by Isaac Asimov
Taking more than enough of the water-soluble vitamins such as the Bcomplex and C, is not likely to do positive harm since they are not stored by the body and are easily excreted. Therefore, if a large dose is not actually needed by the body, the excess merely serves to enrich the urine.
The case is otherwise with the fat-soluble vitamins, particularly A and D. These tend to dissolve in the body fat and to be stored there, and are then relatively immobile, as is the fat itself. Too great a supply may therefore overload the body and disturb its workings, giving rise to hypervitaminoses, as the condition is called. Since vitamin A is stored in the liver, particularly in fish and in animals that live on fish (a whole generation of youngsters had their life made hideous by regular doses of cod-liver oil), there have been horror tales of Arctic explorers who were rendered seriously ill or even killed by dining on polar-bear liver—poisoned by vitamin A.
VITAMINS AS ENZYMES
Biochemists naturally were curious to find out how the vitamins, present in the body in such tiny quantities, exert such important effects on the body chemistry. The obvious guess was that they have something to do with enzymes, also present in small quantities.
The answer finally came from detailed studies of the chemistry of enzymes. Protein chemists had known for a long time that some proteins are not made up solely of amino acids, and that nonamino-acid prosthetic groups might exist, such as the heme in hemoglobin (see chapter 11). In general, these prosthetic groups tended to be tightly bound to the rest of the molecule. With enzymes, however, there were in some cases nonamino-acid portions that were quite loosely bound and might be removed with little trouble.
This was first discovered in 1904 by Arthur Harden (who was soon to discover phosphorus-containing intermediates; see chapter 12). Harden worked with a yeast extract capable of bringing about the fermentation of sugar. He placed it in a bag made of a semipermeable membrane and placed that bag in fresh water. Small molecules could penetrate the membrane, but the large protein molecule could not. After this dialysis had progressed for a while, Harden found that the activity of the extract was lost. Neither the fluid within nor that outside the bag would ferment sugar. If the two fluids were combined, activity was regained.
Apparently, the enzyme was made up not only of a large protein molecule, but also of a coenzyme molecule, small enough to pass through the pores of the membrane. The coenzyme was essential to enzyme activity (it was the cutting edge, so to speak).
Chemists at once tackled the problem of determining the structure of this coenzyme (and of similar adjuncts to other enzymes). The German-Swedish chemist Hans Karl August Simon von Euler-Chelpin was the first to make real progress in this respect. As a result, he and Harden shared the Nobel Prize in chemistry in 1929.
The coenzyme of the yeast enzyme studied by Harden proved to consist of a combination of an adenine molecule, two ribose molecules, two phosphate groups, and a molecule of nicotinamide. Now this last was an unusual thing to find in living tissue, and interest naturally centered on the nicotinamide. (It is called nicotinamide because it contains an amide group, CONH2, and can be formed easily from nicotinic acid. Nicotinic acid is structurally related to the tobacco alkaloid nicotine, but they are utterly different in properties; for one thing, nicotinic acid is necessary to life, whereas nicotine is a deadly poison.) The formulas of nicotinamide and nicotinic acid are:
Once the formula of Harden’s coenzyme was worked out, it was promptly renamed diphosphopyridine nucleotide (DPN): nucleotide from the characteristic arrangement of the adenine, ribose, and phosphate, similar to that of the nucleotides making up nucleic acid; and pyridine from the name given to the combination of atoms making up the ring in the nicotinamide formula.
Soon a similar coenzyme was found, differing from DPN only in the fact that it contained three phosphate groups rather than two. This, naturally, was named triphosphopyridine nucleotide (TPN), Both DPN and TPN proved to be coenzymes for a number of enzymes in the body, all serving to transfer hydrogen atoms from one molecule to another. (Such enzymes are called dehydrogenases.) It was the coenzyme that does the actual job of hydrogen transfer; the enzyme proper in each case selects the particular substrate on which the operation is to be performed. The enzyme and the coenzyme each have a vital function; and if either were deficient in supply, the release of energy from foodstuffs via hydrogen transfer would slow to a limp.
