A number of sterols have vitamin D activity, that is to say they prevent the occurrence of rickets in higher animals. The chemistry of the sterols is very complicated, and books on physiological chemistry may be consulted for structural formulae, etc. When the common sterol of fungi, ergosterol, is irradiated with ultraviolet rays, another sterol, calciferol, is produced. This is commonly referred to as vitamin D 2. (There is no D 1, for the original use of the term was a misnomer.) D 3 is 7-dehydrocholesterol. Numerous other antirachitic (rickets-preventing) substances have also been described. Whether or not a given sterol or sterol derivative has antirachitic action can only be determined by actual test.
Bacteria do not seem to require sterols. Some species of protozoan flagellates do; also Entamoeba histolytica, the ameba that causes human dysentery. Insects, in so far as they have been studied, require sterols, but they seem to need cholesterol, a cholesterol derivative, or a plant sterol; calciferol cannot usually be substituted for these.
In mammals and birds, vitamin D causes an increase in the deposition of calcium and phosphorus in the bones. This is apparently due, in part at least, to an increased absorption through the intestine. Using radioactive isotopes, Greenberg found that vitamin D promotes the absorption of calcium and strontium from the digestive tract. Sterols may act in the same way in promoting the passage of calcium through the walls of the insect intestine. Moreover, they may be concerned with the entrance of calcium into cells. This aspect of the subject has never been investigated.
Wednesday, December 5, 2007
Vitamin A is a yellow viscous oil
Vitamin A is a yellow viscous oil. Chemically, it is related to carotene, the common yellow pigment of plants. The generally accepted formula follows:
When carotene is eaten, it is converted to vitamin A. Because of this, it is sometimes called a provitamin. Another provitamin of vitamin A is kitol, a dihydric alcohol found in whale liver oil. The conversion of carotene to vitamin A occurs in the intestines of higher animals. The vitamin is then stored in the liver. The livers of polar bears and Arctic foxes are so rich in vitamin A that they are toxic. The toxicity is an example of what is known as hypervitaminosis.
Vitamin A has been synthesized in several different laboratories. There are really two forms of the vitamin. One form (A1) constitutes most of the vitamin content of the livers of salt water fishes. In the livers of fresh water fishes vitamin A 2 occurs. The 2 substances give slightly different colors with antimony trichloride and can be distinguished spectroscopically.
In higher animals, vitamin A is important for growth. When rats are deprived of vitamin A, the cornea of the eye becomes horny. This condition is known as xerophthalmia. It can occur also in children. Not only the corneal epithelium, but other types of epithelia are also adversely affected in vitamin A deficiency; this may lead to lessened resistance to infection. Vitamin A has a relation to vision. In man, vitamin A deficiency causes night blindness, but the effect is not as readily produced as formerly supposed.
In so far as they have been investigated, invertebrates, with the apparent exception of the snail Helix, do not seem to require vitamin A. Aside from its relation to vision, little is known as to the reason why vitamin A is necessary. Its presence in cells can be recognized with a fluorescence microscope because of the fact that in ultraviolet light it gives a green fluorescence.
When carotene is eaten, it is converted to vitamin A. Because of this, it is sometimes called a provitamin. Another provitamin of vitamin A is kitol, a dihydric alcohol found in whale liver oil. The conversion of carotene to vitamin A occurs in the intestines of higher animals. The vitamin is then stored in the liver. The livers of polar bears and Arctic foxes are so rich in vitamin A that they are toxic. The toxicity is an example of what is known as hypervitaminosis.
Vitamin A has been synthesized in several different laboratories. There are really two forms of the vitamin. One form (A1) constitutes most of the vitamin content of the livers of salt water fishes. In the livers of fresh water fishes vitamin A 2 occurs. The 2 substances give slightly different colors with antimony trichloride and can be distinguished spectroscopically.
In higher animals, vitamin A is important for growth. When rats are deprived of vitamin A, the cornea of the eye becomes horny. This condition is known as xerophthalmia. It can occur also in children. Not only the corneal epithelium, but other types of epithelia are also adversely affected in vitamin A deficiency; this may lead to lessened resistance to infection. Vitamin A has a relation to vision. In man, vitamin A deficiency causes night blindness, but the effect is not as readily produced as formerly supposed.
In so far as they have been investigated, invertebrates, with the apparent exception of the snail Helix, do not seem to require vitamin A. Aside from its relation to vision, little is known as to the reason why vitamin A is necessary. Its presence in cells can be recognized with a fluorescence microscope because of the fact that in ultraviolet light it gives a green fluorescence.
