/.
`
`l
`
`First published in 1987 by Chapman and Hall Ltd
`11 New Fetter Lane, London EC4P 4EE
`Published in the USA by Chapman and Hall
`29 West 35th Street, New York, NY 10001
`© 1987 Adrien Albert
`Printed in Great Britain at the
`University Press, Cambridge
`ISBN 0 412 28800 1 (hb)
`ISBN 0 412 28810 9 (pb)
`This title is available in both hardbound and
`paperback editions. The paperback edition is sold
`subject to the condition thaf it shall not, by way of
`trade or otherwise, be'lent, resold, hired out, or
`otherwise circulated without the publisher's prior
`consent in any form of binding or cover other than that
`in which it is published and without a similar condition
`including this condition being imposed on the
`subsequent purchaser.
`All rights reserved. No part of this book may be
`reprinted, or reproduced or utilized in any form or by
`any electronic, mechanical or other means, now known
`or hereafter invented, including photocopying and
`recording, or in any information storage and retrieval
`sys~em, without permission in writing from the
`publisher.
`
`British Library Cataloguing in PublicatiQn Data
`
`Albert, Adrien
`Xenobiosis: foods, drugs and poisons
`in the human body.
`l. Xenobiotics
`I. Title
`615.7
`
`QP529
`
`ISBN 0-412-28800--1
`ISBN 0-412-2881Q--9 Pbk
`
`Library of Congress Qataloging in Publication Data
`
`Albert, Adrien.
`Xenobiosis: foods, d_rugs, and poisons in the human body.
`
`Bibliography: p.
`Includes indexes.
`1. Xenobiotics-Metabolism. 2. Biotransformation
`I. Title.
`(Metabolism)
`[DNLM:
`l. Biotransformation.
`2. Drugs-metabolism. 3. Food. 4. Poisons. QU 120
`A333x]
`QP80l.X45A43 1987
`ISBN 0-412-28800-l
`ISBN 0-412- 2881Q--9 (pbk.)
`
`87-11646
`
`615.9
`
`Contents
`
`Preface
`
`1
`
`Introduction to the concept: Xenobiosis
`
`PART ONE: FOODS
`
`2
`Introduction to Part One:Foods as foreign substances
`2.1 Our debt to Early man: the hazardous evolution of man's diet
`2.2
`Food from the beginning of civilization to the prese~t time
`2.3 The gastrointestinal tract and what it does to food
`
`3
`3.1
`
`The metabolism of foods
`The conversion of digested foods into energy: the storage and
`release of energy
`3.2 The conversion of metabolic fragments into human carbohydr-
`ate, lipid and protein
`
`Hidden dangers in wholesome foods
`4
`4.1 Quantitative malnutrition (overeating and undereating)
`4.2 Qualitative malnutrition (unbalanced diets)
`4.3
`Toxican~ occurring naturally in food
`
`5
`5.1
`5.2
`
`Special aspects of wholesome foods
`Inborn sensitivity to a particular food
`Enhancing the enjoyment of meals: herbs, spices and other
`additives
`5.3 Enhancement of mood at table
`
`PART TWO: DRUGS
`
`1
`
`7
`9
`13
`14
`
`18
`
`18
`
`31
`
`37
`39
`45
`57
`
`74
`74
`
`79
`92
`
`97
`
`6
`6.1
`
`Introduction to Part Two: Drugs as foreign substances
`The three classes of drugs currently used in therapy: some useful
`definitions
`
`99
`
`100
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`186 Activity: how actions are related to chemical structure
`
`How one methyl group can change the action 187
`
`Multiple regression analysis has given some very useful results. It functions
`best in industry when a large series of closely related candidate drugs is on hand
`and a speedy indication is needed concerning what should be synthesized next.
`However for the purpose of finding scientific correlations among substances that
`are not closely related chemically, the biological situation is usually found to be
`more complex than such an equation can accommodate. For example the initial
`distribution of a drug need not depend on lipophilicity but on the use of
`facilitated channels that exist for the uptake of such natural products as sugars,
`purines, amino acids and even choline (p. 121). For these reasons those of a
`scholarly cast of mind, provided that they have the time and facilities, will
`continue to examine the connection between phy.okochemical properties and
`pharmacological action, in all their fine details' and rewarding complexity.