What was immediately striking about all this was that the nicotinamide group represents the only part of the enzyme the body cannot manufacture itself. It can make all the protein it needs and all the ingredients of DPN and TPN except the nicotinamide: that it must find ready-made (or at least in the form of nicotinic acid) in the diet. If not, then the manufacture of DPN and TPN stops, and all the hydrogen-transfer reactions they control slow down.
Was nicotinamide or nicotinic acid a vitamin? As it happened, Funk (who coined the word vitamine) had isolated nicotinic acid from rice hulls. Nicotinic acid was not the substance that cured beri-beri, so he had ignored it. But on the strength of nicotinic acid’s appearance in connection with coenzymes, the University of Wisconsin biochemist Conrad Arnold Elvehjem and his co-workers tried it on another deficiency disease.
In the 1920s, the American physician Joseph Goldberger had studied pellagra, a disease endemic in the Mediterranean area and almost epidemic in the southern United States in the early part of this century. Pellagra’s most noticeable symptoms are a dry, scaly skin, diarrhea, and an inflamed tongue; it sometimes leads to mental disorders. Goldberger noticed that the disease struck people who lived on a limited diet (for example, mainly cornmeal) and spared families that owned a milch cow. He began to experiment with artificial diets, feeding them to animals and inmates of jails (where pellagra seemed to blossom). He succeeded in producing blacktongue (a disease analogous to pellagra) in dogs and in curing this disease with a yeast extract. He found he could cure jail inmates of pellagra by adding milk to their diet. Goldberger decided that a vitamin must be involved, and he named it the P-P (pellagra-preventive) factor.
It was pellagra, then, that Elvehiem chose for the test of nicotinic acid. He fed a tiny dose to a dog with blacktongue, and the dog responded with remarkable improvement. A few more doses cured it. Nicotinic acid was a vitamin, all right; it was the P-P factor.
The American Medical Association, worried that the public might get the impression there were vitamins in tobacco, urged that the vitamin not be called nicotinic acid and suggested instead the name niacin (an abbreviation of “nicotinic acid”) or niacinamide. Niacin has caught on fairly well.
Gradually, it became clear that the various vitamins were merely portions of coenzymes, each consisting of a molecular group that an animal or a human being cannot make for itself. In 1932, Warburg had found a yellow coenzyme that catalyzed the transfer of hydrogen atoms. The Austrian chemist Richard Kuhn and his associates shortly afterward isolated vitamin B2, which proved to be yellow, and worked out its structure:
The carbon chain attached to the middle ring is like a molecule called ribitol, so vitamin B2 was named riboflavin, flavin coming from a Latin word meaning “yellow.” Since examination of its spectrum showed riboflavin to be very similar in color to Warburg’s yellow coenzyme, Kuhn tested the coenzyme, for riboflavin activity in 1935 and found such activity to be there. In the same year, the Swedish biochemist Hugo Theorell worked out the structure of Warburg’s yellow coenzyme and showed it to be riboflavin with a phosphate group added. (In 1954, a second and more complicated coenzyme also was shown to have riboflavin as part of its molecule.)
Kuhn was awarded the 1938 Nobel Prize in chemistry, and Theorell received the 1955 Nobel Prize in medicine and physiology. Kuhn, however, was unfortunate enough to be selected for his prize shortly after Austria had been absorbed by Nazi Germany, and (like Gerhard Domagk) was compelled to refuse it.
Riboflavin was synthesized independently by the Swiss chemist Paul Karrer. For this and other work on vitamins, Karrer was a
warded a share of the 1937 Nobel Prize in chemistry. (He shared it with the English chemist Walter Norman Haworth, who had worked on the ring structure of carbohydrate molecules.)
In 1937, the German biochemists Karl Heinrich Adolf Lohmann and P. Schuster discovered an important coenzyme that contains thiamine as part of its structure. Through the 1940s other connections were found between B vitamins and coenzymes. Pyridoxine, pantothenic acid, folic acid, biotin—each in turn was found to be tied to one or more groups of enzymes.
