III. SYNTHESIS OF PURINES AND PYRIMIDINES

Figure 2



Purine synthesis. In this pathway the addition of ribose occurs prior to ring closure and phosphorylation.



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Figure 3



Involvement of the vitamins and other organic nutrients in intermediary metabolism.



nucleus, can repair itself. Occasionally, there is a missense repair and very occasionally this results

in a mutation which is random. That is, the damage and subsequent missense repair can occur

anywhere in the nuclear DNA and the resultant gene product could be one of more than a million

products encoded by the nuclear genome. In addition, this damage might occur in only a few cells

out of the many million within a given tissue or organ. Widespread damage from a single exposure

is certainly possible, but probably not very frequent. Rather, slight but continued and possible

cumulative damage is more likely. Whether degenerative diseases such as cardiovascular disease

could be due to free radical damage to lipid-carrying proteins and/or to vascular tissue has yet to

be documented. This is a very active area of nutrition research. Peroxide or free radical damage to

the nuclear genome is not as serious on an individual genomic basis as damage to the mitochondrial

genome. This genome encodes only 13 products but these products are important components of

the mitochondrial respiratory chain and ATP synthesis. The mitochondrial DNA does not have the

repair capacity of the nuclear genome. In fact, its repair capacity is quite limited. When added to

the fact that the mitochondria consume about 90% of all the oxygen associated with the cell, the



© 1998 by CRC Press LLC



Figure 4



Pyrimidine synthesis. In this pathway the pyrimidine ring is formed before it is attached to ribose and

phosphorylated.



potential for free radical damage is quite large. Fortunately, each cell has many hundreds to

thousands of mitochondria so the loss of a few has little impact on the overall health and wellbeing of the cell or organ or whole animal. Nonetheless, should wholesale destruction of the genome

occur, the results could be quite devastating. This rarely occurs.

Fortunately, there is a very active antioxidant system in place that protects against such damage.

This is described in the sections devoted to vitamin E and selenium. Some of the vitamins and

minerals play an important role in this system. Vitamin E quenches free radicals as they form via

the conversion of tocopherol to the tocopheroxyl radical, which is then converted to its quinone.

Vitamin K serves as an H+/e– donor/acceptor in its role to facilitate the carboxylation of the peptide

glutamyl residues of certain proteins to their epoxide form. Vitamin C and vitamin A are both good

H+/e– donor/acceptors in the suppression of free radical formation. Of course, indirectly, all those

vitamins that serve as coenzymes are involved as well. Shown in Figure 5 is the free radical

suppression system. Note the importance of selenium. In Unit 3, which discusses the antioxidant

function of vitamin E, it is pointed out that there is a complementary role for selenium (see Unit 7).



© 1998 by CRC Press LLC



Figure 5



Roles for micronutrients in the system for the defense against free radical damage. Various agents

can react with fatty acids to produce peroxides and superoxides. These very reactive materials are

suppressed by the system above.



Some of the antioxidant role for vitamin E could be met if there was a sufficient intake of selenium.

This mineral is important to the glutathione peroxidase enzyme which, as can be seen in Figure 5,

is an important component of the free radical suppression system. Selenium plays a role in both

the synthesis of this enzyme and as a required cofactor. As will be discussed in the units on minerals,

several of these have roles in gene expression and these roles have overall importance to the

physiological function of the body.

SUPPLEMENTAL READINGS

Atchison, M.L. 1988. Enhancers: mechanisms of action and cell specificity, Annu. Rev. Cell Biol., 4:127.

Berdanier, C.D. and Hargrove, J.L., Eds. 1993. Nutrition and Gene Expression, CRC Press, Boca Raton, FL, 579 pgs.

Chien, K.R. 1993. Molecular advances in cardiovascular biology, Science, 260:916-917.

Combs, G.F. Jr. 1992. The Vitamins, Academic Press, New York, 528 pgs.