`In fact much steric and electronic information about receptors is available
`from sources other than regression analysis. For example, where the receptor is
`the active site on an enzyme, details (obtained by X-ray diffraction analysis) are
`often available from the Cambridge (UK) or Brookhaven (USA) crystal(cid:173)
`structure databanks. In other cases one can usefully superimpose (on a
`transparent surface) scale drawings of all the drugs that act on the receptor. The
`shared features constitute what is called a·'hyper-molecule' to which the receptor
`should be complementary in outline and charge (Balaban, et al., 1980). If an
`approximate image of the receptor can be generated on the screen of an Evans
`and Sutherland computer-graphics (computer controlled) Picture System the
`images of candidate drugs can then be applied in a contrasting colour. In this
`way the ability of the candidate to make a good fit may be judged (Blaney, et al.,
`1982).
`
`8.4 How one methyl group can significantly change the action of
`a drug
`
`It is quite common to find a pair of closely related molecules where the first has a
`strong biological action whereas the other has none. How can two such
`substances which may differ in composition by only a single methyl group
`perform so differently in a biological test? In this Section a study of methyl groups
`will be made as examples of what are commonly termed 'chemically inert'
`groups. Yet these groups if suitably placed can profoun~ly change the chemical
`behaviour of molecules by well-understood steric and electronic effects. Their
`altered biological properties reflect these changes.
`
`Steric influences
`
`The steric effects introduced by small, inert groups are of two kinds. Some are
`evident even in aqueous solution whereas others require a surface for manifest(cid:173)
`ation, as in enzyme reactions.
`
`(a) Steric inj/uences on solubility. It might be thought that the insertion into a given
`molecule of a methyl group would always lower solubility in water, because a
`methyl group is water-repelling. It usually does lower solubility but there are
`interesting exceptions. In order that a substance may dissolve in water the water
`molecules must be forced apart by breaking their hydrogen bonds. The lower
`alcohols, methanol and ethanol, readily do this because their hydroxyl group
`forms such a large part of each molecule and this group readily becomes
`hydrogen-bonded to water molecules. But in higher alcohols, the paraffinic side(cid:173)
`chain becomes a more dominant feature: it cannot be accommodated in the
`interstices, it cannot force the water molecules apart and hence it tends to be
`squeezed out of the water, dragging the whole molecule with it. This explains the
`low solubility of the higher alcohols. Yet this effect can be considerably lessened
`by shifting the hydroxyl group to the centre of the molecule, as in tertiary amyl
`alcohol, which is consequently more soluble than its lower homologue, normal
`butanol (Ginnings and Baum, 1937). No less surprising, the 2-aminobutyric
`acids are more soluble than 2-aminopropionic acid (alanine), because of chain(cid:173)
`folding.
`Unusual solubilizing effects of methyl groups are found in the antibacterial
`sulfonamides, e.g. sulfadiazine ( 8. 40), and in many other drugs of a similar degree
`of complexity and rigidity. In such molecules, any protruding C-methyl groups
`prevent strainless adsorption of dissolved solute (drug) on to the crystal-lattice of
`the solid phase. This anomaly displaces the final equilibrium in the direction of
`increased solubility (Gilligan and Plummer, 1943).
`A methyl group can hinder adqition of water across an adjacent double bond
`thus greatly increasing the lipophilicity of the substance, a property on which
`activity is apt to depend. Such addition of water is known as covalent hydration
`(Albert, 1976). Several naturally-occurring pteridines such as xanthopterin
`(8.51) which is present in the human kidney, are covalently hydrated, i.e. have
`become secondary alcohols. However the addition of a methyl group, as in ( 8.52),
`largely suppresses the hydration giving a less hydrophilic molecule. Many other
`natural products are covalently hydrated, a characteristic that can be suppressed
`by a neighbouring C-methyl group.
`
`(b) Steric inj/uence on chelation. The antibacterial action of 8-hydroxyquinoline
`(Section 8.3) is seriously decreased if a methyl group is inserted in the 2-position
`(Albert, et al., 194 7). This deactivation is most likely exerted through a steric
`
`H~~
`
`H)(_W,lLNAo
`H H
`Xanthopterin
`(8.&1)
`
`oy~~~
`Me~JlNAO
`
`H
`7-Methylxanthopterin
`(8.&2)
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`188 Activity: how actions are related to chemical structure
`
`How one methyl group can change the action 189
`
`effect at the biological interface. Even in solution this substance (2-methyl-8-
`hydroxyquinoline) has lost its affinity for Al 3 + (while retaining it for Fe3 +)
`because of the steric effect of the methyl group.
