Department of Biology, Memorial University of Newfoundland, St. John's, NL, Canada A1B 3X9.
Morden Research and Development Centre, Agriculture and Agri-Food Canada, Morden, MB, Canada R6M 1Y5.
Free Radic Biol Med. 2018 Jul;122:74-85. doi: 10.1016/j.freeradbiomed.2018.01.016. Epub 2018 Jan 31.
Organic acids play a crucial role in numerous metabolic processes accompanied by transfer of electrons and protons and linked to the reduction/oxidation of major redox couples in plant cells, such as NAD, NADP, glutathione, and ascorbate. Fluxes through the pathways metabolizing organic acids modulate redox states in cell compartments, contribute to generation of reactive oxygen and nitrogen species, and mediate signal transduction processes. Organic acid metabolism not only functions to equilibrate the redox potential in plant cells but also to transfer redox equivalents between cell compartments supporting various metabolic processes. The most important role in this transfer belongs to different forms of malate dehydrogenase interconverting malate and oxaloacetate or forming pyruvate (malic enzymes). During photosynthesis malate serves as a major form of transfer of redox equivalents from chloroplasts to the cytosol and other compartments via the malate valve. On the other hand, mitochondria, via alterations of their redox potential, become a source of citrate that can be transported to the cytosol and support biosynthesis of amino acids. Citrate is also an important retrograde signalling compound that regulates transcription of several genes including those encoding the alternative oxidase. The alternative oxidase, which is activated by increased redox potential and by pyruvate, is, in turn, important for the maintenance of redox potential in mitochondria. The roles of organic acids in establishing redox equilibrium, supporting ionic gradients on membranes, acidification of the extracellular medium, and regulation of production of reactive oxygen and nitrogen species are discussed.
有机酸在伴随电子和质子转移的许多代谢过程中发挥着关键作用，并且与植物细胞中主要氧化还原对（如 NAD、NADP、谷胱甘肽和抗坏血酸）的还原/氧化有关。代谢有机酸的途径通量调节细胞区室中的氧化还原状态，有助于产生活性氧和氮物种，并介导信号转导过程。有机酸代谢不仅有助于平衡植物细胞中的氧化还原电位，而且有助于在支持各种代谢过程的细胞区室之间转移氧化还原当量。在这种转移中最重要的作用属于不同形式的苹果酸脱氢酶，它们可以将苹果酸和草酰乙酸或丙酮酸（苹果酸酶）相互转化。在光合作用中，苹果酸作为从叶绿体到细胞质和其他区室的主要形式的氧化还原当量转移，通过苹果酸门控来实现。另一方面，线粒体通过改变其氧化还原电位，成为可以运输到细胞质并支持氨基酸生物合成的柠檬酸源。柠檬酸也是一种重要的逆行信号化合物，可调节包括编码交替氧化酶的基因的转录。交替氧化酶被增加的氧化还原电位和丙酮酸激活，反过来对于维持线粒体中的氧化还原电位也很重要。有机酸在建立氧化还原平衡、支持膜上的离子梯度、细胞外介质酸化以及调节活性氧和氮物种的产生方面的作用将进行讨论。