Demonacos, C.V., Karayanni, N., Hatzoglou, E., Tsiriyrotes, C., Spandidos, D.A., and Sekerio, C.E. 1996.

Mitochondrial genes as sites of primary action of steroid hormones, Steroids, 61:226-232.

Derman, E. 1982. Transcriptional control in the production of liver specific mRNAs, Cell, 23:731-740.

Evans, R.M. 1988. The steroid and thyroid hormone superfamily, Science, 240:889-891.

Freedman, L.P. and Luisi, B.F. 1993. On the mechanism of DNA binding by nuclear hormone receptors: a

structural and functional perspective, J. Cell. Biochem., 51:140-150.

Johnson, P.F., Sterneck, E., and Williams, S.C. 1993. Activation domains of transcriptional regulatory proteins,

J. Nutr. Biochem., 4:386-398.



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Jump, D.B., Lepar, G.J., and MacDonald, O.A. 1993. Retinoic acid regulation of gene expression in adipocytes.

In Berdanier, C.D. and Hargrove, J.L., Eds., Nutrition and Gene Expression, CRC Press, Boca Raton,

FL, pp. 431-454.

Maniatis, T., Goodbourn, S., and Fischer, J.A. 1987. Regulation of inducible and tissue specific gene expression,

Science, 236:1237.

Semenza, G.L. 1994. Transcriptional regulation of gene expression: mechanisms and pathophysiology, Hum.

Mutat., 3:180-199.

Vogt, J.G. and Vogt, D. 1990. Biochemistry, John Wiley & Sons, New York, pg. 771-986; 1086-1178.

Yamamoto, K. 1985. Steroid receptor regulated transcription of specific genes and gene networks, Annu. Rev.

Genet., 19:209.



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UNIT



3



Fat-Soluble Vitamins

TABLE OF CONTENTS

I. Vitamin A

A. Structure and Nomenclature

B. Chemical Properties

C. Biopotency

D. Sources

E. Metabolism of Vitamin A

1. Absorption

2. Transport

3. Degradation and Excretion

F. Functions of Vitamin A

1. Protein Synthesis

2. Reproduction and Growth

3. Vision

G. Hypervitaminosis A

H. Recommended Dietary Allowances

II. Vitamin D

A. Overview

B. Structure and Nomenclature

C. Physical and Chemical Properties

D. Biopotency

E. Methods of Assay

F. International Units (IU)

G. Metabolism of Vitamin D

1. Absorption

2. Transport

3. Metabolism

4. Function

a. Regulation of Serum Calcium Levels

b. Mode of Action at the Genomic Level

H. Vitamin D Deficiency

I. Hypervitaminosis D

J. Recommended Dietary Allowances



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III. Vitamin E

A. Overview

B. Structure and Nomenclature

C. International Units and Methods of Analysis

D. Chemical and Physical Properties

E. Sources

F. Metabolism

1. Absorption and Transport

2. Intracellular Transport and Storage

3. Catabolism and Excretion

4. Function

G. Hypervitaminosis E

H. Deficiency

I. Recommended Dietary Allowance

IV. Vitamin K

A. Overview

B. Structure and Nomenclature

C. Biopotency

D. Chemical and Physical Properties

E. Chemical Assays

F. Bioassays

G. Biosynthesis

H. Antagonists, Antivitamins

I. Sources

J. Absorption

K. Metabolism and Function

L. Deficiency

M. Recommended Dietary Allowance

Supplemental Readings



There are four vitamins that are soluble only in fat solvents and not in water. As such, these

vitamins are found in lipid extracts of tissues and foods. They are named alphabetically in the order

of their discovery.



I. VITAMIN A

Vitamin A was the first vitamin identified as an essential micronutrient needed by humans.

