Photostability and interaction of ascorbic acid in cream formulations.

The kinetics of photolysis of ascorbic acid in cream formulations on UV irradiation has been studied using a specific spectrophotometric method with a reproducibility of ± 5%. The apparent first-order rate constants (k(obs)) for the photolysis of ascorbic acid in creams have been determined. The photoproducts formed in the cream formulations include dehydroascorbic acid and 2,3-diketogulonic acid. The photolysis of ascorbic acid appears to be affected by the concentration of active ingredient, pH, and viscosity of the medium and formulation characteristics. The study indicates that the ionized state and redox potentials of ascorbic acid are important factors in the photostability of the vitamin in cream formulations. The viscosity of the humectant present in the creams appears to influence the photostability of ascorbic acid. The results show that the physical stability of the creams is an important factor in the stabilization of the vitamin. In the cream formulations stored in the dark, ascorbic acid undergoes aerobic oxidation and the degradation is affected by similar factors as indicated in the photolysis reactions. The rate of oxidative degradation in the dark is about seventy times slower than that observed in the presence of light.

Light-induced byproducts of vitamin C in multivitamin solutions.

BACKGROUND: When solutions of multivitamin preparations (MVPs) are exposed to light, H(2)O(2) as well as organic peroxides are generated and the concentration of vitamin C decreases. The aim of this study was to determine, using mass spectrometry, whether the generation of oxidative byproducts of vitamin C, such as dehydroascorbate (DHA) and 2,3-diketogulonic acid (DKG), accounted for the reported decrease in ascorbic acid in MVPs exposed to light. METHODS: Mass spectrometry was used to document the formation of byproducts of ascorbic acid in solutions containing a MVP, vitamin C + riboflavin, and vitamin C + H(2)O(2) + Fe(2+). The involvement of ascorbic acid and H(2)O(2) in the formation of organic peroxides was tested by measuring peroxide concentrations in solutions containing H(2)O(2) with or without ascorbic acid and with or without Fe(2+) before and after addition of catalase. RESULTS: The loss of ascorbic acid in photo-exposed MVPs was associated with the concomitant generation of byproducts different from DHA and DKG. Among them, one mass fingerprint was particularly observed with solutions of vitamin C + riboflavin exposed to ambient light as well as with the solution of vitamin C + H(2)O(2) + Fe(2+), suggesting a Fenton-like reaction. This fingerprint was associated with the formation of catalase-resistant peroxides. CONCLUSION: Exposure of MVPs to light leads to the rapid loss of ascorbic acid and generation of specific byproducts that differ from DHA and DKG. The conversion of vitamin C into byproducts could be of biological importance in accounting for the decrease in ascorbic acid concentrations and the generation of organic peroxides in light-exposed MVPs.

Analyses of vitamin C in biological samples with an emphasis on recent chromatographic techniques.

AA analyses are rife with problems - but primarily stability of the sample AA and specificity are the most prevalent shortcomings of the assays reviewed. Should one desire to quantify AA alone with no consideration for DHAA or DKG, chromatographic separations such as those described by Nahrwold (1981) with reductants added to samples and standards using UV detection, or Iwata et al. (1985) with fluorescence or Tsao and Salimi (1982) with EC detection would probably be appropriate depending upon the equipment available. The initial preparation of sample must be tested to ensure that no spontaneous oxidation of AA occurs during the sample preparation. The main advantage of these chromatographic assays is simply that one is measuring AA directly. On the other hand, manual assays such as that proposed by Zannoni et al. (1974) or Samyn (1983) have apparently demonstrated sufficient specificity and sensitivity to be used as well. When other vitamin C compounds need to be quantified, chromatographic assays should be considered but the selection is largely dependant on the sample size (and thus sensitivity required), the possible interfering compounds, and the detector available. Ideally HPLC should ensure the specificity by resolving the compounds of interest from artifacts. However, the optical characteristics of DHAA and DKG which are quite different from that of AA and detection limitations have fostered a number of complicated manipulations to quantify the former.