The vitamins beautifully illustrate the economy of the human body’s chemical machinery. The human cell can dispense with making them because they serve only one special function, and the cell can take the reasonable risk of finding the necessary supply in the diet. There are many other vital substances that the body needs only in trace amounts but must make for itself. ATP, for instance, is formed from much the same building blocks that make up the indispensable nucleic acids. It is inconceivable that any organism could lose any enzyme necessary for nucleic-acid synthesis and remain alive, for nucleic acid is needed in such quantities that the organism dare not trust to the diet for its supply of the necessary building blocks. And the ability to make nucleic acid automatically implies the ability to make ATP. Consequently, no organism is known that is incapable of manufacturing its own ATP, and in all probability no such organism will ever be found.
To make such special products as vitamins would be like setting up a special machine next to an assembly line to turn out nuts and bolts for the automobiles. The nuts and bolts can be obtained more efficiently from a parts supplier, without any loss to the apparatus for assembling the automobiles; by the same token the organism can obtain vitamins in its diet, with a saving in space and material.
The vitamins illustrate another important fact of life. As far as is known, all living cells require the B vitamins. The coenzymes are an essential part of the cell machinery of every cell alive—plant, animal, or bacterial. Whether the cell gets the B vitamins from its diet or makes them itself, it must have them if it is to live and grow. This universal need for a particular group of substances is an impressive piece of evidence for the essential unity of all life and its descent (possibly) from a single original scrap of life formed in the primeval ocean.
VITAMIN A
While the roles of the B vitamins are now well known, the chemical functions of the other vitamins have proved rather hard nuts to crack. The only one on which any real advance has been made is vitamin A.
In 1925, the American physiologists L. S. Fridericia and E. Holm found that rats fed on a diet deficient in vitamin A had difficulty performing tasks in dim light. An analysis of their retinas showed that they were deficient in a substance called visual purple.
There are two kinds of cell in the retina of the eye—rods and cones. The rods specialize in vision in dim light, and they contain the visual purple. A shortage of visual purple therefore hampers only vision in dim light and results in what is known as night-blindness.
In 1938, the Harvard biologist George Wald began to work out the chemistry of vision in dim light. He showed that light causes visual purple, or rhodopsin, to separate into two components: the protein opsin and a nonprotein called retinene. Retinene proved to be very similar in structure to vitamin A.
The retinene always recombines with the opsin to form rhodopsin in the dark. But during its separation from opsin in the light, a small percentage of it breaks down, because it is unstable. However, the supply of retinene is replenished from vitamin A, which is converted to retinene by the removal of two hydrogen atoms with the aid of enzymes. Thus vitamin A acts as a stable reserve for retinene. If vitamin A is lacking in the diet, eventually the retinene supply and the amount of visual purple decline, and night-blindness is the result. For his work in this field, Wald shared in the 1967 Nobel Prize for medicine and physiology.
Vitamin A must have other functions as well, for a deficiency causes dryness of the mucous membranes and other symptoms which cannot very well be traced to troubles in the retina of the eye. But the other functions are still unknown.
The same has to be said about the chemical functions of vitamins C, D, E, and K.
Minerals
It is natural to suppose that the materials making up anything as wonderful as living tissue must themselves be something pretty exotic. Wonderful the proteins and nucleic acids certainly are, but it is a little humbling to realize that the elements making up the human body are as common as dirt, and the whole lot could be bought for a few dollars. (It used to be cents, but inflation has raised the price of everything.)
In the early nineteenth century, when chemists were beginning to analyze organic compounds, it became clear that living tissue is made up, in the main, of carbon, hydrogen, oxygen, and nitrogen. These four elements alone constitute about 96 percent of the weight of the human body. Then there is also a little sulfur in the body. If you burned off these five elements, you would be left with a bit of white ash, mostly the residue from the bones. The ash would be a collection of minerals.
It would not be surprising to find common salt, sodium chloride, in the ash. After all, salt is not a mere condiment to improve the taste of food—as dispensable as, say, basil, rosemary, or thyme. It is a matter of life and death. You need only taste blood to realize that salt is a basic component of the body. Herbivorous animals, which presumably lack sophistication as far as the delicacies of food preparation are concerned, will undergo much danger and privation to reach a salt lick, where they can make up the lack of salt in their diet of grass and leaves.