Although it has been recognized as a chemical entity for about 60 years, foods rich in this vitamin

have long been prescribed as treatment for night blindness. Ancient Egyptian physicians recommended the consumption of ox or chicken liver for people unable to see at night. In India it was

recognized that inadequate diets were related to night blindness and, in France, that diets consisting

of sugar, starch, olive oil, and wheat gluten fed to animals resulted in ulcerated corneas. Mori, in

Japan, reported on the curative power of cod-liver oil in the treatment of conjunctivitis and, later,

Hopkins in the U.S., reported on the importance of whole milk in such treatments. During the

1920s, Osborne and Mendel at Yale and McCollum’s group in Wisconsin identified a substance in

cod-liver oil, egg yolk, and butterfat that cured night blindness and which was essential for normal

growth. They called this substance “fat soluble A.”



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Table 1



Nomenclature of Major Compounds in the Vitamin A Group



Recommended Name



Synonyms



Retinol

Retinal

Retinoic acid

3-Dehydroretinol

3-Dehydroretinal

3-Dehydroretinoic acid

Anhydroretinol

Retro Retinol

5,6-Epoxyretinol

Retinyl palmitate

Retinyl acetate

Retinyl β-glucuronide

11-cis-Retinaldehyde

4-Ketoretinol

Retinyl phosphate

β-Carotene

α-Carotene

γ-Carotene



Vitamin A alcohol

Vitamin A aldehyde, retinene, retinaldehyde

Vitamin A acid

Vitamin A2 (alcohol)

Vitamin A2 aldehyde; retinene2

Vitamin A2 acid

Anhydrovitamin A

Rehydrovitamin A

5,6-Epoxyvitamin A alcohol

Vitamin A palmitate

Vitamin A acetate

Vitamin A acid β-glucuronide

11-cis or Neo b vitamin A aldehyde

4-Keto vitamin A alcohol

Vitamin A phosphate

Provitamin A

Provitamin A

Provitamin A



A. Structure and Nomenclature

Vitamin A is not a single compound. It exists in several forms and is found in a variety of foods

such as liver and highly colored vegetables. The IUPAC-IUB Commission on Biochemical Nomenclature has proposed the following rules for naming the compounds having vitamin A activity. The

parent substance, all-trans vitamin A alcohol, is designated “all-trans retinol.” Derivatives of this

compound are named accordingly. In Table 1 are listed the major vitamin A compounds.

In foods of animal origin the vitamin usually occurs as the alcohol (retinol). However, it can

also occur as an aldehyde (retinal) or as an acid (retinoic acid). In foods of plant origin, the precursor

to the vitamin is associated with the plant pigments and is a member of the carotene family of

compounds. These latter compounds can be converted to vitamin A in the animal body and are

known as provitamins. Of the carotenes, β-carotene is the most potent.

Figure 1 gives the structure of vitamin A as all-trans retinol. Some of the biologically important

compounds having vitamin A activity are shown in Figure 2.

Note that all of these compounds have a β-ionone ring to which an isoprenoid chain is attached.

This structure is essential if a compound is to have vitamin activity. If any substitutions to the chain

or ring occur, then the activity of the compound as vitamin A is reduced. For example, substitution

of methyl groups for the hydrogen on carbon 15 of the side chain results in a derivative that has



Figure 1



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Structure of all-trans retinol.



Figure 2



Structures of Vitamin A compounds.



no vitamin activity. However, the preparation of a methyl ester or other esters at carbon 15 results

in a very stable compound with full vitamin activity. In addition to improving the chemical stability

of the compound, these ester forms confer an improved solubility in food oils. These vitamin ester

forms are frequently used in food products for vitamin enrichment.

If the side chain is lengthened or shortened, vitamin activity is lost. Activity is also reduced if

the unsaturated bonds are converted to saturated bonds or if the side chain is isomerized. Oxidation

of the β-ionone ring and/or removal of its methyl groups likewise reduces vitamin activity. Some

of these substituted or isomerized forms are potent therapeutic agents. For example, 13-cis retinoic

acid has been used in the treatment of certain kinds of cancer. Other analogs, notably the fluoro

and chloro derivatives, have been synthesized with the hope of providing chemotherapeutic agents

for the treatment of certain skin diseases and cancer.



