18bitoriaI. MODERN VIEWS ON VITAMINS AND THEUi FUNCTIONS.

Transcription

1 119 18bitoriaI. MODERN VIEWS ON VITAMINS AND THEUi FUNCTIONS. IN the July number of tbe Journal, 1932, we wrote an editorial on recent work on vitamins. At that time a good deal of evidence had accumulated showing that carotene may be converted into vitamin A. More than ten years ago Steenbok found that when foodstuffs of plant origin were arranged according to their carotene content, their vitamin A potency was in the same order. But when he tested the efficiency of carotene obtained from plants as a source of vitamin A for animals he did not obtain conclusive results. It has now been shown that carotene will replace vitamin A if vitamin D be supplied as well. Moore showed that the ingestion of carotene led to the storage in the liver of a substance resembling vitamin A. Drummond and Heilbron and his colleagues found that a substance could be isolated from the liver oil of the sturgeon and halibut having properties similar to those of vitamin A and which appe!l.red to be an alcohol derived from the breaking up of the carotene molecule. Drummond believes that the vitamin A in the liver of fish is derived from the ingestion of minute marine plants, the phytoplankton, which contain carotene in about the same proportion as green land plants. As regards the functions of vitamin A, the work of Green and Mellanby appeared to suggest that during pregnancy an extra supply of vitamin A would confer an increased resistance to bacterial infection. Bnt Moore has found abundant vitamin A reserves in many infective conditions so that it cannot be considered as an anti-infective agent indiscriminate in its action. Griffith observed that the resistance to tuberculosis is not diminished by vitamin A deficiency. At Sheffield Mellanby discovered that vitamin A deficiency in combination with a high cereal intake produced demyelination of the cord. He is continuing his experimental work on the nutritional control of the nervous system. He has found that many afferent nerves, including the second, fifth, and eighth cranial nerves, show degenerative changes when the diet is deficient in vitamin A and rich in cereals. Mrs. Mellanby's experimental work has demonstrated that a deficiency of vitamin A or carotene plays an important part in the development of the periodontal tissues. Up to the present time these observations have not been applied to man, but that they can. be so extended is strongly supported by the distribution of periodontal disease in different races. It is found that natives in many tropical regions are nearly free from caries but are subject to pe~iodontal disease when their diet consists largely of white maize and little green vegetables and their. bodies are exposed to sunshine. In Denmark during the Great War outbreaks of xerophthalmia in

2 420 Modern Views on Vitamins and their Functions children appeared to be due to the consumption of margarine instead of butter and when the Government ordered the addition of a certain amount of butter to margarine xerophthalmia promptly disappeared. The characteristic change in vitamin A de6ciency is a development of stratified keratinizing epithelium and furth~r observations on xerophthalmia have indicated a probable relationship between degeneration of the nerves and a tendency to infection of epithelial surfaces. It has been shown that xerophthalmia is always accompanied by degeneration of the afferent fibres of the trigeminal nerve and it appears likely that.thepathological changes in the eye in this condition are,secondary too. loss oftrophic control. Much work has been done on the vitamin B complex, and animal experiment3 seem to show that there are five or six entities, bu.t Bl and B2 are the most important from a clinical point of view. Impure crystalline preparations of Bl were first obtained by J ansen and Donath from rice polishings. In the last two years crystals of greater purity have been obtained, and in two laboratories, including that of Professor Peters at Oxford, from an original source of yeast. The hydrochloride of the base separates from acid absolute alcohol solution in glistening plates. The crystals prepared by different workers are alike in general properties, but differ in potency and chemical composition. They contain sulphur and those from rice polishings appear to have the formula C12H1S02N4S 2HCl. Peters thinks that the differences between preparations in diffefent laboratories can be explained by the presence of varying admixtures of inactive and active vitamin. Activity appears to follow the sulphur content, but itis not certain: that pure vitamin Bl has yet been obtained. Vitamin B2 has been divided by German workers into two fractions. The crystals are calledflavines; ovoflavine from egg white has the formula Cl7H20N406' is yellow with a green fluorescence and is highly sensitive to light. Peters thinks this last point may be of importance in relation to the light sensitivity of pellagrines and if confirmed it is a striking development of the discovery of vitamin B2, unaccompanied by vitamin Bl, in egg white by Chick and her colleagues. Peters considers the most important point to be an apparent relation between vitamin B2 and the essential constituent of Warburg and Christian's new oxidizing enzyme. Such a relation between vitamins and enzymes has often been sought before in vain. A colourless form of apparently the same substance has been prepared by White in Professor Drummond's laboratory. Both the forms are stated to be equally effective in animal experiments. Vitamin Bl action is very complicated and much work has been done on the "B complex" by Peters and others at Oxford. At first it was considered that vitamin B deficiency led to retardation of growth, atrophy of organs, more especially those cif the digestive system, anremia, failure of reproduction, loss of appetite,reduced oxidative processes arid polyneuritis. It- was found that loss of appetite and subsequent inanition were the cause of some of these symptoms; there was no evidence of

