Synthesis and antioxidant properties of a new lipophilic ascorbic acid analogue

4-(4-Hydroxyphenyl)-5-(4-hydroxyphenylmethyl)-2-hydroxyfurane-2-one 1 was prepared by an acidic dimerisation of 4-hydroxyphenylpyruvic acid and some of its antioxidant and spectroscopic properties have been measured and compared to that of ascorbic acid. 1 is as good an antioxidant as ascorbic acid in the DPPH (2,2-diphenyl-1-picryl hydrazyl radical) test and the inhibition of hydroxyl radical and a powerful inhibitor of the Cu(2+) or AAPH (2,2'-azobis-(2-amidinopropane) dihydrochloride) induced oxidation of human LDL. 1 gives a stable radical characterised by its ESR spectrum similarly to ascorbic acid but in lower concentration and with a different reactivity towards nitroxides. Theoretical calculations allow us to propose the structure for the radical formed from 1, to explain its lower stability than ascorbyl radical and to evaluate the lipophilicity of 1.     

Interplay between ascorbic acid and lipophilic antioxidant defences in chloroplasts of water-stressed Arabidopsis plants

Nitric oxide (NO) is a bioactive molecule involved in diverse physiological functions in plants. Here we demonstrate that NO is capable of regulating the activity of photophosphorylation in chloroplasts. The electron transport activity in photosystem II determined from chlorophyll a fluorescence was inhibited by NO. NO also inhibited light-induced DeltapH formation across the thylakoid membrane. High concentrations of nitrite and nitrate did not show such inhibitory effects, suggesting that the inhibition is not due to uncoupling effects of the oxidized products of NO. ATP synthesis activity upon illumination was severely inhibited by NO (IC(50)=0.7 microM). The inhibition was found to be temporary and the activity was completely recovered by removing NO. Bovine hemoglobin and bicarbonate were effective in preventing NO-dependent inhibition of photophosphorylation. These results indicate that NO is a reversible inhibitor of photosynthetic ATP synthesis.     

Enhancement of antioxidant enzyme activities in rice callus by ascorbic acid under salinity stress

Ascorbic acid (AsA) is naturally occurring compound with antioxidant activity and plays a pivotal role in plant cell adaptation to salinity stress. The objective of this work was to assess the influence of exogenous AsA on the embryogenic callus of indica rice (Oryza sativa L.) cv. MRQ74 cultivated under saline conditions. NaCl (200 mM) decreased callus fresh and dry masses, relative growth rate, and K⁺ and Ca⁺² content, and increased Na⁺ content and Na⁺/K⁺ ratio. Application of AsA (0.5 or 1 mM) alleviated these effects of salinity. Activities of peroxidase, catalase, superoxide dismutase, as well as content of proline increased due to the NaCl treatment, and these parameters were mostly further increased by 0.5 mM AsA. Thus, AsA can increase callus tolerance to NaCl stress.     

Glutathione and ascorbic acid metabolism and antioxidant enzyme activity in the tissues of vitamin E-deficient rats

In has been shown in the experiments on male rats that alimentary vitamin E deficit causes the decrease of reduced glutathione and ascorbic acid concentration in the liver and lungs and that of glutathione-S-transferase, glutathione reductase in the liver and lungs, catalase in the liver and glutathione peroxidase in the heart activity, but increases the amount of glutathione disulfide in the liver and lungs and superoxide dismutase and gamma-glutamyltransferase activity in the liver. The data obtained show the selective character of reaction participants of the antioxidant system of rats' organism to the deficit of one of the antioxidant factors--vitamin E and also testify to complex interrelation between separate members of this system.     

Biologically significant scavenging of the myeloperoxidase-derived oxidant hypochlorous acid by ascorbic acid. Implications for antioxidant protection in the inflamed rheumatoid joint

Ascorbic acid, at physiological concentrations, can scavenge the myeloperoxidase-derived oxidant hypochlorous acid at rates sufficient to protect alpha 1-antiprotease against inactivation by this molecule. The rapid depletion of ascorbic acid at sites of inflammation, as in the inflamed rheumatoid joint, may therefore facilitate proteolytic damage.     