Vitamins, riboflavin, nicotinic acid, pantothenic acid form
Most of the work on vitamins has been stimulated by the practical needs of medicine and industry. The physician and the drug manufacturer are not ordinarily interested in the vitamin requirements of lower organisms, unless these organisms can conveniently be used in vitamin assay. Thus, as might be expected, there are vast gaps in our knowledge of the vitamin requirements of many large groups of animals. We know almost nothing of the vitamin needs of metazoan invertebrates other than insects. And we know very little about the vitamins necessary for fish, amphibia and reptiles. A wide field is open for such study, and many facts of interest may well be discovered. Some vitamins are required by practically all living organisms from bacteria to man; others are needed by only some forms and not others.
There has been much interest in the avitaminoses, that is to say the diseases caused by the lack of specific vitamins. Our knowledge in this field is almost entirely confined to higher animals. Thus we know that when the vitamins D are omitted from the diet, a child will develop rickets and so will a pig or a rabbit, a dog or a chicken. Accordingly, the vitamins D have been called the antirickets vitamins. But animals without bones such as insects or protozoa may also require vitamin D. From a knowledge of the reasons why this is so, we may be able to obtain additional information as to the way that vitamins function.
Much as is known about the vitamins, their chemistry, their occurrence in various foods, the diseases that follow their absence from the diet, there is still almost complete ignorance as to the exact nature of their action. A beginning has been made in the recognition of the fact that some vitamins such as riboflavin, nicotinic acid, and pantothenic acid form essential parts of some enzyme systems. We know also the end results of many types of vitamin deficiency. But to say that a vitamin is necessary for growth or that it prevents anemia tells us very little as to what it really does or why it is necessary. If we were to know how certain vitamins influence growth or prevent cell deterioration in the epithelial or nervous system, we would have basic information of great importance. Such information would be useful not only in the better understanding of the vitamins but also in the elucidation of various basic vital processes. The final interpretation of vitamin action must lie in an understanding of how the vitamins affect the cell protoplasm.
It is this aspect of the subject that will be stressed in the following discussion of individual vitamins. The general physiologist is not primarily interested in the relation of vitamins to human welfare, the foods a man should eat, or the benefits to be derived from adding this or that vitamin to the diet. Such information can be obtained in books on physiological chemistry, nutrition, or medicine. In discussing the individual vitamins, we shall not proceed in alphabetical order. This order has no other basis than the chronology of discovery. We shall consider first the fat-soluble and then the water-soluble vitamins. In general, especially in the case of the water-soluble vitamins, true chemical names have tended to supplant letters and subscripts. But in some instances, several chemical substances can and normally do supply a given vitamin need. In these instances the alphabetical name is sometimes more convenient.
There has been much interest in the avitaminoses, that is to say the diseases caused by the lack of specific vitamins. Our knowledge in this field is almost entirely confined to higher animals. Thus we know that when the vitamins D are omitted from the diet, a child will develop rickets and so will a pig or a rabbit, a dog or a chicken. Accordingly, the vitamins D have been called the antirickets vitamins. But animals without bones such as insects or protozoa may also require vitamin D. From a knowledge of the reasons why this is so, we may be able to obtain additional information as to the way that vitamins function.
Much as is known about the vitamins, their chemistry, their occurrence in various foods, the diseases that follow their absence from the diet, there is still almost complete ignorance as to the exact nature of their action. A beginning has been made in the recognition of the fact that some vitamins such as riboflavin, nicotinic acid, and pantothenic acid form essential parts of some enzyme systems. We know also the end results of many types of vitamin deficiency. But to say that a vitamin is necessary for growth or that it prevents anemia tells us very little as to what it really does or why it is necessary. If we were to know how certain vitamins influence growth or prevent cell deterioration in the epithelial or nervous system, we would have basic information of great importance. Such information would be useful not only in the better understanding of the vitamins but also in the elucidation of various basic vital processes. The final interpretation of vitamin action must lie in an understanding of how the vitamins affect the cell protoplasm.
It is this aspect of the subject that will be stressed in the following discussion of individual vitamins. The general physiologist is not primarily interested in the relation of vitamins to human welfare, the foods a man should eat, or the benefits to be derived from adding this or that vitamin to the diet. Such information can be obtained in books on physiological chemistry, nutrition, or medicine. In discussing the individual vitamins, we shall not proceed in alphabetical order. This order has no other basis than the chronology of discovery. We shall consider first the fat-soluble and then the water-soluble vitamins. In general, especially in the case of the water-soluble vitamins, true chemical names have tended to supplant letters and subscripts. But in some instances, several chemical substances can and normally do supply a given vitamin need. In these instances the alphabetical name is sometimes more convenient.
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