallate and ascorbate. It is suggested that superoxide anions play important roles in the enzymatic chlorophyll a bleaching reactions.

Breakdown of Chlorophylls by Soluble Proteins Extracted from Leaves of Chenopodium album

Summary Chlorophylls were degraded by soluble proteins extracted from leaves of Chenopodium album . The bleaching (oxidative cleavage) of chlorophylls and accumulation of pheophorbides were observed. The accumulation of pheophorbides was not inhibited by anoxygenic conditions or by ascorbate, but the bleaching of chlorophylls was inhibited. It appears that there are two distinct degradation pathways: chlorophyll bleaching and pheophorbide accumulation in the process of chlorophyll breakdown. The bleaching reaction was not affected by hydrogen peroxide or hydrogen peroxide and 2,6-dichloroindophenol, but was slightly inhibited by linolenic acid. Catalase had no effect, but superoxide dismutase inhibited weakly. This bleaching was strongly inhibited by tiron and ascorbate. These results suggest that active oxygen, probably the superoxide radical, may be involved, but the enzymes catalase, peroxidase, and lipoxidase are not responsible for the bleaching reaction. The accumulation of pheophorbide derivatives is discussed in relation to the inhibition of bleaching activity.

Ethylene Formation from Ethyl Moiety of Ethionine

In plants, ethylene is formed in the presence of light and flavin mononucleotide from ethionine and S-ethylcysteine. The ethylene is formed from the ethyl moiety of ethionine.

Effect of ethylene on degreening of intact radish (Raphanus sativus L.) cotyledons

Abstract Ethylene enhanced the degreening of intact radish ( Raphanus sativus L.) cotyledons held in the dark for 36 h. Ethylene treatment caused a significant decrease in chlorophyll (Chl) content, and Chl b was degraded preferentially. No chlorophyllase (Chlase) activity was detected in the cotyledons but Chl a bleaching enzyme activity was detected; the enzyme required H 2 O 2 and 2,4-dichlorophenol (2,4-DCP). Activities of this enzyme and of guaiacol peroxidase were higher in ethylene-treated radish seedlings.