Ca^(2+)对叶绿体中超氧物自由基产生以及由ACC形成乙烯的影响

Effects of Calcium on the Generation of Superoxide Free Radical and the Conversion of ACC to Ethylene by Wheat Chloroplasts

高浓度Ca^(2+)(0.1 mol/L)使叶绿体产生O_2^+的能力下降,旋转相关时间(τ_c)增大34.3％,即膜的流动性降低,并抑制ACC形成乙烯;衰老时细胞内的Ca^(2+)作用却与此相反;O_2^+的生成与乙烯的产量成正相关(r=0.941)。EGTA,吩噻嗪和W_7等加入到叶绿体反应体系中,可使O_2^+的产量下降,ACC形成乙烯减少;相反,亚油酸作为Ca^(2+)载体,却使之明显升高,但亚油酸本身产生乙烯的量比ACC少得多。因此推测:高浓度Ca^(2+)可能影响叶绿体膜的状态,从而影响EFE的构象或者减少O_2^+的生成,抑制ACC的转化,衰老时细胞内的Ca^(2+)可启动钙信使系统,使O_2^+的产量升高,而其中膜脂过氧化是衰老的中心环节,因此O_2^+的升高可能是诱发衰老启动的重要因素。

The effects of calcium in the propensity to generate superoxide radical and to form ethylene from ACC were investigated with electron-paramagnetic-resonance spectroscopy. (1). The rate of ethylene formation from ACC, superoxide radical production and membrane fluidity were decreased by treatment with exogenous Ca^(2+) (10^(-1) mol /L), but during senescence the opposite effects were observed by endogenous Ca^(2+) of chloroplasts, isolated from wheat seedlings irrigated with Ca^(2+) (10^(-1) mol/L). (2). The effects of endogenous Ca^(2+) concentration in increasing O_^- and ethylene production were inhibited by CaM antagonists, such as phenothiazine (50 μmol / L) and W_7 (50 μmol / L), and promoted by exogenous linoleic acid, which was considered an endogenous Ca^(2+) ionophore. Linoleic acid was probably the precursor of ethylene, but much less ethylene was produced from it than from ACC. O_2^- levels was positively correlated to ethylene production regulated by endogenous Ca^(2+)(r=0.941). These results implied that exogenous Ca^(2+) may affect membrane state, and hence change the configuration of ethylene forming enzyme or decrease O_2^- production and inhibit the conversion of ACC to ethylene. These also suggested that during senescence internalized Ca^(2+) may induce the increase of superoxide radical production, and then enhance the ethylene formation from its precursor. According to above findings, lipid peroxidation caused by O_2^- and other activated oxygen play a key role in triggering senescence in chloroplasts.