3 Modern Views on Vitamins and their Functions 421 impairment of the oxidative system of the tissues. Professor Woollard on examining experimental animals living on a diet deficient in vitamin Bl concluded that the "polyneuritis" symptoms were less those of a peripheral neuritis than of a central lesion. He also showed that many of the "degenerations" in nerve specimens were probably due to faulty technique in their preparation. "Head retraction" in pigeons was found not to be a characteristic of vitamin B deficiency, for it was found in cases of starvation without deficiency of vitamin B, and Hess showed that the condition could be produced by sublethal doses of cyanide. It was discovered by Professor Peters and his co-workers that an accumulation.of lactic acid in the brain invariably accompanied the head symptoms in pigeons deprived of vitamin. B; they then found that in such cases there was an inhibition of the oxidative process in the brain. -They also made the important discovery that administration of the vitamin either to the living animal or to in vitro preparations restored the oxygen uptake to normal. Subsequent work showed that the action of vitamin Bl is particularly related to the metabolism of lactic acid in the brain. The action of vitamin B2 is still little known, but deprivation of the vitamin leads to retardation in the growth of animals and to the appearance of symptoms which some regard as being those of pellagra, though others consider them to be "pseudo-pellagrous." The experimental study of vitamin B2 deficiency has been made chiefly on young rats. These animals when fed on a diet deficient in B2 develop a symmetrical dermatitis which was considered by Goldberger to be analogous to.that seen in human pellagra. Dogs fed on diets.deficiei:t~ in B2 develop severe stomatitis, "black tongue," which resembles another phase of pellagra symptomatology. Akroyd and Roscoe arranged a list of cereals and other foods in the order in which they have been found effective for the prevention and cure: (a) of vitamin B2 deficiency in rats ; (b) of human pellagra in communities where the disease is endemic; (0) of black tongue in dogs. The striking similarity in the quantitative distribution among these foods of the dietary factor concerned in the prevention of these three diseases led irresistibly to the conclusion that they must be regarded as analogous. It is, however, difficult.to understand the almost universal association of epidemic pellagra with unbalanced diets containing maize as the staple cereal. Maize has not been found inferior to wheat or rice in content of vitamin B 2, yet a population subsisting too exclusively on these cereals does not develop pellagra, but beriberi. Rice endosperm was found to be the poorest in vitamin B2 of all the.cereals tested, yet among rice eaters pellagra is somewhat of a rarity. Mellanby has shown that the nervous symptoms of pellagra can be produced by deficiency in vitamin A combined witbtbe toxic action of some substance present in white maize. The nervous symptoms are not produced by yellow maize as this contains vitamin A.

4 422 Modern Views on Vitamins and their Functions Wilson maintaim; that shortage of protein of good biological value is the main cause of pellagra, though he admits that many of the sources of protein may contain moderateiy large amounts of vitamin B 2 Dr. M. Fixsen working under the direction of Dr. Chick on the proteins. of wheat and maize in the support of growth and development has {omid them to be nearly equal in this respect. This result confirmed that obtained in previous work where maintenance of nitrogenous equilibrium was the criterion used, and does not suppl'lrt the theory that pellagra results from the low nutritive value of th~ protein in diets containing maize as a staple consti tu en t. Dr. Chick considers that the known fads concerning the retiology of pellagra are in accord with the theory postula'ting a toxic substance in maize and other pellagra-producing diets (or produced therefromby metabolic processes in the body) for the neutralization of which poison an abundant supply of vitamin B2 is required. The identification of vitamin C is of much scientific interest, as well as of great practical importance. It has been known for years that in the juices of the citrus plants there is a substance which in quite small quantities prevents scurvy. In 1924, Dr.S. S.Zilva, working for the Medical Research Council at the Lister Institute, discovered that a strong affinity for oxygen-or reducing power-was invariably shown by active concentrates of the vitamin. In 1928, Dr. A. Szent-Gyorgi, while studying a different problem, isolated a substance" hexuronic acid., from the cortex of the suprarenal gland. He showed that the reducing power of this acid was similar'to that described by Zilva for vitamin C concentrates. He also demonstrated its presence in' various vegetable tissues. In 1932, Professor Tillmann and his co-workers in Germany suggested on the basis of the constant relation of reducing power' andantiscorbutlc potency of natural lemon juice that the properties described by Zilva were those of the vitamin itself, and that the vitamin was identical with the hexuronic acid of Szent-Gyorgi. ' In April, 1932, Svil'bely and Szent-Gyorgi showed that hexuronic acid (in 1983 called ascorbic acid by Szent-Gyorgi) had an antiscorbutic action. They found that hexuronic acid fed at a level of one milligranime per day protected guinea-pigs over the usual ninety-day test period. They therefore concluded that vitamin C is a single substance identical with hexuronic acid. Zilva, however, pointed out that he had obtained fractions active in doses of 0'5 milligramme which were evidently grossly contaminated, a fact which militated against the contention that hexuronic acid and the antiscorbutic factor are identical. In 1932 King and Waugh of the University of Pittsburgh announced that they had succeeded in isolating vitamin C, and that it had the properties of hexuronic acid.

