IV.Other Compounds with Biologic Activity

B. Pyrroloquinoline Quinone

Pyrroloquinoline quinone, sometimes called methoxatin, serves as a cofactor for certain methane-forming bacteria. The structure of pyrroloquinoline quinone is shown in Figure 11. It is a

tricarboxylic acid with a fused heterocylic (o-quinone) ring system. Its C-5 carbonyl group is very

reactive towards nucleophiles and it is this action that allows this substance to function in metabolic

reactions. At present, information is scarce with respect to its food sources and essentiality. Likely,

it can be endogenously synthesized in organisms that can use it, but whether this compound meets

the definition of a vitamin has yet to be established for any species.



Figure 11



Structure of pyrroloquinoline quinone.



C. Ubiquinone

Ubiquinone is an essential component of the mitochondrial respiratory chain and it has been

shown to be synthesized endogenously. Actually, ubiquinone is one of a group of related substances.

They are a group of tetra-substituted 1,4-benzoquinone derivatives with isoprenoid side chains of

various lengths. The biochemists have termed these substances as coenzyme Q. They function as

reversible donors/acceptors of reducing equivalents from NAD, passing electrons from flavoproteins

to the cytochromes via cytochrome b5. Because the ubiquinones can be synthesized endogenously

in large amounts even when diets lacking the ubiquinones are offered, these substances fail to meet

the definition of the word vitamin.

D. Orotic Acid

Orotic acid is an important metabolic intermediate in the synthesis of pyrimidines. Its structure

is shown in Figure 12. It is synthesized endogenously from N-carbamyl phosphate by dehydration

and oxidation. It is another of those substances that fails to meet the definition of a vitamin. When

used as a dietary supplement (0.1% of diet) orotic acid had deleterious effects. It resulted in a fatty,

enlarged liver and increased the levels of hepatic uracil, presumably due to an influence on

pyrimidine synthesis. Orotic acid-induced fatty liver is accompanied by falling plasma cholesterol

levels and falling activity of HMG CoA reductase. This hepatic enzyme is greatly influenced by

its product (cholesterol) and so if the liver does not export it, it feeds back to inhibit synthesis.



Figure 12



© 1998 by CRC Press LLC



Structure of orotic acid.



E. Para-Aminobenzoic Acid (PABA)

p-Aminobenzoic acid is an essential growth factor for a number of bacteria which use it as a

precursor of folacin. Animals, however, cannot synthesize folacin so p-aminobenzoic acid does not

meet the definition of a vitamin. Its involvement in folacin use is discussed in Unit 4, Section VIII.

F. Lipoic Acid

Lipoic acid is essential to oxidative decarboxylation of α-keto acids. It participates in the

pyruvate dehydrogenase complex (see Figures 7 and 8 in Unit 4 on pages 84 and 85). This is a

multienzyme complex where lipoic acid is linked to the ε-amino group of a lysine residue from

the enzyme dihydrolipoyl transacetylase. As the lipoamide undergoes reversible acylation/deacylation it transfers acyl groups to coenzyme A and results in reversible ring opening/closing in the

oxidation of the α-keto acid. Lipoic acid can be synthesized endogenously in amounts sufficient

to meet the need. Therefore, it does not meet the definition of the word vitamin.

G. Bioflavinoids

Bioflavinoids are a group of compounds that are presumed by some to augment the action of

ascorbic acid in the prevention of scurvy . They are mixtures of phenolic derivatives of 2-phenyl-1,4benzopyrone. The bioflavinoids are present in a large variety of foods and were first isolated by SzentGyorgy from lemon juice and red peppers. More than 800 different flavinoids have been found. They

occur naturally as glycosides which are hydrolyzed by the gut flora prior to absorption. No single

unique deficiency syndrome has been found or reported in animals fed a bioflavinoid-free diet. Furthermore, there has not been a unique response to the addition of bioflavinoids to the diet. On this

basis, despite their activity as potentiators of ascorbic acid, bioflavinoids can not be considered vitamins.

