By the end of the 1930s, serious research had commenced on the biological role of vitamin C. In particular, biochemical reductionists sought to explain the nature of the relationship between the clinical manifestations of scurvy and the biochemical involvements of vitamin C. It was recognized that vitamin C was a powerful biological reductant, and there were early attempts to explain its nutritional significance in terms of its involvement in oxidation-reduction systems - a major theme in prewar biochemistry. But the first clear advance in the biochemistry of vitamin C came from studies of its biosynthesis, and by the early 1950s, the pathway for its formation from simple sugars had been worked out. L. W Mapson and a colleague at the Low Temperature Research Station at Cambridge (U. K.) fed different possible precursor molecules to cress seedlings and measured the formation of vitamin C. And in the United States, King and co-workers used labeled glucose to chart the biosynthetic pathway in rats (Mapson 1967: 369-80).
The biosynthetic pathway proved to be a comparatively simple one. D-glucuronate (formed from glucose) is converted to L-gluconate and then to L-gulono-gamma-lactone, which in turn is further reduced (via L-xylo-hexulonolactone) to L-ascorbic acid (2-oxo-L-gulono-gamm-lactone). The final enzymatic step is catalyzed by L-gulonolactone oxidase (EC 1.1.3.8.) - in the liver in evolutionarily “advanced” species such as the cow, goat, rat, rabbit, and sheep and in the kidney in other species such as the frog, snake, toad, and tortoise - and it is this enzyme that is lacking in those species unable to synthesize vitamin C.
To date, this biochemical “lesion” has been detected in a small, and disparate, number of species
- higher primates (including, of course, humans), guinea pigs, certain bats, birds, insects, and fish (Chat-terjee 1973; Sato and Uderfriend 1978). Whether all these species are necessarily scurvy-prone is not quite so clear. A survey of 34 species of New World microchiropteran bats showed that L-gulonolactone oxidase was apparently absent from the livers of all of them (and from the kidneys of at least some of them), but nevertheless, the tissue levels of ascorbic acid (even in species that were fish-eaters or insect-eaters) were similar to those in species that could biosynthesize the vitamin (Birney, Jenness, and Ayaz 1976).
This finding would suggest that the vitamin was being synthesized in organs other than the liver and kidney; or that the metabolic requirement for it was remarkably low; or that there were extremely efficient mechanism(s) for its protection against degradative changes. The whole question of the evolutionary significance of vitamin C - in plants as well as in animals
- remains a largely uncharted area.
The rate of endogenous biosynthesis of vitamin C in those species capable of producing the vitamin shows considerable interspecies variation, ranging from 40 milligrams (mg) per kilogram (kg) body weight daily for the dog to 275 for the mouse (Levine and Morita 1985). These values are well in excess of the amounts of the vitamin required to prevent the appearance of scurvy in species unable to synthesize it - a finding that has frequently been used to buttress the claim that vitamin C has a number of “extraantiscorbutic” roles requiring daily intakes well in excess of the recommended daily amounts.
The total body pool of ascorbic acid in a 70 kg man has been estimated at about 1.5 grams (g) (but according to Emil Ginter it could be three times as great as this [Ginter 1980]), which is attainable in most people by the sustained daily intake of 60 to 100 mg. A daily intake of 10 mg vitamin C results in a body pool of about 350 mg. Scorbutic signs do not appear until the pool falls to below 300 mg (Kallner 1981;“Experimental Scurvy” 1986).
Plasma (and less conveniently, leucocyte) concentrations of ascorbic acid are often taken as an index of the body status of the vitamin. The normal concentration range in the plasma of healthy persons on an adequate plane of nutrition is 30 to 90 micromoles per liter (pmol/L) (0.5-16 mg/100 ml). The Nutrition Canada Interpretive Guidelines are often referred to in this respect; these guidelines suggest that values between 11 and 23 pmol/L are indicative of marginal deficiency and that values below 11 pmol/L point to frank severe deficiency - but differences of sex, race, metabolism, smoking habits, and, particularly, of age (factors known to influence plasma ascorbic acid concentrations) reduce the validity of such a generalization (Basu and Schorah 1981; Hughes 1981b).