Effect of exogenous ascorbic acid intake on biosynthesis of ascorbic acid in mice

The effect of exogenous ascorbic acid intake on biosynthesis of ascorbic acid in mice has been studied. After the mice were on diets containing added ascorbic acid for two months, the activities of ascorbic acid synthesizing enzymes in the mouse liver homogenates were measured using L-gulono-gamma-lactone as a substrate. Exogenous ascorbic acid intake (0.5, 1 or 5% in the diet) was able to increase the concentration of ascorbic acid in the blood and to decrease the activities of ascorbic acid synthesizing enzymes in mouse liver. The results suggest that ascorbic acid synthesis was controlled by local regulatory mechanism or by the concentration of ascorbic acid in the hepatic portal blood. Ingestion of dietary erythorbic acid, a stereoisomer of ascorbic acid, had no effect on the activities of ascorbic acid synthesizing enzymes.     

The ascorbic acid paradox

Ascorbic acid (AA) is a common culture medium and dietary supplement. While AA is most commonly known for its antioxidant properties, it is also known to function as a pro-oxidant under select conditions. However, the complexity and often unknown composition of biological culture systems makes prediction of AA behaviour in supplemented cultures challenging. The frequent observation of outcomes inconsistent with antioxidant behaviour suggests that AA may be playing a pro-oxidant role more often than appreciated. In this work we explored the intracellular and extracellular impact of AA supplementation on KG1a myeloid leukaemia cells over a 24-h culture period following the addition of the AA supplement. At 24 h we found that supplementation of AA up to 250 μM resulted in intracellular antioxidant behaviour. However, when these same cultures were evaluated at 2 or 4 h we observed pro-oxidant activity at the higher AA concentrations indicating that the outcome was very much time and dose dependent. In contrast, pro-oxidant activity was never observed in the extracellular medium. Paradoxically, and to our knowledge not previously reported, we observed that intracellular pro-oxidant activity and extracellular antioxidant activity could occur simultaneously. These results indicate that the precise activity of AA supplementation varies as a function of dose, time and cellular location. Further, these results demonstrate how in the absence of careful culture characterization the true impact of AA on cultures could be underappreciated.     

Cellular functions of ascorbic acid

It has long been suspected that ascorbic acid is involved in many cellular reactions. This is evident from the multitude of seemingly unrelated symptoms seen in scurvy. However, until recently, our understanding of its involvement was confined to its role in the synthesis of collagen. Studies in the past few years have unveiled mechanisms of its actions in collagen formation and many other enzymatic reactions. In addition, numerous physiological responses are reportedly affected by ascorbic acid. From the well-characterized enzymatic reactions involving ascorbic acid, it has become clear that in animal cells the ascorbate does not seem to be directly involved in catalytic cycles. Rather its major function seems to keep prosthetic metal ions in their reduced form. The role of ascorbate as a reductant in these enzymatic reactions complements its other antioxidant functions which have been recently appreciated, including that as a scavenger of free radicals. Therefore, it seems that the major function of ascorbate is to protect tissues from harmful oxidative products and to keep certain enzymes in their required reduced forms. However, it remains unclear how the deficiency of ascorbate leads to the pathological symptoms found in scurvy.     

Genetic toxicology of ascorbic acid

The activation mechanism of emodin, a fungal anthraquinone and constituent of rhubarb, into a direct mutagen to Salmonella typhimurium TA1537 was investigated by using the S9 and microsomes of rat livers. Upon incubating emodin with the hepatic S9 derived from PCB-pretreated rats, this anthraquinone exhibited mutagenicity in the presence of NADPH or NADH, and this enzymatic activation, maximal at pH 7.0 and occurring in the microsomes, was induced by the pretreatment of rats with PCB, 3-methylcholanthrene or phenobarbital and was inhibited by α-naphthoflavone, SKF 525A and carbon monoxide. Thin-layer chromatographic analysis revealed that emodin was biotransformed by the microsomal enzymes into at least 5 quinonoid metabolites, among which one pigment, identified as 2-hydroxyemodin (1,2,3,8-tetrahydroxy-6-methyl-anthraquinone), was proved to be a direct mutagen to the test strain, and the remaining 4 quinonoid metabolites were negative or far less active than this active principle.