Table Of ContentThe Biochemistry
of Foreign Compounds
BY
DENNIS V. PARKE, B.SC, PH.D., F.R.I.C.
Professor of Biochemistry
at the University of Surrey.
With a Foreword by
R. TECWYN WILLIAMS, F.R.S., PH.D., D.SC, DOC HON. CAUSA (PARIS)
Professor of Biochemistry in the University of London,
at St. Mary's Hospital Medical School.
PERGAMON PRESS
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Copyright © 1968
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First edition 1968
Reprinted 1974
Library of Congress Catalog Card No. 67-22825
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ISBN 0 08 012202 7
PREFACE
ONE of the most characteristic features of life in the present era is the
widespread use of selectively-toxic chemicals, as medicines, insecticides,
food and cosmetic additives, etc. These synthetic chemicals, together with
the many naturally-occurring anutrient substances, are known collectively
as foreign compounds, xenobiotics or simply as "drugs". The first of these
terms has been adopted for the text of this book, because, in addition to
being the original terminology, it also appears to be the most readily under
stood.
The study of the biochemistry of foreign compounds, like many other
branches of Biochemistry, has advanced rapidly in recent years, and has
become a basic scientific discipline of both Pharmacology and Toxicology.
A concise, unifying text for students of these subjects, which would also
serve to introduce this aspect of Biochemistry to those engaged in related
fields of medicine and science, seemed both opportune and necessary.
The book is divided into two sections. The first, Biochemical Mechanisms,
is concerned with the absorption and excretion of foreign compounds and
with their metabolism. The second, Applications, deals with illustrative
examples from the major classes of these compounds, namely, natural
products, food additives, drugs, pesticides and industrial chemicals.
Publications in this field are very numerous, and it has been necessary
therefore to limit references to works of a review nature, which are given
in the bibliographies at the end of each chapter, and to recent developments,
which are cited in the text and collected in the List of References at the end
of the book. One work of reference which must, however, receive special
mention, is Detoxication Mechanisms (2nd edition), the authoritative treatise
by R. T. Williams, which has been consulted in the preparation of most of
the chapters. It was indeed Professor R. T. Williams who first aroused my
interest in this subject more than fifteen years ago, and who has, since then,
been my mentor and a continuous source of inspiration.
In the preparation of this book many individuals have helped generously
with advice and assistance, for which I am extremely grateful. Among those
deserving especial mention are Professor K. S. Dodgson, who first persuad
ed me to consider writing this book, and who has been a particular source
of encouragement; Dr. J. N. Bridges, Professor P. N. Campbell and Dr.
P. J. Creaven who read the text in its various stages andhave made innumer
able, valuable suggestions; Dr. Alan Fisk who assisted with his expert
advice in the preparation of the illustrations, and Mr. W. R. Buchanan and
VII
Viü PREFACE
his staff who interpreted to perfection my original drawings; Mr. R. G. For
tune, the editor, who patiently acceded to my many modifications of the text
to keep it abreast of modern developments; and lastly, but perhaps the most
important, my wife, who has assiduously typed and edited the various drafts
of the manuscript, prepared the indexes, and sustained me by her continuous
encouragement. To these, and the many others who have helped make this
book possible, I express my sincere and grateful thanks.
London, May 1967
FOREWORD
MAN'S environment has always contained small amounts of a very large
variety of substances normally regarded as foreign to the body. These
substances are often toxic and occur frequently in food, but in the past we
have not been aware of this. Modern man is becoming increasingly depen
dent upon the use of synthetic chemicals in the form of drugs, pesticides,
food additives, etc., for his well-being. It is thus apparent that a large num
ber of foreign compounds are being ingested daily by man and it is important
to know what happens to them when they enter the body. We now know
that the body possesses a biochemical defence mechanism which can protect
us against the harmful effect of these substances, but we must also know
how this defence system works and what are its limitations. The intelligent
use of our knowledge of the biochemistry of foreign compounds should
permit us to predict what compounds are safe to use and to avoid those,
like thalidomide, which produce adverse effects. The science of Biochemistry
has been, and is, concerned largely with compounds which are regarded as
normal to the body and usually derived from carbohydrates, fats and pro
teins, but it is now clear that more attention is being, and must be, paid to
compounds which are foreign to the body.
Most of the agricultural and medical sciences draw on Biochemistry for
explanations of many of their observations, and there is no doubt that the
study of the biochemistry of foreign compounds is providing a similar
service particularly to Toxicology, Pharmacology and Pathology.
Some undergraduate and postgraduate studies of this aspect of Bio
chemistry are now under way in various universities in this country and
abroad and in view of the social and economic implications of the subject
its study will increase. For these reasons this book is to be welcomed since
it forms a most valuable introduction to this field.
London, 1967 R. TECWYN WILLIAMS.
IX
CHAPTER 1
INTRODUCTION
THE materials which animals ingest for food rarely contain nutrients in a
pure state and are generally associated with varying amounts of non-
nutrients, substances which are utilized neither for the production of energy
nor for the elaboration of tissue components. Such compounds are conse
quently foreign to the normal metabolic pathways of the organism and are
known as foreign compounds. They are present in natural foodstuffs in
at least as great a diversity and number as the nutrients themselves, and if
allowed to remain within the animal would probably result in inhibition of
the normal processes of metabolism, producing poisoning and even death.
