Arrigoni O
Istituto di Botanica, University of Bari, Italy.
J Bioenerg Biomembr. 1994 Aug;26(4):407-19. doi: 10.1007/BF00762782.
By using lycorine, a specific inhibitor of ascorbate biosynthesis, it was possible to demonstrate that plant cells consume a high quantity of ascorbate (AA). The in vivo metabolic reactions utilizing ascorbate are the elimination of H2O2 by ascorbate peroxidase and the hydroxylation of proline residues present in the polypeptide chains by means of peptidyl-proline hydroxylase. Ascorbate acts in the cell metabolism as an electron donor, and consequently ascorbate free radical (AFR) is continuously produced. AFR can be reconverted to AA by means of AFR reductase or can undergo spontaneous disproportion, thus generating dehydroascorbic acid (DHA). During cell division and cell expansion ascorbate consumption is more or less the same; however, the AA/DHA ratio is 6-10 during cell division and 1-3 during cell expansion. This ratio depends essentially on the different AFR reductase activity in these cells. In meristematic cells AFR reductase is very high, and consequently a large amount of AFR is reduced to AA and a small amount of AFR undergoes disproportionation; in expanding cells the AFR reductase activity is lower, and therefore AFR is massively disproportionated, thus generating a large quantity of DHA. Since the transition from cell division to cell expansion is marked by a large drop of AFR reductase activity in the ER, it is suggested here that AFR formed in this compartment may be involved in the enlargement of the ER membranes and provacuole acidification. DHA is a toxic compound for the cell metabolism and as such the cell has various strategies to counteract its effects: (i) meristematic cells, having an elevated AFR reductase, prevent large DHA production, limiting the quantity of AFR undergoing disproportionation (ii) Expanding cells, which contain a lower AFR reductase, are, however, provided with a developed vacuolar system and segregate the toxic DHA in the vacuole. (iii) Chloroplast strategy against DHA toxicity is efficient DHA reduction to AA using GSH as electron donor. This strategy is usually poorly utilized by the surrounding cytoplasm. DHA reduction does play an important role at one point in the life of the plant, that is, during the early stage of seed germination. The dry seed does not store ascorbate, but contains DHA, and several DHA-reducing proteins are detectable. In this condition, DHA reduction is necessary to form a limited AA pool in the seed for the metabolic requirements of the beginning of germination. After 30-40 h ascorbate ex novo synthesis starts, DHA reduction declines until a single isoform remains, as is typical in the roots, stem, and leaves of seedlings.(ABSTRACT TRUNCATED AT 400 WORDS)
通过使用抗坏血酸生物合成的特异性抑制剂石蒜碱，得以证明植物细胞会消耗大量抗坏血酸（AA）。利用抗坏血酸的体内代谢反应包括通过抗坏血酸过氧化物酶消除过氧化氢，以及借助肽基脯氨酸羟化酶对多肽链中存在的脯氨酸残基进行羟化。抗坏血酸在细胞代谢中作为电子供体起作用，因此会持续产生抗坏血酸自由基（AFR）。AFR可通过AFR还原酶重新转化为AA，或者发生自发歧化反应，从而生成脱氢抗坏血酸（DHA）。在细胞分裂和细胞扩张过程中，抗坏血酸的消耗量大致相同；然而，细胞分裂期间AA/DHA的比例为6 - 10，细胞扩张期间为1 - 3。该比例主要取决于这些细胞中不同的AFR还原酶活性。在分生组织细胞中，AFR还原酶活性非常高，因此大量AFR被还原为AA，少量AFR发生歧化反应；在正在扩张的细胞中，AFR还原酶活性较低，因此AFR大量歧化，从而产生大量DHA。由于从细胞分裂到细胞扩张的转变以内质网中AFR还原酶活性的大幅下降为标志，因此本文认为在内质网中形成的AFR可能参与内质网膜的扩大和前液泡的酸化。DHA是细胞代谢中的有毒化合物，因此细胞有多种策略来抵消其影响：（i）分生组织细胞具有较高的AFR还原酶，可防止大量DHA产生，限制发生歧化反应的AFR的量；（ii）然而，含有较低AFR还原酶的正在扩张的细胞具有发达的液泡系统，并将有毒的DHA隔离在液泡中。（iii）叶绿体对抗DHA毒性的策略是利用谷胱甘肽作为电子供体将DHA高效还原为AA。这种策略通常很少被周围的细胞质利用。DHA还原在植物生命的一个阶段即种子萌发早期确实起着重要作用。干燥的种子不储存抗坏血酸，但含有DHA，并且可以检测到几种DHA还原蛋白。在这种情况下，DHA还原对于在种子中形成有限的AA库以满足萌发开始时的代谢需求是必要的。30 - 40小时后，抗坏血酸从头合成开始，DHA还原下降，直到只剩下一种异构体，这在幼苗的根、茎和叶中是典型的。（摘要截断于400字）
