The Effect of Superoxide Radical on the ACC Synthase from Etiolated Mungbean Seedlings

Abstract:

The activity of ACC synthase(ACS, EC 4.4.1.14) from etiolated mungbean seedlings was elevatedobviously in treatment for 20 min with 0.5, 5 or 50 mmol/L of sodium dithionite (Na₂S₂O₄) which is generally used as an exogenous generator of superoxide radical (O₂^-·). Superoxide dismutase (SOD) or 1,4-diazabicycol (2,2,2) octane (DABCO), a special scavenger of O₂^-·, could reduce the increase of ACC synthase activity induced by Na₂S₂O₄, showing the promotion role of superoxide radical on the activity of ACC synthase. However，the activity of ACC synthase decreased as the treatm ent time extended．The activity of ACC synthase was even lower than the control when treated for 60 min with 5 or 50 mmol／L of Na₂S₂O₄．The decline of activity of ACC synthase could be inhibited by adding SOD or DABCO．These results showed the dual-efect of superoxide radical on the activity of ACC synthase．The further studies indicated that Krn of ACC synthase，which substrate is S-adenosylmethionine (SAM)，decreased in treatment for 20 min with exogenous generator of superoxide radica1．However，in the treatment for 60 min with exogenous O₂^-·，the value of Km of ACC synthase increased obviously．The results showed that superoxide radical could change the properties of the ACC synthase，i.e.，decreased its Km value，increased the affinity to SAM，and thus，to alter the activity of ACC synthase．This might be the mechanism of the dual effect of superoxide radical on the activity of ACC synthase.