Neuhaus H E, Schulte N
Fachbereich Biologie/Chemie, Universität Osnabrück, Germany.
Biochem J. 1996 Sep 15;318 ( Pt 3)(Pt 3):945-53. doi: 10.1042/bj3180945.
C3 or crassulacean acid metabolism (CAM)-induced Mesembryanthemum crystallinum plants perform nocturnal starch degradation which is linear with time. To analyse the composition of metabolites released by isolated leaf chloroplasts during starch degradation we developed a protocol for the purification of starch-containing plastids. Isolated chloroplasts from C3 or CAM-induced M. crystallinum plants are also able to degrade starch. With respect to the endogenous starch content of isolated plastids the rate of starch degradation in intact leaves. The combined presence of Pi, ATP, and oxaloacetate is identified to be the most positive effector combination to induce starch mobilization. The metabolic flux through the oxidative pentose-phosphate pathway in chloroplasts isolated from CAM-induced M. crystallinum is less than 3.5% compared with other metabolic routes of starch degradation. Here we report that starch-degrading chloroplasts isolated from CAM-induced M. crystallinum plants use exogenously supplied oxaloacetate for the synthesis of malate. The main products of starch degradation exported into the incubation medium by these chloroplasts are glucose 6-phosphate, 3-phosphoglyceric acid, dihydroxyacetone phosphate and glucose. The identification of glucose 6-phosphate as an important metabolite released during starch degradation is in contrast to the observations made on all other types of plastids analysed so far, including chloroplasts isolated from M. crystallinum in the C3 state. Therefore, we analysed the transport properties of isolated chloroplasts from M. crystallinum. Surprisingly, both types of chloroplasts, isolated from either C3 or CAM-induced plants, are able to transport glucose 6-phosphate in counter exchange with endogenous Pi, indicating the presence of a glucose 6-phosphate translocator as recently demonstrated to occur in other types of plastids. The composition of metabolites released and the stimulatory effect of oxaloacetate on the rate of starch degradation are discussed with respect to the acidification observed for CAM leaves during the night.
C3 或景天酸代谢（CAM）诱导的冰花植物在夜间进行淀粉降解，且降解过程与时间呈线性关系。为了分析淀粉降解过程中分离的叶片叶绿体释放的代谢物组成，我们开发了一种纯化含淀粉质体的方案。从 C3 或 CAM 诱导的冰花植物中分离的叶绿体也能够降解淀粉。就分离的质体内源淀粉含量而言，完整叶片中淀粉降解的速率。已确定 Pi、ATP 和草酰乙酸的共同存在是诱导淀粉动员的最有效效应物组合。与淀粉降解的其他代谢途径相比，从 CAM 诱导的冰花中分离的叶绿体中通过氧化戊糖磷酸途径的代谢通量小于 3.5%。在此我们报告，从 CAM 诱导的冰花植物中分离的淀粉降解叶绿体利用外源提供的草酰乙酸合成苹果酸。这些叶绿体输出到孵育培养基中的淀粉降解主要产物是 6-磷酸葡萄糖、3-磷酸甘油酸、磷酸二羟丙酮和葡萄糖。6-磷酸葡萄糖被鉴定为淀粉降解过程中释放的重要代谢物，这与迄今为止对所有其他类型质体（包括从处于 C3 状态的冰花中分离的叶绿体）的观察结果形成对比。因此，我们分析了冰花分离叶绿体的转运特性。令人惊讶的是，从 C3 或 CAM 诱导的植物中分离的两种类型的叶绿体都能够以与内源 Pi 反向交换的方式转运 6-磷酸葡萄糖，这表明存在一种 6-磷酸葡萄糖转运体，正如最近在其他类型质体中所证明的那样。结合夜间 CAM 叶片观察到的酸化现象，讨论了释放的代谢物组成以及草酰乙酸对淀粉降解速率的刺激作用。