As early as the mid-eighteenth century, the Swedish chemist Johann Gottlieb Gahn had shown that bones are made up largely of calcium phosphate; and an Italian scientist, Vincenzo Antonio Menghini, had established that the blood contains iron. In 1847, Justus von Liebig found potassium and magnesium in the tissues. By the mid-nineteenth century, then, the mineral constituents of the body were known to include calcium, phosphorus, sodium, potassium, chlorine, magnesium, and iron. Furthermore, these are as active in life processes as any of the elements usually associated with organic compounds.
The case of iron is the clearest. If it is lacking in the diet, the blood becomes deficient in hemoglobin and transports less oxygen from the lungs to the cells. The condition is known as iron-deficiency anemia. The patient is pale for lack of the red pigment and tired for lack of oxygen.
In 1882, the English physician Sidney Ringer found that a frog heart could be kept alive and beating outside its body in a solution (called Ringer’s solution to this day) containing, among other things, sodium, potassium, and calcium in about the proportions found in the frog’s blood. Each is essential for functioning of muscle. An excess of calcium causes the muscle to lock in permanent contraction (calcium rigor) whereas an excess of potassium causes it to unlock in permanent relaxation (potassium inhibition). Calcium, moreover, is vital to blood clotting. In its absence blood would not clot, and no other element can substitute for calcium in this respect.
Of all the minerals, phosphorus was eventually discovered to perform the most varied and crucial functions in the chemical machinery of life (see chapter 13).
Calcium, a major component of bone, makes up 2 percent of the body; phosphorus, 1 percent. The other minerals I have mentioned come in smaller proportions, down to iron, which makes up only 0.004 percent of the body. (That still leaves the average adult male 1/10 ounce of iron in his tissues.) But we are not at the end of the list; there are other minerals that, though present in tissue only in barely detectable quantities, are yet essential to life.
The mere presence of an element is not necessarily significant; it may be just an impurity. In our food we take in at least traces of every element in our environment, and some small amount of each finds its way into our tissues. But elements such as titanium and nickel, for instance, contribute nothing. On the other hand, zinc is vital. How does one distinguish an essential mineral from an accidental impurity?
The best
way is to show that some necessary enzyme contains the trace element as an essential component. (Why an enzyme? Because in no other way can any trace component possibly play an important role.) In 1939, David Keilin and Thaddeus Robert Rudolph Mann of England showed that zinc is an integral part of the enzyme carbonic anhydrase. Now carbonic anhydrase is essential to the body’s handling of carbon dioxide, and the proper handling of that important waste material, in turn, is essential to life. It follows in theory that zinc is indispensable to life, and experiment shows that it actually is. Rats fed on a diet low in zinc stop growing, lose hair, suffer scaliness of the skin, and die prematurely for lack of zinc as surely as for lack of a vitamin.
In the same way it has been shown that copper, manganese, cobalt, and molybdenum are essential to animal life. Their absence from the diet gives rise to deficiency diseases. Molybdenum is a constituent of an enzyme called xanthine oxidase. The importance of molybdenum was first noticed in the 1940s in connection with plants, when soil scientists found that plants would not grow well in soils deficient in the element. It seems that molybdenum is a component of certain enzymes in soil microorganisms that catalyze the conversion of the nitrogen of the air into nitrogen-containing compounds. Plants depend on this help from microorganisms because they cannot themselves take nitrogen from the air. (This is only one of an enormous number of examples of the close interdependence of all life on our planet. The living world is a long and intricate chain which may suffer hardship or even disaster if any link is broken.)
Not all trace elements are universally essential. Boron seems to be essential in traces to plant life but not, apparently, to animals. Certain tunicates gather vanadium from sea water and use it in their oxygen-transporting compound, but few, if any, other animals require vanadium for any reason. Some elements, such as selenium and chromium, are suspected of being essential, but their exact role has not been determined.