`
`(c) Steric influences on receptors and enzymes. Most molecules that fit the mqscarine
`receptor for acetylcholine have a quaternary nitrogen atom of which one
`substituent is a straight chain of five atoms in length. Addition of one more
`methylene group to this chain causes a dramatic loss of biological effect. At least
`two of the other substituents on the nitrogen atom must be methyl groups to
`achieve maximal action. If one of these is substituted by either hydrogen or ethyl,
`a sharp drop in activity takes place. On p. 100 we-noted how the addition of a
`methyl group to the molecule of acetylcholine ""(6.2) to give methacholine ( 6.1)
`hindered hydrolysis ofthe molecule by acetylcholinesterase so strong~y that the
`momentary pharmacological action exerted by ACh became a durable, and
`clinically valued, one. The biological effect of the vitamin thiamine (8.53) is very
`sensitive to addition or loss of a methyl group. When tested on pigeons, the
`activity drops to 5% if the methyl group is removed from the pyrimidine ring,
`and to less than l% if the methyl group is removed from the thiazole ring. Finally
`if an extra methyl group is inserted into "the thiazole ring, between nitrogen and
`sulfur, the vitamin action is completely lost (Schultz, 1940).
`Sometimes a methyl group increases the biological effect of a drug by making it
`a poorer fit for a destructive enzyme. Thus amphetamine (7.24), which is l(cid:173)
`methyl-2-phenylethylamine, has a much more prolonged hypertensive effect
`than 2-phenylethylamine. This has been traced to the resistance of amphetamine
`to monoamine oxidase, the enzyme that quickly destroys the lower homologue
`(Blaschko, 1952). Similarly, the action of corticosteroids and the steroid sex
`hormones can be intensified by inserting a methyl, or a fluorine, substituent-a
`steric device that has produced several clinically valuable drugs. Such seemingly
`inert substituents turn the steroids into poorly-fitting substrates for their natural
`destructive enzymes (Ringold, 1961).
`
`Electronic influences
`
`The methyl group is the commonest substituent that releases electrons no matter
`whether inductive or mesomeric mechanisms·are operating.
`
`(a) Electronic influences on ionization•. Because of its electron-releasing nature a
`methyl group, if attached to a nearby carbon atom, strengthens a base and
`weakens an acid. Also a methyl group attached to nitrogen, to give a secondary
`amine, is base-strengthening although most tertiary amines are weaker than
`secondary amines. Such changes in strength are usually less than one pK unit but
`can influence biological results if the pK falls within one unit of the pH at which
`
`• For more on ionization see Albert and Seijeant (1984) .
`
`the biological test is made. When as usually happens one ionic species (e.g. the
`cation) is far more biologically active than another (e.g. the neutral species) this
`change in ionization can decide whether a substance is biologically active or not.
`The triphenylmethane dyestuffs (8.54), which show a large increase in basic
`strength upon N-alkylation, illustrate how antibacterial action is correlated with
`ionization in this series as Table 8.5 illustrates. Thus antibacterial activity is
`virtually cr~ated here by the insertion of 'chemically inert groups'.
`Although it is obvious that methylation of an acidic group must abolish its
`ability ~o ionize, the consequences of such a methylation in the barbituric acid
`series are particularly interesting. In aqueous solution, barbituric acid exists in
`the trioxo form ( 8.55) and forms the mono-anion by loss of a proton from C-5. It is
`
`Table 8.5 Connexion between ionization and antibacterial activity in .a series of
`triphenylmethane bases.
`
`Substance
`
`Formula
`
`pKequil·
`
`Percentage
`ionized
`at
`pH7.3
`
`Doebner's violet
`Malachite green
`Brilliant green
`
`(8.54a)
`(8.54b)
`(8.54c)
`
`5.38
`6.90
`7.90
`
`2
`28
`80
`
`From Gold.acre and Philips, 1949
`
`Minimal
`bacteriostatic
`concentration
`(Staph. aureus;
`24 hat 37"C
`and pH 7.3)
`
`I in
`20000
`I in
`80000
`I in 1280000
`
`Thiamine
`(8.53)
`
`2
`a.n
`
`NR 2
`R2 N+
`(8.54) a. R = H: Doebner's violet
`b. R = CH 3 : Malachite green
`c. A= C2 H 5 : Brilliant green
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`How one methyl group can change the action 191
`
`selectivity can be introduced into a molecule, and what rules govern it, will be
`discussed in Chapter 9.
