超氧阴离子自由基对绿豆黄化幼苗ACC合酶的影响

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

摘要:

以0.5、5和50 mmol/L的连二亚硫酸钠(Na₂S₂O₄)为外源超氧阴离子自由基(O₂^-·)源，处理20 min能明显提高绿豆黄化幼苗ACC合酶(ACC synthase, ACS, EC 4.4.1.14)的活性。超氧阴离子自由基的特异性清除剂超氧化物歧化酶(superoxide dismutase, SOD)和1,4-二氮杂二环(2,2,2)辛烷[1,4-diazabicyclo(2,2,2) Octane, DABCO]能抑制Na₂S₂O₄的这种作用,显示Na₂S₂O₄产生的O₂^-·引起了ACC合酶活性的升高。但Na₂S₂O₄处理较长时间,ACC合酶活性开始下降,处理60 min的ACC合酶活性明显低于对照,SOD和DABCO的加入有助于抑制ACC合酶活性的降低,表明超氧阴离子自由基对ACC合酶活性具有促进和抑制作用并存的“双重性”影响。研究表明,外源Na₂S₂O₄处理20 min能明显降低以S-腺苷蛋氨酸(S-adenosylmethionine, SAM)为底物的ACC合酶的Km值,处理60 min反而提高了ACC合酶的Km值,表明O₂^-·是通过改变ACC合酶对底物SAM的亲和力,从而影响其活性的。

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.