5 Modern.views on Vitamins and their Functions 423 In 1933 Svirbely and Szent-Gyorgi made experiments to show that the antiscorbutic activity of their hexur~nic acid (ascorbic acid) preparations is due to the acid itself and not to contaminations_ They obtained fairly large quantities of ascorbic acid from paprika (Capsicum annuum) and prepared a mono-acetone derivative. This substance was moderately active as an. antiscorbutic. The ascorbic acid recovered from the monoacetone derivative was, however, fully active. This was regarded as definite evidenceconcernillg the identity of ascorbic acid and vitaminc. 'rhey also found that ascorbic acid from adrenal glands when recrystallized five times retained its activity. In 193H Dr. Zilva and Dr. Hirst examined the activity of ascorbic acid derived from various sources and their results now support the view that ascorbic acid is the pure vitamin C. Dr. Zilva and Dr. Gough have found ascorbic acid present in several organs of different animal species. The anterior lobe of the pituitary gland is very rich in the acid and has strong vitamin C activity. On the other hand the absence of ascorbic acid from the cortex of the suprarenal gland is not necessarily followed by scurvy. At the Nutritional Laboratory, University of Cambridge, quantitative experiments have been made to determine whether hexuronic acid (ascorbic acid) is pure vitamin C,and the results show that it is so. By a new microchemical metbod of determining the amount of hexuronic acid applied to many different materials it has been found-that the hexuronic acid content of each substance corresponds to its vitamin C activity, and destruction of the acid by chemical means causes a parallel loss of vitamin power in the material treated. Dr. Rarris has found that dogs are able to form vitamin C when fed for long periods on vitamin-c-free diets. Liver from such dogs when fed to scorbutic guinea-pigs cured them of scurvy. He also ascertained that hexuronic acid is.synthesized concurrently by plants on germination. It has been shown that _ rats after adrenalectomy are still able to form the vitamin. Obviously it must be formed in other places than the adrenal gland. The conversion of ergosterol to vitamin D by ultra-violet radiation has been investigated at the National Institute of Medical Research at Hampstead and the workers there isolated a crystalline compound possessing great antirachitic properties. They n~med the substance "calciferol." Later it was found thali this" calciferol., consisted of two substances, one of which, "pyrocalciferol," is physiologically inactive, while the other, "calciferol ii " is more active than any other substance and is now considered to be the pure vitamin. Professor Windaus also isolated the pure vitamin in the Institute of Chemistry at Gottingen. Experiments have been in progress at the.n ational Institute on the stability of the pure Sl1 bstance with a view to its acceptance as an international standard in place of the empirical solution of mixed irradiation products hitherto in use. Data

6 424 Modern Views on Vitamins and their Functions accumulated over one and a half years show a remarkable stability of pure calciferol when protected from light and oxygen. Mr. Webster has been investigating the distribution and fate of vitamin D in the apimal body. In animals it is found, in c'ontrast with vitamin A, t'o be distributed almost unif'ormly in the fat extracted fr'om all the major organs, with the exception of the fatty constituents of the brain, from which it appears to be completely absent.. Much work has been done on the action of vitamin D on the deposition of calcium in bony tissues" which appears to depend 'On the concentration 'Of calcium and phosphate ions. In normal tissues this c'oncentrati'on is such that calcium is not deposited and some mechanism must come into action to allow of the deposition of calcium in bones and teeth. Robison and his colleagues found that at centres of ossification and at sites where it is necessary to transport calcium and phosphates across vital boundaries an enzyme, phosphatase, can he detected, which by the hydr'olysis of certain organic phosph'oric esters in the blood leads to a local concentration of phosphoric acid ions and this in growing animals causes a deposition of calcium. In rickets there is a deficiency in the blood of calcium or phosphate ions, or both, and the bone enzyme cannot raise them to a sufficiently high level for the deposition of calcium. The mode of action of vitamin D in calcification is still unkn'own, but it is believed to cause a retenti'on 'Of calcium or of phosphate. Miss Bruce and.dr. Callow have found in experimental rickets that on a diet in which the proportion of calcium is excessive in relation to phosphate the addition of some source of available phosphorus will by itself cause some degree of healing of the rachitic condition, but with the addition 'Of calciferol the combined curative effect is much greater. The addition of cereal phosphorus, such as oatmeal, is less effective than an equivalent addition of inorganic phosphorus. This is explained by the fact that the phosphorus in oatmeal and other cereals is,in the unavailable form of phytic acid. Dr. Parks, of Vermont, D.S.A., has shown that contrary to the theory that vitamin D acts by stimulating the parathyroid glands, parathyroid hormone has no effect in increasing the net absorption of calcium and phosphorus in rickets.