`
`Further reading
`
`For the biological effects of inserting chemically-inert substituents, see Albert,
`1985, pp. 43-5Q.
`
`Follow-up
`
`Consider the traditional (and apparently unsinkable) phrase 'structure-activity
`relationships' (SAR) and discuss the extent to which the word 'structure' retains
`its original meaning. Could you think of a better phrase?
`
`190 Activity: how actions are related to chemical structure
`
`a fairly strong acid (pKa 3.9). The insertion of two alkyl groups into the 5-position
`removes any possibility of an anion being formed in the 5-position. Consequently
`the anion is formed from N-3 but is much weaker. Thus barbital (5,5-
`diethylbarbituric acid) has a pKa of7 .9 and hence is 104 times weaker as an acid
`than barbituric acid! The consequences of the insertion of these ethyl groups on
`the structure-activity relationship is momentous. A substance with a pKa of3.9 is
`completely ionized at pH 7 .3, and hence unlikely to pass the blood-brain barrier.
`However when as in barbital the pKa is 7.9 the substance is 80% non-ionized at
`pH 7.3, and hence passes through without difficulty.
`
`(b) Electronic influences on reduction-oxidation potentials. '!'he electrons released to
`the rest ofthe molecule by a C-methyl substituent lower the redox potential (E0
`).
`As a result the affected substance becomes a more active reducing agent (and is
`more easily oxidized) than the unmethylated homologue. Redox potentials are
`used to record the equilibrium between oxidized and reduced forms.
`An example of this lowering of E 0 is the insertion of a methyl group into the
`2-position of 1,4"-naphthaquinone which depresses the potential (by 76 mV) to
`408 m V (Fieser and Fieser, 1935). In another example the reduction potential of
`NAD (p. 20) is -180m V, a value so low ·that a substituted NAD of slightly
`lower potential could, most likely, not become reduced to its NADH. Any
`analogue that cannot be reduced in the living cell cannot act as a hydrogen
`carrier. It is apparently for this reason that 2-methyl-nicotinamide has no
`biological activity, even if the effect of the methyl group may be partly steric.
`
`(c) Electronic irifluences on reactions where a covalent bond is broken. The electron(cid:173)
`releasing effects of a methyl group described above were of an instantaneously(cid:173)
`appearing character. Some time-dependent, i.e. kinetically controlled, effects
`will now be mentioned. Methyl groups, because of their electron-releasing
`properties, promote electrophilic substitution, e.g. they make neighbouring
`amino groups readier to be acylated or to form an azomethine (Schiff base). A
`methyl group also constitutes a side-chain that is conveniently biodegraded.
`Thus the metabolic oxidation of a methyl group to a carboxylic acid confers
`hydrophilic properties on a highly lipophilic molecule and leads to rapid
`excretion in the urine.
`
`(tf) Solubility. In an aromatic nitrogen-heterocycle such as pyridine replacement
`by methyl of the hydrogen atom in an-OH, -NH 2 or-C(: O)NH- group usually
`increases solubility in water dramatically. Thus 6-aminopurine (adenine) is
`soluble to the extent of only I part in 1100, whereas 6-dimethylaminopurine
`dissolves 1 in 120 (Albert and Brown, 1954); countless similar examples are
`known.
`
`In conclusion it must be pointed out that this Chapter deals only with activity,
`which is the quality that makes a molecule more biologically active than a food.
`Yet activity can create only a poison unless selectivity is also incorporated. How
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`9
`Selectivity: designing drugs
`without side-effects.