During a period of vitamin C depletion there is a comparatively rapid loss of vitamin C (a reduction of about 3 percent in the body pool daily) resulting from the continued catabolism of the vitamin and the excretion of its breakdown products in the urine. In humans, the main pathway identified involves the conversion of the ascorbic acid to dehydroascorbic acid, diketogulonic acid, and oxalic acid (in that order), with the two latter compounds accounting for the bulk of the urinary excretion of breakdown products. Smaller amounts of other metabolites, such as ascorbic acid-2-sulphate also occur, and in the guinea pig there is substantial conversion of part of the ascorbic acid to respiratory COj. It has sometimes been argued that the excess formation of these catabolites (particularly oxalic acid) should signify caution in the intake of amounts of vitamin C substantially in excess of the amount required to prevent scurvy.
Biochemical Role of Vitamin C
It was noted in the early experiments of Holst and Frohlich, and confirmed by many subsequent workers, that defective formation of connective tissue was a primary pathological feature of experimental scurvy, and at one time it was believed that this lesion could account for most of the known pathological sequelae of the disease - the petechial hemorrhages, the breakdown of gum tissue, and the impairment of wound repair tissue. Attempts to characterize the biochemical modus operandi of vitamin C in preventing scurvy, therefore, centered initially on the metabolism of collagen - the essential glycoprotein component responsible for imparting strength to connective tissue.
By the 1970s, there was suggestive evidence that the biochemical lesion was located in the hydroxyla-tion of the proline and lysine components of the collagen polypeptide and that vitamin C had an essential role in the process (Barnes and Kodicek 1972). The hydroxylases involved in collagen biosynthesis (prolyl 4-hydroxylase, prolyl 3-hydroxylase, and lysyl hydroxylase) require ferrous iron as a cofactor, and it appears that vitamin C, a powerful biological reductant, has an almost obligatory role in maintaining the ferrous iron in the reduced form. Thus emerged a simplistic and reductionist explanation for the role of vitamin C in preventing the emergence of the main clinical features of scurvy.
Yet although there can be little doubt that vitamin C plays a critical role in the biosynthesis of collagen, recent studies have suggested that the simple “defective hydroxylation” theory is, perhaps, not the complete story. Studies have indicated that the activity of prolyl hydroxylase and the formation of collagens by fibroblast cultures is not influenced by ascorbic acid; furthermore, ascorbic acid deficiency does not always result in severe underhydroxylation of collagen in scorbutic guinea pigs (Englard and Seifter 1986).
There is increasing evidence that vitamin C may also influence the formation of connective tissue by modifying the nature and formation of the extracellular matrix molecules (Vitamin C Regulation 1990). B. Peterkofsky (1991) has recently suggested that the role of vitamin C in collagen biosynthesis is a dual one - a direct influence on collagen synthesis and an indirect one (mediated perhaps via appetite) on proteoglycan formation.
The complement component Clq, which has a central role in disease resistance, contains a collagenlike segment that is rich in hydroxyproline, and it has been suggested that this segment could offer a link with the putative anti-infective powers widely suggested for vitamin C (Pauling 1976; see also the section on megatherapy). Studies over the last 15 years, however, have failed to demonstrate that the complement system, unlike connective tissue collagen, reflects vitamin C availability (Thomas and Holt 1978; Johnston 1991). Indeed, the belief that vitamin C had anti-infection powers probably stemmed from reports by Harris in 1937 of lowered vitamin C in persons suffering from certain diseases, particularly tuberculosis.
During the 1960s and the 1970s, however, some 25 epidemiological studies were completed in different parts of the world to assess the validity of claims that vitamin C had anti-infection powers, particularly with respect to the common cold. The general conclusion drawn from the results of these studies was that the evidence for a protective/curative role for vitamin C in the common cold was far from convincing (Hughes 1981b: 22-6;Carpenter 1986: 213-16).
There is accumulating evidence, however, that vitamin C may have additional involvements in a range of enzymatic changes unrelated to the formation of collagen. There are three systems of considerable physiological significance in which vitamin C plays an important, and possibly obligatory, role: (1) as the immediate donor for dopamine B-hydroxylase, a key reaction in the conversion of tyrosine to norepinephrine (Englard and Seifter 1986; Fleming and Kent 1991); (2) in the peptidylglycine alpha-amidating monooxygenase system, whereby peptidyl carboxyl-terminal residues are amidated, a process that requires molecular oxygen, copper, and ascorbate and is important in the biosynthesis of a number of neuroendocrine peptides (Englard and Seifter 1986; Eipper and Mains 1991); (3) in the hydroxylation reactions in the biosynthesis of carnitine from lysine and methionine (Englard and Seifter 1986;Rebouche 1991).
