L- Galactono-gamma-lactone dehydrogenase: partial characterization, induction of activity and role in the synthesis of ascorbic acid in wounded white potato tuber tissue
1994
Oba, K. | Fukui, M. | Imai, Y. | Iriyama, S. | Nogami, K.
The total initial vitamin C content (23 mg%) of sliced white potato tuber tissue incubated at 20 degrees C 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 activity of L-galactono-gamma-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 Km values for L-galactono-gamma-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-galactono-gamma-lactone.
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