Humans are unable to synthesise L-ascorbic acid (L-AA, ascorbate, vitamin C), and are thus entirely dependent upon dietary sources to meet needs. In both plant and animal metabolism, the biological functions of L-ascorbic acid are centred around the antioxidant properties of this molecule. Considerable evidence has been accruing in the last two decades of the importance of L-AA in protecting not only the plant from oxidative stress, but also mammals from various chronic diseases that have their origins in oxidative stress. Evidence suggests that the plasma levels of L-AA in large sections of the population are sub-optimal for the health protective effects of this vitamin. Until quite recently, little focus has been given to improving the L-AA content of plant foods, either in terms of the amounts present in commercial crop varieties, or in minimising losses prior to ingestion. Further, while L-AA biosynthesis in animals was elucidated in the 1960s, 1 it is only very recently that a distinct biosynthetic route for plants has been proposed. 2 The characterisation of this new pathway will undoubtedly provide the necessary focus and impetus to enable fundamental questions on plant L-AA metabolism to be resolved. This review focuses on the role of L-AA in metabolism and the latest studies regarding its bio- synthesis, tissue compartmentalisation, turnover and catabolism. These inter-relationships are considered in relation to the potential to improve the L-AA content of crops. Methodology for the reliable analysis of L-AA in plant foods is briefly reviewed. The concentrations found in common food sources and the effects of processing, or storage prior to consumption are discussed. Finally the factors that determine the bioavailability of L-AA and how it may be improved are considered, as well as the most important future research needs. # 2000 Society of Chemical Industry

Plant L‐ascorbic acid: chemistry, function, metabolism, bioavailability and effects of processing

Birds are the most species-rich class of tetrapod vertebrates and have wide relevance across many research fields. We explored bird macroevolution using full genomes from 48 avian species representing all major extant clades. The avian genome is principally characterized by its constrained size, which predominantly arose because of lineage-specific erosion of repetitive elements, large segmental deletions, and gene loss. Avian genomes furthermore show a remarkably high degree of evolutionary stasis at the levels of nucleotide sequence, gene synteny, and chromosomal structure. Despite this pattern of conservation, we detected many non-neutral evolutionary changes in protein-coding genes and noncoding regions. These analyses reveal that pan-avian genomic diversity covaries with adaptations to different lifestyles and convergent evolution of traits.

/pdf/comparative-genomics-reveals-insights-into-avian-genome-3gamkx0zqj.pdf

Comparative genomics reveals insights into avian genome evolution and adaptation.

Ascorbic acid: much more than just an antioxidant.

ASCORBIC acid, originally called vitamin C, is required for human health.1 In human beings deprived of ascorbic acid, the deficiency disease scurvy develops and can be life threatening. Although a disease remarkably similar to scurvy was described by the ancient Egyptians,2 , 3 it was not until 1753 that a Scottish physician, James Lind, systematically described scurvy and its prevention by dietary means.4 Even then, the dietary requirements were controversial. For four decades the British navy refused to accept Lind's findings, and countless sailors continued to die unnecessarily from scurvy until lemon juice was finally included in sailors' rations. Research since Lind's . . .

New concepts in the biology and biochemistry of ascorbic acid.

Vitamin C, a reducing agent and antioxidant, is a cofactor in reactions catalyzed by Cu(+)-dependent monooxygenases and Fe(2+)-dependent dioxygenases. It is synthesized, in vertebrates having this capacity, from d-glucuronate. The latter is formed through direct hydrolysis of uridine diphosphate (UDP)-glucuronate by enzyme(s) bound to the endoplasmic reticulum membrane, sharing many properties with, and most likely identical to, UDP-glucuronosyltransferases. Non-glucuronidable xenobiotics (aminopyrine, metyrapone, chloretone and others) stimulate the enzymatic hydrolysis of UDP-glucuronate, accounting for their effect to increase vitamin C formation in vivo. Glucuronate is converted to l-gulonate by aldehyde reductase, an enzyme of the aldo-keto reductase superfamily. l-Gulonate is converted to l-gulonolactone by a lactonase identified as SMP30 or regucalcin, whose absence in mice leads to vitamin C deficiency. The last step in the pathway of vitamin C synthesis is the oxidation of l-gulonolactone to l-ascorbic acid by l-gulonolactone oxidase, an enzyme associated with the endoplasmic reticulum membrane and deficient in man, guinea pig and other species due to mutations in its gene. Another fate of glucuronate is its conversion to d-xylulose in a five-step pathway, the pentose pathway, involving identified oxidoreductases and an unknown decarboxylase. Semidehydroascorbate, a major oxidation product of vitamin C, is reconverted to ascorbate in the cytosol by cytochrome b(5) reductase and thioredoxin reductase in reactions involving NADH and NADPH, respectively. Transmembrane electron transfer systems using ascorbate or NADH as electron donors serve to reduce semidehydroascorbate present in neuroendocrine secretory vesicles and in the extracellular medium. Dehydroascorbate, the fully oxidized form of vitamin C, is reduced spontaneously by glutathione, as well as enzymatically in reactions using glutathione or NADPH. The degradation of vitamin C in mammals is initiated by the hydrolysis of dehydroascorbate to 2,3-diketo-l-gulonate, which is spontaneously degraded to oxalate, CO(2) and l-erythrulose. This is at variance with bacteria such as Escherichia coli, which have enzymatic degradation pathways for ascorbate and probably also dehydroascorbate.

Vitamin C. Biosynthesis, recycling and degradation in mammals.

The ability to synthesize ascorbic acid is absent in the insects, invertebrates, and fishes. The biosynthetic capacity started in the kidney of amphibians, resided in the kidney of reptiles, became transferred to the liver of mammals, and finally disappeared from the guinea pig, the flying mammals, monkey, and man. A similar transition in the biosynthetic ability was observed in the branched evolution of the birds.

https://science.sciencemag.org/content/sci/182/4118/1271.full.pdf

Evolution and the biosynthesis of ascorbic acid

The ability of several species of birds to synthesize L-ascorbic acid is correlated with their phylogeny. In the more primitive species, synthesis of L-ascorbic acid occurs in the kidney. Among the highly evolved passeriform species, kidney and liver can synthesize L-ascorbic acid in some, whereas in others synthesis occurs in the liver. In still others, the capacity for the synthesis of L-ascorbic acid is apparently lost. The pattern of evolution of the ascorbic acid pathway among birds is thus similar to that among mammals.

L-Ascorbic Acid Synthesis in Birds: Phylogenetic Trend

Estimation of dehydroascorbic acid in blood of diabetic patients

Synthesis of l-ascorbic acid from d-glucurono-1,4-lactone conjugates by different species of animals

http://science.sciencemag.org/content/sci/126/3274/608.full.pdf

I. B. Chatterjee

Papers

Evolution and the biosynthesis of ascorbic acid

L-Ascorbic Acid Synthesis in Birds: Phylogenetic Trend

Estimation of dehydroascorbic acid in blood of diabetic patients

Synthesis of l-ascorbic acid from d-glucurono-1,4-lactone conjugates by different species of animals

Effect of cyanide on biosynthesis of ascorbic acid in vitro.