The exact physiological significance of these and other reactions vis-a-vis the clinical manifestations of scurvy is unclear. The first two, having obvious involvements in the endocrine and nervous systems, could well be causally related to various functional derangements of scurvy; and as carnitine has an important role in the transport of fatty acids into the mitochondria, where they may be oxidized to provide energy, it has been suggested that the carnitine involvement could account for the lassitude and fatigue that have been invariably noted as an early feature of scurvy (Hughes 1981a).
Should such involvements require an availability of ascorbic acid greater than that required to prevent the emergence of “classical” scurvy - and there is some evidence that this is so, at least in the case of carnitine biosynthesis - then a revision of the currently accepted Recommended Dietary Allowance/Refer-ence Value would be called for (Hughes 1981a). The current recommended daily intake of vitamin C (60 mg in the United States and recently raised from 30 to 40 in the United Kingdom) is, after all, the amount estimated to prevent the emergence of the classic (“collagen”) features of scurvy - and in the United Kingdom, it is based, essentially, on a single experiment completed almost half a century ago on a nonrepresentative population sample.
Source of Vitamin C
Vitamin C is a heat-labile, water-soluble, and readily oxidizable molecule, and its distribution among foodstuffs and the losses resulting from processing and food preparation have been well documented. Studying the losses induced in the vitamin C content of various foodstuffs by simple culinary procedures must be one of the commonest and oft-repeated projects in basic college and university courses, and the amount of unpublished data resulting from these studies must be immense.
The mean daily intake of vitamin C in the United Kingdom (based on noncooked purchases) is about 60 mg daily with potatoes, citrus fruits, and cabbage accounting for 20, 12, and 6 percent, respectively, of the intake. The losses resulting from cooking are substantial, and these are further increased if the cooked food is allowed to stand around before being eaten. Nevertheless, because of the comparatively widespread distribution of the vitamin in plant foodstuffs, and the role of technology in increasing the availability of uncooked plant and vegetable material during the whole year, very few persons today appear to suffer from clinically defined hypovitaminosis C; consequently, frank scurvy is an almost unknown condition.
A recent survey of vitamin C intakes in European countries revealed an interesting, and almost providential, reciprocity between the consumption of two important sources of vitamin C. Of 27 countries studied, Iceland, Switzerland, and France had the lowest annual consumption of cabbage (less than 5 kg per capita) but a high consumption of citrus fruit (over 20 kg); Romania, Poland, and the former Soviet Union, in contrast, had the lowest consumption of citrus fruit (less than 4 kg) but the highest consumption of cabbage (more than 30 kg) (Kohlmeier and Dortschy 1991). Only where a person, for ideological, economic, or supposed “health” reasons subsists on a diet devoid of fruit and vegetables (such as one based on nuts, grain, and/or cooked meat/fish) is scurvy likely to emerge.
The foliage of many flowering plants has an unexpectedly high concentration of vitamin C, with concentrations of up to 1 percent wet weight being attained in some members of the Primulaceae family. The mean concentration for 213 species examined (162 mg per 100 g) was some three times that of those culinary vegetables usually regarded as good sources of the vitamin; and the mean value for the leaves of 41 woody shrubs and trees examined was 293 mg per 100 g - significantly higher than black currants, which are usually cited as the dietary source par excellence of vitamin C (Jones and Hughes 1983, 1984; see also Table fVA.3.1).
Note: Values in parentheses are for cooked meals taken by elderly patients in hospitals (Jones, Hughes, and Davies 1988 and additional unpublished material).
Source: Based on Jones and Hughes (1983, 1984) and Hughes (1990).
The historically important “antiscorbutic herbs” are among the poorest sources of vitamin C (Hughes 1990). William Perry, who stowed boxes of mustard greens and cress on board his ship in an attempt to fend off scurvy during his. Arctic expedition of 1818 (Lloyd and Coulter 1963: 108), would have done better to adorn his stateroom with primrose plants, a single leaf of which, chewed in the mouth daily, would have sufficed to offer complete protection. The exact reason, if any, for these high (and often disparate) concentrations of ascorbic acid in angio-sperms is not known; nor is the role of ascorbic acid in plant biochemistry understood. It has been suggested that there is a positive correlation between the concentration of ascorbic acid in plants and corresponding concentrations of phenolic compounds, but the extent to which this reflects a biochemical relationship is a matter of conjecture (E. C. Bate-Smith, personal communication).
World History