H. Pseudovitamins

The term vitamin was coined many years ago to designate those organic dietary compounds

that cannot be made endogenously but are needed in small amounts to sustain normal growth and

metabolism throughout life. This term, developed by nutritional biochemists and physiologists, has

been used commercially as well as scientifically. Unfortunately, there have been (and continue to

be) commercial uses of the term that are inappropriate. Hence we have compounds such as laetrile

(an extract from fruit pits), pangamic acid or vitamin B15, and methylsulfonium salts of methionine

called vitamin U, and gerovital. Gerovital, also called vitamin H3 or CH3, is a buffered solution of

procaine hydrochloride (Novocaine™), a local anesthetic. To be effective as an anesthetic it must

be injected. Gerovital is advertised as an antiaging substance but claims of its effects have not been

substantiated. The advertised use of methylsulfonic salts of methionine to prevent peptic ulcers

likewise has not been substantiated. Pangamic acid, another of these pseudovitamins, is not a

chemically defined substance. Rather it is a mixture of compounds. Of the materials labeled

pangamic acid, one of the compounds is N,N-diisopropylamine dichloroacetate. This is a drug, and

when administered to normal rats it caused death preceded by respiratory failure, extreme hypotension, and hypothermia. There is no evidence of essentiality for pangamic acid.

Lastly, laetrile is included in this list of pseudovitamins. This compound has been the focus of

a number of litigations due to the claim by its suppliers that it can serve as an anticancer drug.

This claim was investigated and found wanting by the U.S. Food and Drug Administration (FDA).

The term laetrile has several synonyms: amygdalin and vitamin B17. Amygdalin is a β-cyanogenic

glucoside and is a major constituent in preparations named laetrile. Amygdalin is a substance found

in peach pits, apricot pits, and the kernels and seeds of many fruits. Neither the U.S. FDA or the

Canadian equivalent of this regulatory agency recognize laetrile as a vitamin.



© 1998 by CRC Press LLC



SUPPLEMENTAL READINGS

Choline

Chan, M.M. 1991. Choline. In: Handbook of Vitamins, Machlin, L.J., Ed., Marcel Dekker, New York, pp. 537-556.

Mehlman, M.A., Therriault, D.G., and Tobin, R.B. 1971. Carnitine-14C metabolism in choline-deficient, alloxan

diabetic choline deficient and insulin treated rats, Metabolism, 20:100-107.

Zeisel, S.H. 1990. Choline deficiency, J. Nutr. Biochem., 1:332-349.



Carnitine

Borum, P. 1991. Carnitine. In: Handbook of Vitamins, Machlin, L.J., Ed., Marcel Dekker, New York, pp. 557-563.



Inositol

Best, L. and Malaise, W.J. 1983. Phospholipids and islet function, Diabetologia, 25:299-305.

Farese, R.V. 1990. Lipid derived mediators in insulin action, Proc. Soc. Exp. Biol. Med., 312:324.

Flier, J.S. and Underhill, L.H. 1990. Sorbitol, phosphoinositides, and sodium-potassium-ATPase in the pathogenesis of diabetic complications, N. Engl. J. Med., 316:599-606.

Han, O., Failla, M., Hill, A.D., Morris, E.R., and Smith, J.C. 1994. Inositol phosphates inhibit uptake and

transport of iron and zinc by a human intestinal cell line, J. Nutr., 124:580-587.

Holub, B.J. 1986. Metabolism and function of myoinositol and inositol phospholipids, Annu. Rev. Nutr., 6:563-597.

Martin, T.F.J. 1991. Receptor regulation of phosphoinositidase C, Pharmacol. Ther., 49:329-345.

Olgemoller, B., Schwaabe, S., Schleicher, E.D., and Gerbitz, K.D. 1993. Upregulation of myoinositol transport

compensates for competitive inhibition by glucose, Diabetes, 42:1119-1125.

