PREFACE
Downloaded by 147.158.188.2 on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.pr001
THE BRILLIANT SUCCESS OF ASCORBIC ACID RESEARCH in the
1930s led
to
the commercial production of inexpensive ascorbic acid in large quanti ties. The wide distribution of ascorbic acid, and its incorporation into many food products, so completely solved the problem of scurvy in both general and special populations that pressure for a more complete scientific understanding of this vitamin was sharply reduced. As a result many questions concerning ascorbic acid's chemistry, biochemistry, physiological roles and kinetics, and its nutritional requirements were deferred for more pressing scientific problems. In spite of a relaxed position for ascorbic acid studies, our technical knowledge regarding this vitamin has greatly increased during the second half of the 20th century. Much remains to be done. The results described in this book will contribute to that development by summarizing current work in many areas. The metabolism of ascorbic acid has yet to be clarified. Only one of the major water-soluble urinary metabolites of ascorbic acid has been isolated and characterized; the others and the matabolic pathways in volved are yet to be defined. When these pathways and their inter mediates are understood, there will be good possibilities for significant nutritional and medical innovation. The physiological roles of ascorbic acid have not yet been described in a manner that is scientifically satisfactory. The presence of ascorbic acid in all eucaryote organisms suggests fundamental roles that are not understood. The absence of ascorbic acid in procaryote organisms sug gests an unknown fundamental difference between these two classes of organisms in which ascorbic acid plays an essential role. Perhaps more than any other nutritional factor, ascorbic acid has been the focus of the questions, "How much ascorbic acid is required in humans for optimum health and well-being? and "What factors can change this requirement?" There seem to be no simple answers to these questions, but good progress has been made in fundamentals related to this problem, such as measurement of pool sizes and turnover in humans as a function of environmental variables. The lack of such physiological data for children, women, and pregnant women needs to be corrected. ,,
Ascorbic acid has been implicated in a great variety of biological phenomena, such as the immune reactions, the cytochrome P450 system, ix Seib and Tolbert; Ascorbic Acid: Chemistry, Metabolism, and Uses Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Downloaded by 147.158.188.2 on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.pr001
cell division, neurological function, atherosclerosis, and free radical reactions in biological fluids. Unfortunately none of these different proposals have led to well defined and accepted uses of ascorbic acid. Even the demonstrated requirement for ascorbic acid in the formation of connective tissues is not sufficient to mandate post-operational supple mentation of a diet with ascorbic acid. To prove beneficial effects from supplemental dietary ascorbic acid has been extremely difficult. Large quantities of ascorbic acid are used by the pharmaceutical and food industries. Ascorbic acid is added to food as a nutrient or as a processing acid in brewing, wine making, bread making, meat curing, and freezing of fruits. Losses of the vitamin in processing, storing and cooking of foods are substantial. There is still a need for a stable form of vitamin C to use in foods. Recently, C. F. Klopfenstein, E . Varriano-Marston and R. C. Hoseney (Nutrition Reports International 1981, 24: 1017-1028) found that a diet of 50% grain sorghum gave poor growth in guinea pigs receiving 2 mg of ascorbic acid/day. When the level of ascorbic acid was increased to 40 mg/day, the pigs grew normally. Wheat, millet, and oats did not depress the growth of pigs. The impact of these findings in human nutri tion is not clear. Much progress has been made in recent years in the chemistry of ascorbic acid. At least six new syntheses of L-ascorbic acid have been devised since 1971. One of those methods was used to prepare specifically labeled L-ascorbic acid to investigate its biosynthesis in plants. Proton magnetic resonance at 600.2 M H z has shown that the side chain of L-ascorbic acid and its sodium salt in aqueous solution adopt the same conformation as crystalline L-ascorbic acid. The conformation of crystal line sodium L-ascorbate, on the other hand, is different. Assay methods for ascorbic acid have steadily improved. High performance liquid chromatography can now be used to separate and quantitate ascorbic, dehydroascorbic, and 2-ketogulonic acids. Improved analytical techniques are needed to identify and quantitate the oxidation products of ascorbic acid. New derivatives of L-ascorbic acid have been synthesized including saccharoascorbic acid, 5-ketoascorbic acid, 2-phosphoric-ascorbic acid, 6-bromo-6-deoxyascorbic acid, 6-chloro-6-deoxyascorbic acid, the 5,6dehydro-5,6-dideoxy derivative, the 4,5-dehydro-5-deoxy derivative, the 5,6-dideoxy derivative, and numerous nitrogen derivatives. Dehydroascorbic acid is now readily prepared in pure form by the Ohmori-Takagi method in which ascorbic acid is dissolved in ethanol and oxygenated in the presence of charcoal. The structure of dehydroL-ascorbic acid is solvent dependent. In water, dehydro-L-ascorbic acid exists almost exclusively in the bicyclic hydrated monomer in which 6-OH x Seib and Tolbert; Ascorbic Acid: Chemistry, Metabolism, and Uses Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Downloaded by 147.158.188.2 on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.pr001
bonds to C3 through a hemiketal linkage. In 2V,IV-dimethylformamide and acetonitrile, dehydroascorbic acid forms a mixture containing a symmetrical and asymmetrical dimer. Monodehydroascorbic acid is present in tissue as a relatively stable free radical. That radical anion has been postulated as an intermediate in the oxidation of ascorbic acid by metal ions. The stability of the radical anion and its disproportionation into dehydroascorbic acid and ascorbic acid helps explain the antioxidant role that ascorbate plays in biological systems and in foods. There continues to be a fascination in the challenge of the scientific problems related to ascorbic acid. The authors of the chapters in this book have probably felt an intellectual curiosity about this commonplace substance: Why is it so unique? What does it do? The excitement and challenge will lead us on and intrigue the next generation of scientists working in this field. It is our pleasure to thank Myron Brin, Frank Loewus, Dietrich Hornig, and Jack Bauernfeind for helpful discussions in planning the symposium and this book. We are grateful to the chairmen of the various sessions: Frank Loewus, Linus Pauling, and Benjamin Borenstein. The work of Suzanne B. Roethel and Robin Giroux of the ACS Books Department to assemble the manuscripts is gratefully acknowledged.
PAUL A. SEIB
Kansas State University Manhattan, KS 66506 BERT M. TOLBERT
University of Colorado Boulder, CO 80309 June 28, 1982
xi Seib and Tolbert; Ascorbic Acid: Chemistry, Metabolism, and Uses Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
