L-Galactono-y-Lactone Dehydrogenase : Partial Characterization, Induction of Activity and Role in the Synthesis of Ascorbic Acid in Wounded White Potato Tuber Tissue : ENVIRONMENTAL AND STRESS RESPONSES

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The total initial vitamin C content (23 mg%) of sliced white potato tuber tissue incubated at 20℃ decreased slightly during the first 12 h and increased thereafter by 80% during a 48-h incubation. The ascorbic acid (AsA) content (21 mg%) decreased slightly during the first 12 h and then increased by 40% within 36 h and slowly decreased thereafter. The level of dehydroascorbic acid (DHA) was low initially, being only 2 mg%; at 36 h it started to increase and reached 10 mg% at 48 h. The activty of L-galactono-γ-lactone dehydrogenase (EC 1.3.2.3, GLDHase) in a 500 x g supernatant from the white potato tuber increased 2.3-fold from 0 to 12 h and remained at the elevated level for the 48-h observation period. Similar results were obtained with two different assays of GLDHase activity. Thin-layer chromatography on silica gel of the 2,4-dinitrophenyl hydrazone of the reaction product showed that AsA-hydrazone could be detected only after the reaction had been allowed to proceed for 10 min and not at 0 min. GLDHase was localized in the mitochondria by sucrose density gradient centrifugation and could be partly solubilized by sonication. Maximum activity was observed at pH 7.9. Two apparent K_m values for L-galactono-γ-lactone were obtained for GLDHase, 0.08 and 0.23 mM. Substrate inhibition was observed at concentrations greater than 8.4 mM. The most effective substrate for the GLDHase was L-galac-tono-γ-lactone.
日本植物生理学会の論文