Saltiel, A.R. 1990. Signal transduction in insulin action, J. Nutr. Biochem., 1:180-188.



© 1998 by CRC Press LLC



UNIT



6



Minerals and Living Systems

TABLE OF CONTENTS

I.

II.

III.

IV.



Overview

Bioavailability

Apparent Absorption

The Periodic Table and Mineral Function

A. Lewis Acids and Bases

V. Mineral Absorption as Related to RDA

Supplemental Readings



I. OVERVIEW

Minerals are found in every cell, tissue, and organ. They are important constituents of essential

molecules such as thyroxine, hemoglobin, and vitamin B12. They serve as critical cofactors in

numerous enzymatic reactions, and form the hard mineral complexes that comprise bone. Minerals

serve in the maintenance of pH, osmotic pressure, nerve conductance, muscle contraction, energy

production, and in almost every aspect of life. While minerals are essential to normal health and

development, they can also be toxic. The body defends itself against such toxicity through a variety

of mechanisms. For the microminerals, the protective mechanisms center primarily around the

regulation of uptake by the mucosal cells of the intestine. Many of the minerals are poorly absorbed

and this, in itself, can be viewed as a protection against lethality. This protection is absolutely

essential because in many of these same instances the means for excretion is very inefficient, if it

exists at all.

Optimal intake is a balance between an intake that is too little and one that is toxic. With some

minerals, the range of intake for optimal benefit is very large; for others it is quite small. This is

illustrated in Figure 1 that arbitrarily plots a generic function against an intake of a mineral upon

which that function depends. This plot has a typical bell-shaped curve with an optimal range in

the middle. Almost any mineral function can be plotted in this way. In hemoglobin synthesis, too

much iron results in a condition known as hemosiderosis; too little iron results in anemia. Other

mineral-related functions likewise can be demonstrated, with the caveat that the body protects itself

from excess intake through a reduction in mineral absorption, through deposition in the mineral

apatite of bone and through a variety of excretions such as bile, urine, sweat, expired air, hair, and

desquamated epithelial cells. Some of these excrements are not usually considered important

pathways of excretion, but under toxic conditions they become avenues of loss of the excess mineral.



© 1998 by CRC Press LLC



Figure 1



Dependence of biological function on intake of a mineral.



The illustration in Figure 1 is indeed quite simplistic of the need for a given mineral. Just as was

discussed in the vitamin units, there are numerous interactions that affect mineral uptake and use.

The ratio of calcium to phosphorus, the ratio of iron to copper and to zinc, the ratio of calcium to

magnesium, and other factors both mineral and nonmineral affect the mineral status. Some of these

interactions are mutually beneficial while others are antagonistic. Most of these interactions occur

at the level of the gut in that most are concerned with mineral absorption. For example, zinc

absorption is impaired by high iron intakes; high zinc intake impairs copper absorption. Molybdenum and sulfur antagonize copper, tungsten interferes with molybdenum, and so forth. These

interactions are itemized in the individual mineral sections. These antagonisms contribute to the

relative inefficiency of absorption of minerals that are poorly absorbed and just as poorly lost once

absorbed.



II. BIOAVAILABILITY

One concept that nutritionists have developed relates not only to absorption efficiency but also

to mineral interactions at the site for absorption and the site of use. This concept is that of

bioavailability. Bioavailability is defined as the percent of the consumed mineral that enters via the

intestinal absorptive cell, the enterocyte, and is used for its intended purpose. Thus, bioavailability

includes not only how much of a consumed mineral enters the body, but also how much is retained

and available for use. An example might be the comparison of iron from red meat to the same

amount of iron in spinach. Iron from red meat has a greater bioavailability than iron from spinach

because it is an integral component of the protein heme. It is this form (heme iron) that is efficiently

absorbed and used. The iron in the spinach is bound to an oxalate, and, even though some of this

iron can be released from the oxalate, it is in the ferric state and poorly absorbed.