`The three sources of--selectivity
`
`9.1 Selectivity through comparative distribution 193
`9.2 Selectivity through comparative biochemistry 198
`9.3 Selectivity through comparative cytology 207
`
`In Chapter 8 we saw how a biologically inert molecule could be redesigned to
`endow it with biological activity. However that would be only the first step in the
`creation of a useful drug because a biologically active substance remains only a
`toxicant (poison) until it is provided with selectivity also. In other words, it must
`be further designed to confine its action to the uneconomic cells (p. lO 1). The
`extent to which a drug can differentiate between economic and uneconomic cells
`is the measure of its selectivity. Toxicity in a drug is no drawback, in fact it is the
`very core of its usefulness. What is important is to arrange for this toxicity to be
`selective. The present Chapter lists and examines the properties from which
`selectivity can be derived.
`Realization of the importance of selectivity dates from about 191 I when Paul
`Ehrlich introduced his chemotherapeutic index as the first means of measuring it
`(p. 111). Today the drug designer's goal is complete selectivity and this has been
`closely approached in several chemotherapeutic agents such as the penicillins,
`the antibacterial sulfonamides and several anthelminthics such as piperazine.
`However for some diseases the best available drugs still have only partial
`selectivity although current research is ·Steadily improving on this position.
`Since 1948 I have been seeking and publicly discussing the principles that can
`introduce selectivity into a biologically-active molecule. This search led me to
`conclude that three main principles govern this phenomenon:
`
`1. Comparative accumulation, by choice of a toxicant that accumulates pre(cid:173)
`ferentially in the uneconomic cells;
`2. Comparative biochemistry, by choice of a toxicant that injures a biochemical
`process found only in the uneconomic cells;
`
`I
`
`Comparative distribution 193
`
`3. Comparative cytology, by choice of a toxicant that interferes with a cytological
`feature peculiar to the uneconomic cells.
`How these principles, singly or jointly, can establish selectivity in otherwise
`unselective toxicants will now be discussed under these three headings.
`
`9.1 Selectivity through comparative distribution
`
`Many substances that could be toxic for almost all kinds of cells can nevertheless
`be made highly selective by favourable differences in distribution. This applies
`even to the hydrogen ion (H+) surely the simplest of all biologically-active
`agents. In the form of 10% sulfuric acid, it can safely be sprayed on emerging
`cereal crops to destroy weeds, as was discovered in France by Rabate and
`confirmed in the University of California's field trials. Of course, sulfuric acid is
`injurious to the cytoplasm of both wheat and weed, but two factors restrain it
`from penetrating the cereal. Firstly the exterior of the cereal, a monocotyledon, is
`smooth and waxy whereas that of the weeds (mainly dicotyledons) is roughand
`absorbent; hence the acid runs off the former but is accumulated by the latter.
`Secondly the tender new shoot of the cereal arises from the soil and is protected by
`a leaf-sheath whereas the growing point of the dicotyledon is exposed and
`vulnerable because it forms the apex of the shoot. Hence the weeds die and the
`economic crop persists because of a selective action that depends entirely on
`distribution. (Unfortunately, acidification of the soil limits this type of weeding
`to a single season).
`Human medicine provides many similar examples, notably the tetracyclines
`(e.g. 9.1) which are, after the penicillins, the most frequently prescribed of all
`<;Lntibiotics. Franklin, working in Manchester, observed that the tetracyclines are
`accumulated by all bacteria whereas they hardly penetrate mammalian cells
`thanks to a difference in the cytoplasmic membranes of these two forms oflife. As
`a result the synthesis of proteins by bacterial ribosomes is repressed and the
`bacteria die. Yet when both the economic and uneconomic cells were fractiona(cid:173)
`ted it was found that the ribosomes of the host were just as sensitive to these
`antibiotics as those.of the parasites. However so selective is the distribution of
`these drugs that the tetracyclines do not normally reach the ribosomes in
`mammalian cells. Hence the high . therapeutic index of these antibiotics
`(Franklin, 1971).
`Selective partitioning is possible between the tissues of a single organism. In a
`rare example from anticancer therapy, 5-fluorouracil (9.2) is used by dermatolo(cid:173)
`gists to eliminate two malignant growths- basal and squamous cell carcinomas.
`So selective is this drug that patients are encouraged to rub a solution of it daily
`into the affected area. The eventual action is on thymidylate synthetase which is
`present in both healthy and malignant tissues. However the malignant tissue in
`this treatment is the only one to become inflamed. It finally disintegrates by
`necrosis and is replaced by healthy granulation tissue followed by new skin
`(Klein et al., 1972).
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