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Figure 3



Structures of carotenes having vitamin A activity.



The provitamin A group consists of members of the carotene family. Shown in Figure 3 are the

structures of some of these compounds. Also shown are structures of related compounds that,

although highly colored, have little potential as a precursor of retinol. More than 600 members of

the carotenoid family of pigments exist. However, only 50 or so can be converted (or degraded)

into components that have vitamin activity. All these compounds have many conjugated double

bonds and thus each can form a variety of geometric isomers. β-Carotene, for example, can assume

a cis or a trans configuration at each of its double bonds and in theory could have 272 isomeric

forms. The asymmetric carotene, α-carotene, can, in theory, appear in 512 forms. The vitamin A



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Table 2



Carotenoids With Vitamin A Activity



Compound

β-Carotene

α-Carotene

γ-Carotene

Cryptoxanthin

Lycopene

Zeaxanthin

Xanthophyll

a



a



Relative Potency

100

53

43

57

0

0

0



Reference compound for subsequent compounds. Note that β-carotene compared to alltrans retinol is only half as active.



activity of the provitamin members of the carotene family is variable. Theoretically, β-carotene

should provide two molecules of retinol. However, in living systems this does not always happen.

The β-carotene content of food varies with the growing conditions and the post-harvest storage of

the food. In addition, the digestibility of the food affects the availability of the vitamin. Even when

fully available, β-carotene and other provitamin A compounds may not be absorbed efficiently and,

further, the enzymes responsible for cleaving the β-carotene into two equal parts may not be active.

In general, the β-carotene molecule will provide about 50% of its quantity as vitamin A. During

its cleavage by the enzyme β-carotenoid 15,15′-dioxygenase, there is some oxidative conversion

of the cleavage product to retinal and some oxidation to retinoic acid. This retinoic acid is rapidly

excreted in the urine. Other carotenes are less potent than β-carotene, due not only to a decrease

in their absorption, but also to their chemical structures, which do not meet the requirements

described above for vitamin activity. These compounds are listed in Table 2. Note that some of

these compounds, i.e., xanthophylls and lycopenes, have no vitamin activity even though they are

highly colored and are related chemically to β-carotene.

B. Chemical Properties

Through the careful work of Karrer and associates, the structures of both β-carotene and alltrans retinol were determined. It was only after this work was completed that it was realized that

β-carotene was a precursor for retinol.

With the structures now known, the next step was the crystallization of the compounds. This

was accomplished for vitamin A from fish liver by Holmes and Corbet. Ten years later, in 1947,

Arens and van Drop and also Isler et al. were able to synthesize pure all-trans retinol. The chemical

synthesis of β-carotene was achieved shortly thereafter. With the crystallization, structure identification, and synthesis came the understanding of the physical and chemical properties of these

compounds and the development of techniques to measure their presence in food. All-trans retinol

is a nearly colorless oil and is soluble in such fat solvents as ether, ethanol, chloroform, and

methanol. While fairly stable to the moderate heat needed to cook foods, it is unstable to very high

heat, to light, and to oxidation by oxidizing agents. Alpha-tocopherol or its acetate (vitamin E),

through its role as an antioxidant, prevents some of the destruction of retinol.

The above properties of vitamin A allow for the removal of the vitamin from foods by fatsolvent extraction and its subsequent determination using agents such as antimony trichloride (the

Carr-Price reaction), which produce a blue color. The intensity of the color is directly proportional

to the amount of retinol in the material being analyzed. More recently, the development of high

resolution (high pressure) chromatography (HPLC) has made possible the separation and quantification of each of the vitamers A. This technique is very sensitive and can detect microgram

amounts of the vitamers. β-Carotene is also soluble in fat solvents such as acetone or ethanol. It

is bright yellow in color and it, too, is stable to moderate heat but unstable to light or oxidation.



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