III. APPARENT ABSORPTION

There is another term referring to absorption that is frequently used. That is the term “apparent

absorption”. This term refers to the difference between the amount of mineral consumed and that

which appears in the feces. Some minerals are recirculated via the bile while others are not. This

recirculation, especially in a poorly absorbed mineral, can contribute to the mineral content of the

feces, but there is no correction for the biliary contribution to the fecal mineral content. The term

“apparent absorption” refers only to the difference between intake and fecal excretion.



© 1998 by CRC Press LLC



Table 1



Periodic Table of the Elements



Group



I



II



III



IV



V



VI



VII



0



Period

1



H

1

2

Li

Be

3

4

3

Na Mg

11 12

4

K

Ca

19 20

5

Rb Sr

37 38

6

Cs Ba

55 56

7

Fr Ra

87 88

*Lanthanide series

**Actinide series



Ti

22

Zr

40

Hf

72



V

23

Nb

41

Ta

73



Cr

24

Mo

42

W

74



Mn

25

Tc

43

Re

75



Fe

26

Ru

44

Os

76



Co

27

Rh

45

Ir

77



Ni

28

Pd

46

Pt

78



Cu

29

Ag

47

Au

79



Zn

30

Cd

48

Hg

80



B

5

Al

13

Ga

31

In

49

Ti

81



La

57

Ac

89



Ce

58

Th

90



Pr Nd

59 60

Pm U

91 92



Pm

61

Np

93



Sm

62

Pu

94



Eu Gd

63 64

Am Cm

95 96



Tb

65

Bk

97



Dy

66

Cf

98



Transition elements

Sc

21

Y

39

*

57–71

**

89–102



C

6

Si

14

Ge

32

Sn

50

Pb

82



N

7

P

15

As

33

Sb

51

Bi

83



O

8

S

16

Se

34

Te

52

Po

84



F

9

Cl

17

Br

35

I

53

At

85



He

2

Ne

10

Ar

18

Kr

36

Xe

54

Rn

86



Ho

67

Es

99



Er

68

Fm

100



Tm

69

Md

101



Yb

70



Lu

71



IV. THE PERIODIC TABLE AND MINERAL FUNCTION

Of the 109 known elements in the periodic table (Table 1) 30 are essential to life — 19 of these

are trace elements and, of these, 12 are transition elements. Transition elements are those which

have more than one charged state. For example, iron can exist as the ferrous ion (Fe++) and the ferric

ion (Fe+++), while chromium has several oxidation states, as does copper. However, biological systems

usually use only one of these states. The multivalent characteristic of iron is unique in that it allows

it to serve as an oxygen carrier in hemoglobin or as an hydrogen carrier in enzymatic reactions using

an iron-sulfur center within a large structure. Most transition elements are not this variable.

Because of their ionic nature, minerals can form electrovalent bonds with a variety of substances.

Although ingested as salts, minerals ionize to their component parts and it is the resultant ions that

are absorbed, used, stored, or excreted. For some ions there are very efficient retainment cycles.

Sodium, potassium, chloride, calcium, and phosphorus fall into this category. For sodium, potassium, and chloride, conservation is energy driven via the sodium-potassium ATPase. There are

several ATPases that serve in this role. The Ca2+Mg2+ATPase of the mitochondrial membrane works

to optimize the ion content of this organelle. These ATPases are proteins and illustrate a further

mechanism of mineral metabolism. As ions, minerals react with charged amino acid residues of

intact proteins and peptides. Table 2 provides a list of minerals and the amino acids with which

they react. Depending on their valence state these electrovalent bonds can form very strong,

moderate, or very weak associations. The marginally charged ion (either an electron acceptor or

an electron donor) will be less strongly attracted to its opposite number than will an ion with a

strong charge.

