Wheeler G L, Jones M A, Smirnoff N
School of Biological Sciences, University of Exeter, Hatherly Laboratories, UK.
Nature. 1998 May 28;393(6683):365-9. doi: 10.1038/30728.
Vitamin C (L-ascorbic acid) has important antioxidant and metabolic functions in both plants and animals, but humans, and a few other animal species, have lost the capacity to synthesize it. Plant-derived ascorbate is thus the major source of vitamin C in the human diet. Although the biosynthetic pathway of L-ascorbic acid in animals is well understood, the plant pathway has remained unknown-one of the few primary plant metabolic pathways for which this is the case. L-ascorbate is abundant in plants (found at concentrations of 1-5 mM in leaves and 25 mM in chloroplasts) and may have roles in photosynthesis and transmembrane electron transport. We found that D-mannose and L-galactose are efficient precursors for ascorbate synthesis and are interconverted by GDP-D-mannose-3,5-epimerase. We have identified an enzyme in pea and Arabidopsis thaliana, L-galactose dehydrogenase, that catalyses oxidation of L-galactose to L-galactono-1,4-lactone. We propose an ascorbate biosynthesis pathway involving GDP-D-mannose, GDP-L-galactose, L-galactose and L-galactono-1,4-lactone, and have synthesized ascorbate from GDP-D-mannose by way of these intermediates in vitro. The definition of this biosynthetic pathway should allow engineering of plants for increased ascorbate production, thus increasing their nutritional value and stress tolerance.
维生素C（L-抗坏血酸）在植物和动物中都具有重要的抗氧化和代谢功能，但人类以及其他一些动物物种已经失去了合成它的能力。因此，植物来源的抗坏血酸盐是人类饮食中维生素C的主要来源。尽管动物体内L-抗坏血酸的生物合成途径已为人熟知，但植物的合成途径仍然未知——这是少数几种情况如此的主要植物代谢途径之一。L-抗坏血酸盐在植物中含量丰富（在叶片中的浓度为1-5 mM，在叶绿体中为25 mM），可能在光合作用和跨膜电子传递中发挥作用。我们发现D-甘露糖和L-半乳糖是抗坏血酸合成的有效前体，并由GDP-D-甘露糖-3,5-表异构酶相互转化。我们在豌豆和拟南芥中鉴定出一种酶，L-半乳糖脱氢酶，它催化L-半乳糖氧化为L-半乳糖-1,4-内酯。我们提出了一条涉及GDP-D-甘露糖、GDP-L-半乳糖、L-半乳糖和L-半乳糖-1,4-内酯的抗坏血酸生物合成途径，并在体外通过这些中间体从GDP-D-甘露糖合成了抗坏血酸。这条生物合成途径的确定应该能够通过基因工程提高植物的抗坏血酸产量，从而提高其营养价值和抗逆性。