Ascorbate transport by AtT20 mouse pituitary corticotropic tumor cells: uptake and secretion studies.

Ascorbate is an important cofactor in the biosynthesis of alpha-amidated endocrine and neural peptides. Peptidylglycine alpha-amidating monooxygenase (PAM) is the enzyme responsible for the generation of mature COOH-terminal alpha-amidated peptides from COOH-terminal glycine-extended peptides, and this enzyme requires ascorbate for full activity in vitro. Also, cultured intermediate pituitary lobe cells contain PAM and require ascorbate for the COOH-terminal alpha-amidation of alpha MSH. Since pituitary cells are not capable of synthesizing ascorbate, the ability of the cells to accumulate the cofactor must play an important role in the biosynthesis of alpha-amidated peptides. The AtT20 corticotropic pituitary tumor cell line also contains PAM and a potential site for COOH-terminal alpha-amidation of the pro-ACTH/endorphin-derived hinge peptide and was, thus, used for the study of cellular ascorbate transport. Radiolabeled L-[1-14C]ascorbate ([1-14C]ascorbate) was incubated with the cells under various conditions, and the accumulation of radioactivity by the cells was followed. Reverse phase HPLC was used to identify the integrity of the labeled ascorbate, both intra- and extracellular, during the course of the experiments. The uptake of [1-14C]ascorbate was saturable (Km = 31.5 microM), sodium and temperature dependent, and stereoselective. The products of ascorbate autooxidation, dehydroascorbate and 2,3-diketogulonic acid, did not inhibit [1-14C]ascorbate uptake. To study the presence of ascorbate in the secretory granules, cells were incubated with [1-14C]ascorbate and then induced to secrete with isoproterenol or 8-bromo-cAMP. A 2- to 6-fold stimulation of ACTH secretion over the basal secretion rate was observed; however, the secretion of intracellular [1-14C]ascorbate did not change significantly with stimulation, suggesting that very little of the cellular ascorbate was contained within secretory granules.

Changes in ascorbic acid content and acetylcholinesterase activity in the muscle of frog following sciatectomy.

Unilateral-sciatectomy for three months in the frog, Rana cyanophlictis resulted in a substantial increase on unit weight basis in the ascorbic acid (ASA) and dehydroascorbic acid (DHA) contents of the sciatectomized gastrocnemius muscle. Diketogulonic acid (DKA) levels did not vary. On whole muscle-weight basis only the ASA level increased. The AChE activity in sciatectomized muscle is significantly lower than that of the control. Partially purified preparation of the AChE from the sciatectomized muscle showed different kinetics compared to that from innervated control. In vitro additions of ASA in physiological concentration to the enzyme assay medium inhibited the AChE activity significantly and the inhibition was an un-competitive type. Reduced activity of the enzyme has been correlated to the increased concentration of ASA in the sciatectomized muscle.

[Relation between ascorbic acid metabolism and the body' supply of vitamin K]. / Zavisimost' obmena askorbinovoi kisloty of obespechennosti organizma vitaminom K.

Experiments on rats were made to study the content of ascorbic (AA), Dehydroascorbic (DAA) and diketogulonic (DKGA) acids in the blood serum, daily urine, liver, adrenal, kidney, spleen and lung tissues under varying body supply with vitamin K. The rats with vitamin K deficiency manifested the decreased content of AA, DAA and the elevated content of DKGA in the blood serum, daily urine and test tissues. The percentage of AA absorption by blood proteins was found to be increased upon vitamin K deficiency in the body. The changes in AA metabolism correlated with the reduced capillary resistance. Administration of AA to rats with vitamin K deficiency led to an increase in the content of AA, DAA and to a lowering of DKGA in the blood serum and tissues promoting the normalization of capillary resistance. The data obtained attest to the dependence of AA metabolism on the body supply with vitamin K, pointing to the necessity of exercising the control over AA metabolism in different forms of vitamin K deficiency.