In plants, vitamin C (L-ascorbic acid : AsA) can accumulate to millimolar concentrations in both photosynthetic and nonphotosynthetic tissues. Three main types of biological activity can be defined for AsA : its function as an enzyme cofactor, as a radical scavenger, and as a donor/acceptor in electron transport either at the plasma membrane or in the chloroplasts. Even under optimal conditions, many metabolic processes, including the chloroplastic, mitochondrial, and plasma membrane-liked electron transport systems of higher plants, produce active oxygen species (AOS). Furthermore, the imposition of various environmental stresses can cause excess concentrations of AOS, resulting in oxidative damage at the cellular level. Therefore, antioxidants and antioxidantive enzymes function to interrupt the cascades of uncontrolled oxidation in each organelle. It is clear that AsA is a major primary antioxidant, reacting directly with AOS and a powerful secondary antioxidant, reducing the oxidized form of α-tocopherol. Plants and algae have evolved the AsA-glutathione cycle, consisting of ascorbate peroxidase (APX) and the enzymes related to the regeneration of AsA. APX isoenzymes are distributed in stromal APX and thylakoid membrane-bound APX in chloroplasts, microbody membrane-bound APX, and cytosolic APX and play important roles in the metabolism of H_2O_2 in plants. In this paper, the focus will be on recent molecular and physiological findings concerning APX isoenzymes in higher plants and algae. We also introduce a new biosynthetic pathway of AsA which proceeds via GDP-_D-mannnose and DGP-_L-galactose.

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The ODS(osteogenic disorder Shionogi) rat, which has a hereditary in ascorbic acid biosynthesis, was a useful model for investigating the physiological function of this vitamin. To find out a novel function of ascorbic acid in vivo, we looked for proteins whose levels were changed by ascorbic acid deficiency in serum and tissues of ODS rats. Male ODS rats (6-week old, body weight 〜140g) were fed a basal diet containing ascorbic acid (300mg/kg) or a diet without ascorbic acid for 14 days. As a result of screening, ascorbic acid deficiency significantly elevated the serm concentration of haptoglobin and significantly lowered those of apolipoprotein A-I and albumin. In addition, ascorbic acid deficiency significantly lowered the renal levle of kidney fatty acid-binding protein, which is synthesized as α2u-globulin in liver and thereafter located in kidney. The hepatic mRNA levels of haptoglobin and α1-acid glycoprotein (positive acute phase proteins) in the ascorbic acid-deficient rats were significantly elevated. On the contrary, the hepatic mRNA levels of apolipoprotein A-I, albumin and α2-globulin (negative acute phase proteins) in the ascorbic acid-deficient rats weresignificantly lowered. The serum concentration of interleukin 6, an inflammatory cytokine that stimulates gene expression of some positive acute phase proteins, was elevated in the ascorbic acid-deficient rats. These results demonstrate for the first time that ascorbic acid deficiency changes the acute phase protein gene expressions as seen in inflammatory response, and the mechanism of action of ascorbic acid in this phenomenon should be clarified in further studies.

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In 1989, we have developed a novel vitamin C derivative, 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G), which is highly stable to heat and oxidative stress. This stable form of ascorbate, of which hydroxy moiety at the C-2 position is regiospecifically substituted with glucose, was enzymatically synthesizes with mammalian intestinal and rice seed α-glucosidase, and cyclodextrin glucanotans-ferase from Bachillus stearothermophylus. In 1999, we have also reported a series of 6-acylated forms of AA-2G, 6-Acyl-AA-2Gs, of which hydroxy moiety at the C-6 position of AA-2G is substituted with fatty acids having a different length of carbon chains. Exposure both of AA-2G and 6-Acyl-AA-2G to the mammalian biomembrance results in the formation of active vitamin C by α-glucosidase and/or esterase. AA-2G has been admitted as a medicinal additive for whitening and is widely used in domestic and foreign countries for years. Our of the various acylated derivatives, 6-O-dodecanoyl-2-O-α-D-glucopyranosyl-L-ascorbic acid is considered to be the most expected candidate in application for medicinal use, for examples, preservation solution for the grafts, whitening ingredient, food additives and medicines, because it is the most prominent one in terms of the stability, effectiveness and skin permeability. The two types of ascorbate derivatives are very useful vitamin C source, especially in the tissue culture experiments and contribute the finding of unknown physiological action, and to the clarification of the mode of action of ascorbate. It has been found that vitamin C stimulates the cytokine generation and enhances the cytokine-induced biological activity in immune and neurological responses, although ascorbate alone is without effect. These findings suggest that AA-2G and 6-Acyl-AA-2G not only may serve as useful agents for the medicinal and cosmetic use, but also contributes to shed some light on a new aspect of the biological activity of ascorbate.

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[in Japanese]

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