L-GALACTONO-GAMMA-LACTONE DEHYDROGENASE - PARTIAL CHARACTERIZATION, INDUCTION OF ACTIVITY AND ROLE IN THE SYNTHESIS OF ASCORBIC-ACID IN WOUNDED WHITE POTATO-TUBER TISSUE

The total initial vitamin C content (23 mg%) of sliced white potato tu ber tissue incubated at 20 degrees C decreased slightly during the fir st 12 h and increased thereafter by 80% during a 48-h incubation. The ascorbic acid (AsA) content (21 mg%) decreased slightly during the fir st 12 h and then increased by 40% within 36 h and slowly decreased the reafter. The level of dehydroascorbic acid (DHA) was low initially, be ing only 2 mg%; at 36 h it started to increase and reached 10 mg% at 4 8 h. The activty of L-galactono-gamma-lactone dehydrogenase (EC 1.3.2. 3, GLDHase) in a 500 x g supernatant from the white potato tuber incre ased 2.3-fold from 0 to 12 h and remained at the elevated level for th e 48-h observation period. Similar results were obtained with two diff erent 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 cou ld be partly solubilized by sonication. Maximum activity was observed at pH 7.9. Two apparent K-m values for L-galactono-gamma-lactone were obtained for GLDHase, 0.08 and 0.23 mM. Substrate inhibition was obser ved at concentrations greater than 8.4 mM. The most effective substrat e for the GLDHase was L-galactono-gamma-lactone.