Table 2



Mineral-Amino Acid Interactions



Minerals

Calcium

Magnesium

Copper

Selenium

Zinc



© 1998 by CRC Press LLC



Amino Acid

Serine, carboxylated glutamic acid (GLA)

Tyrosine, sulfur-containing amino acids

Histidine

Methionine, cysteine

Cysteine, histidine



Table 3



Hard and Soft Acids and Bases: Some Properties that Can Be Used as Guidelines

for Classifying Species



Hart

Low

High

Large

Small

Ionic,

electrostatic

Few and not

easily

excited



ACID

(Electron acceptor)

Property

Polarizability

Electropositivity

Positive charge or

oxidation state

Size

Types of bond

usually associated

with the acid

Outer electrons on

donor atoms



Soft



Hard



BASE

(Electron donor)

Property



High

Low

Small



Low

High

Large



Polarizability

Electronegativity

Negative charge



High

Low

Small



Large

Covalent, π



Small

Ionic,

electrostatic



Large

Covalent, π



Several,

easily

excited



High energy and

inaccessible



Size

Types of bond

usually associated

with the base

Available empty

orbitals or donor

atom



Soft



Low lying and

accessible



Note: The entire column need not be true before a species is called hard or soft. The more factors that are true,

the greater the degree of hardness or softness.



A. Lewis Acids and Bases

In biological systems, the princples of Lewis acids and bases apply. According to this concept,

a Lewis base is an ion which has at least one pair of valence electrons available for sharing (an

electron donor). A Lewis acid is an ion that can accept or share at least one pair of valence electrons

(an electron acceptor). Thus, an acid-base reaction which produces a product is represented as

A+:B → A:B. The product of this reaction can be called a coordination complex, an adduct, or an

acid-base complex. Examples of this type of reaction have already been shown in Units 3 and 4.

In particular, the reader should reexamine the structure of vitamin B12 where cobalt is held in a

coordinate structure involving pterin rings.

Within the Lewis system there is a subdivision of hard and soft acids and bases. The hydrochloric

acid released by the gastric parietal cells is an example of a hard acid. It meets the definition

because the constituent ions, H+ and Cl–, readily polarize, have high electropositivity, have a large

positive and negative charge, are small in size, are almost exclusively ionic in their bonding, and

have few outer electrons to be donated or accepted. The constituent ions of hydrochloric acid are

single-valence ions. In contrast, consider a number of compounds that are soft Lewis acids. These

have properties that are the opposite of those listed above. In many instances the electron donor is

one of the multivalent ions, i.e., copper or iron, and the electron acceptor is an amino acid or an

organic ring structure. Some organic substances can be both an acid and a base. Consider ethyl

acetate — it can be a Lewis base when it forms complexes through one of its oxygen atoms to a

proton or other Lewis acid. It acts as a Lewis acid when it adds bases such as the hydroxide ion.

Tables 3 to 5 provide further information about Lewis acids and bases. These reactions are important

mechanisms for the hydrolysis of an ester bond (as in triacylglyceride metabolism) or in understanding the basis for ligand (mineral-protein) formation. Throughout the individual units dealing

with the minerals the reader will encounter instances of ligand binding. Many minerals are carried

in the blood by specific transport proteins. This is an example of a Lewis acid-base reaction.

The specificity of the transport protein has yet to be unraveled. There are preferred ligand

bonding groups, as shown in Table 6, but why these are the preferred groups is not known. There

are instances where a transport protein will carry more than one ion. An example is metallothionein,

which will carry both zinc and copper. It will also carry some of the heavy metals, but its affinity

is greater for zinc and copper. Many of the ions can be chelated by organic materials. Ethylenediaminetetraacetic acid (EDTA) is a potent chelator and is used to remove lead or other heavy metals

from the body. It will also chelate calcium and magnesium, so the clinician using EDTA to treat



© 1998 by CRC Press LLC