Hatch M D, Agostino A, Burnell J N
Division of Plant Industry, CSIRO, Canberra City ACT, Australia.
Arch Biochem Biophys. 1988 Mar;261(2):357-67. doi: 10.1016/0003-9861(88)90351-7.
Mitochondria from bundle sheath cells of the phosphoenolpyruvate carboxykinase-type C4 species Urochloa panicoides were shown to have metabolic properties consistent with a role in C4 photosynthesis predicted from earlier studies. The rate of O2 uptake in response to added malate plus ADP was at least five times the activity observed with NADH, glycine, or succinate. With malate plus ADP the O2 uptake rate averaged about 150 nmol O2 min-1 mg-1 protein, equivalent to about 0.6 mumol min-1 mg-1 of extracted chlorophyll. About half of this activity was apparently phosphorylation-linked with ADP/O2 ratios of about 4. Studies with electron transport inhibitors suggested that about 65% of this malate oxidation is cytochrome oxidase-terminated with a minor component mediated via the alternative oxidase. These mitochondria supported rapid rates of pyruvate production from malate and this activity was also stimulated by ADP but blocked by inhibitors of electron transport. Adding oxaloacetate increased pyruvate production but inhibited O2 uptake. The results were consistent with the notion that in this subgroup of C4 species mitochondrial-located NAD malic enzyme contributes substantially to total C4 acid decarboxylation. This enzyme is apparently also the primary source of NADH necessary to generate the ATP required for phosphoenolpyruvate carboxykinase-mediated oxaloacetate decarboxylation.
研究表明，来自磷酸烯醇式丙酮酸羧激酶型C4植物类黍尾草叶肉细胞的线粒体具有的代谢特性，与早期研究预测的在C4光合作用中的作用一致。添加苹果酸和ADP后，氧气摄取速率至少是添加NADH、甘氨酸或琥珀酸时所观察到活性的五倍。添加苹果酸和ADP时，氧气摄取速率平均约为150 nmol O2 min-1 mg-1蛋白质，相当于约0.6 μmol min-1 mg-1提取叶绿素。该活性的大约一半显然与磷酸化有关，ADP/O2比率约为4。用电子传递抑制剂进行的研究表明，这种苹果酸氧化的约65%由细胞色素氧化酶终止，一小部分由交替氧化酶介导。这些线粒体支持从苹果酸快速产生丙酮酸，并且该活性也受到ADP的刺激，但被电子传递抑制剂阻断。添加草酰乙酸增加了丙酮酸的产生，但抑制了氧气摄取。结果与以下观点一致：在这个C4植物亚组中，位于线粒体的NAD苹果酸酶对总C4酸脱羧有很大贡献。该酶显然也是磷酸烯醇式丙酮酸羧激酶介导草酰乙酸脱羧所需ATP生成所需NADH的主要来源。
