Ascorbic acid phosphorylation

A remarkably selective phosphorylation at HO-3 in L-ascorbic acid and its 5,6-O-isopropylidene derivative has been achieved with phosphoryl chloride in aqueous pyridine204 the use of acetone-pyridine as the solvent led to a much less selective reaction. 

Because the preceding chromogenic assay rely on choline quantitation, the hydrolysis of substrates with headgroups other than choline cannot be followed. To circumvent this problem, another useful protocol was devised whereby the phosphorylated headgroup produced by the PLCBc hydrolysis is treated with APase, and the inorganic phosphate (Pi) that is thus generated is quantitated by the formation of a blue complex with ammonium molybdate/ascorbic acid 5 nmol of phosphate may be easily detected. This assay, which may also be performed in a 96-well format, has been utilized to determine the kinetic parameters for the hydrolysis of a number of substrates by PLCBc [37,38]. 

It is not clear whether V(V) or V(IV) (or both) is the active insulin-mimetic redox state of vanadium. In the body, endogenous reducing agents such as glutathione and ascorbic acid may inhibit the oxidation of V(IV). The mechanism of action of insulin mimetics is unclear. Insulin receptors are membrane-spanning tyrosine-specific protein kinases activated by insulin on the extracellular side to catalyze intracellular protein tyrosine phosphorylation. Vanadates can act as phosphate analogs, and there is evidence for potent inhibition of phosphotyrosine phosphatases (526). Peroxovanadate complexes, for example, can induce autophosphorylation at tyrosine residues and inhibit the insulin-receptor-associated phosphotyrosine phosphatase, and these in turn activate insulin-receptor kinase. 

The Mechanism of Oxidative Phosphorylation 1066 Box 18-D Vitamin C Ascorbic Acid... 

Ascorbic acid, thiamine, riboflavin, and vitamin B12 requirements increase in hyperthyroidism (issue concentrations reduced Vitamin A massive doses of vitamin A inhibit secretion of TSH thyroid hormones required for carotene and retimene conversions Vitamins A, D, E. and K requirements increased in hyperthyroidism tissue concentrations reduced in Vitamin B, . niacin conversion to phosphorylated reactive forms impaired in hyperthyroidism... 

Selective phosphorylation of 5,6-O-isopropylidene D-erythorbic acid (D-erpt/zro-hex-2-enono- 1,4-lactone), the 5-epimer of L-ascorbic acid, was performed with phosphoryl chloride at high pH in the presence of pyridine. D-Erythorbate 2-phosphate (155) is obtained as the crystalline magnesium or cyclohexylammonium salts following removal of the 5,6-acetal.351... 

The phosphorylation of ascorbic acid under basic conditions has been studied extensively. The synthesis of ascorbic acid 2-phosphate, 22, has been reported by Seib et al. (9) via treatment of 5,6-O-isopropylidene ascorbic acid, with phosphorus oxychloride under highly basic conditions (pH 12). Hydrolysis of the ketal protecting group of the 2-phosphate intermediate 20 affords the 2-O-phosphate 22, whose structure has been... 

Ascorbic acid will reduce cytochrome c, via the dye tetramethyl-/>-phenylenediamine (TMPD) hence it misses out on the first two phosphorylation sites and therefore has a P 0 ratio of unity. 

The structure of the phosphate salt of ascorbic acid obtained by direct phosphorylation has been established by independent synthesis. ... 

It is hoped that further work on the plant mitochondria preparations studied by Young and Conn (25) may help to clarify the possible role of GSH in plant respiration. Preliminary experiments have shoAvn that the mitochondria from avocados catalyze oxidation of ascorbic acid by O2, and that GSH is also oxidized by O2 when a trace of ascorbic acid is added to the particles. Some evidence has also been obtained that the particles catalyze a reduction of dehydroascorbic acid by citrate in the presence of TPN and GSH. Further work on these systems is in progress. Experiments are also in progress to determine whether oxidative phosphorylations occur during reactions of the type described. 

Many attempts have been made to demonstrate phosphorylation coupled to oxidation of reduced cytochrome c. Judah has presented evidence which suggests that phosphorylation occurs when cytochrome c is reduced in situ by a non-enzymatic reaction with ascorbic acid, and then subsequently oxidized enzymatically. Slater, on the other hand, has reported experiments which he interprets as meaning that phosphorylation does not occur when cytochrome c is oxidized. Slater s experiments seem to be open to some criticism on technical grounds. In any event, it is difficult to explain the P/0 ratio of 2.0 which has been observed in the oxidation of succinate unless one assumes that phosphorylation occurs coupled to oxidations above the level of cytochrome c. The potential span between succinate and cytochrome c is only about 0.3 v., which is not sufficient to generate the 24,000 cal. needed to bring about the formation of 2 moles of ADP to ATP. 

Enzymic transglycosylation, with maltose as glucosyl donor, can be used to make 2-0-a-D-glucopyranosyl-L-ascorbic acid, which may have potential as a storage form of ascorbate in commercial formulations. 3,74 Microorganisms have also been identified which can phosphorylate ascorbic acid specifically at 0-2P ... 

Certain natural compounds possess anti-apoptotic effects. For example, ascorbic acid and a-tocopherol prevent apoptosis caused by serum withdrawal in HL-60 cells with the antioxidative effects (76). Caffeic acid inhibits ceramide-induced apoptosis in U937 cells through the inhibition of protein tyrosine kinase activity 17). Quercetin inhibits hydrogen peroxide-induced apoptosis via intervention in the activator protein 1 (AP-1) -mediated apoptotic pathway 18). Epigallocatechin-3-gallate and theaflavins inhibits arsenite-induced apoptosis through the decrease in phosphorylation of Erks and JNKs (79). Therefore, it is important to know whether these natural compounds show inhibitory effects on Trp-P-1-induced apoptosis. In this study, we demonstrated that Trp-P-1 induced caspase-8-initiated apoptosis in rat MNCs, and that certain food components inhibited Trp-P-l-induced apoptosis. 

The product of phosphorylation of 5,6-protected ascorbic acid has been shown to be the 2- -phosphate by X ray analysis of its dipiperazinium salt dihydrate.Four 2-acylamino-2-deoxy-1,3 4 tri- -dodecanoyl- -D-glucopyranose 6-phosphate analogues (25) of the repeating unit of the glycophospholipid "Lipid A" have been synthesized. An alternative route has been described for the... 

Experiments carried out by Arnon (11,12,15,395) and his collaborators demonstrated that chloroplasts or suitably prepared chloroplast fragments carry out ATP formation in the light. This phosphorylation requires ascorbic acid, flavin mononucleotide, and menadione (vitamin Kx) as cufactors and proceeds without the evolution or consumption of oxygen. A role of vitamin K in photosynthesis... 

Nothing is known with certainty about metabolic pathways for the degradation of ascorbic acid or of its use in the plant. Although some biologists consider that it is a catalyst in photosynthetic phosphorylation, this is by no means certain, and until more positive evidence is advanced it is safer to consider that this acid fulfils some as yet imperfectly known metabolic function in the active plant cell. 

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