The animal is unable to separate nutrients from the contaminating foreign
compounds during the processes of ingestion, but it has the means of acceler
ating the elimination of these unwanted substances and frequently also of
neutralizing any pharmacological activity they may possess. This vital
function of chemical defence is known as detoxication and is effected
principally by the liver. Foreign compounds enter the body mostly by ab
sorption from the gastrointestinal tract and are taken via the portal vein to
the liver where they are detoxicated. The detoxication products are then
excreted in the bile to be voided in the faeces, or are taken to the kidneys
and excreted in the urine. Secondary sites of detoxication are located in
other tissues, such as the lungs, the gastrointestinal tract, the kidneys and
the skin.
Although foreign compounds include inorganic as well as organic sub
stances, little is known of the metabolism of the former. Moreover, highly
sensitive analytical methods, such as activation analysis, have shown that
many metals are normally present in tissues in trace amounts, although
their biological function, if any, is not always known. The following review
of the metabolism of foreign compounds is therefore largely confined to
organic compounds.
THE FATE OF FOREIGN COMPOUNDS
Foreign compounds are ingested in the food and drink, in the inspired
air, or through the skin. They are metabolized by two phases of reaction,
3
4 BIOCHEMICAL MECHANISMS
metabolic transformation and conjugation, giving rise to metabolites! and
conjugates which are then excreted in the urine, the bile and the expired
air.
Metabolic transformations are reactions in which the foreign compound
undergoes one or more of a wide variety of oxidations, reductions, hydro
lyses, etc., usually resulting in the introduction of functional groups which
increase the polarity of the molecule and act as centres for the second phase
of reaction.
Conjugations are syntheses by which the foreign compound or any of
its metabolites is combined with endogenous molecules or groups, such as
glucuronic and sulphuric acids, amino acids, methyl and other alkyl groups,
generally making the molecule more polar and less lipid-soluble, and there
fore more readily excreted.
Most foreign compounds are metabolized, at least to some extent, by
both phases of reaction. Thus, for example, benzene undergoes oxidative
transformation into phenol which is subsequently conjugated with glucuronic
and sulphuric acids :
metabolic
Benzene ^ " ^, Pheno. "*"*" , "henylg^ronide
Phenyl sulphate
Some compounds are metabolized predominantly by reactions of one
phase only. Benzoic acid, for example, is almost entirely metabolized by
conjugation, yielding benzoylglycine (hippuric acid) and benzoylglucuronide,
and only traces undergo an oxidative transformation to give the isomerie
hydroxybenzoic acids :
Benzoylglycine 4- Benzoylglucuronide
conjugation ^+
(major) ^ s^
Benzoic acid
metabolic ^v.
transformation ^*
(minor)
conjugation Hydroxybenzoic
Hydroxybenzoic .
acids ' - 1- acid conjugates
Many of the enzymes metabolizing foreign compounds are located in
the endoplasmic reticulum (microsomal fraction) of the cells of the liver and
other tissues. These microsomal enzymes do not metabolize endogenous
compounds, such as phenylalanine, tryptophan and kynurenine, which are
hydroxylated by specific enzymes occurring in other parts of the hepatic
cell, although tryptamine and tyramine(218) have been shown to be meta
bolized by hepatic microsomal enzymes. Foreign compounds may also be
t This term is frequently used in a wider sense to be inclusive of conjugates.
INTRODUCTION 5
metabolized by some of the normal enzymes of intermediary metabolism,
such as alcohol dehydrogenase, aldehyde dehydrogenase, xanthine oxidase
and esterases.
Some foreign compounds, such as diethyl ether, phthalic acid and barbi-
tone, are largely excreted unchanged and have been considered to be
biochemically inert. This would be expected with highly polar substances
such as phthalic acid (pK 3-0) which could not readily penetrate the tissues
&
and moreover would be rapidly excreted by the active transport mechanisms
of the kidney; or with highly volatile compounds, such as ether, which would
be rapidly excreted via the lungs. In many cases, however, the metabolic
inertness is only relative and the use of more sensitive techniques has revealed
that metabolism does occur, even if only to a minor extent. For example,
when barbitone labelled with radioactive carbon-14 was administered to
rats 95 % of the dose was excreted in the urine unchanged, but 5 % was
accounted for as metabolites (see Chapter 10). Compounds which are meta-
bolically inert and are also non-polar, such as hexachlorobenzene and many
chlorinated hydrocarbon insecticides, are not readily excreted and become
stored in fatty tissues of the body.
SYNTHETIC CHEMICALS
Detoxication mechanisms have most probably evolved to protect the
animal from toxic effects of naturally-occurring foreign compounds of the
diet. However, in this present era a vast number and variety of synthetic
chemicals are in everyday use and have considerably increased the number
of foreign compounds that are ingested. These chemicals include food and
cosmetic additives, drugs,(14) agricultural chemicals and pesticides.(204)
Industrial chemicals, which may present special hazards to those engaged in
manufactures, are found also in paints, polishes, detergents, plastics, and
many other products in common domestic use. Foreign chemicals from
industrial gases and effluents, the products of incomplete combustion of
carbonaceous fuels, pesticides(1) and "fallout" from nuclear reactions(155)
pollute the atmosphere(121) and contaminate the soil and natural waters.(125)
These synthetic foreign compounds find their way into the animal body in
foods, medicines, and as contaminants of the environment(344) and are
metabolized by the same mechanisms which detoxicate foreign compounds
of natural origin.
It is often believed that only synthetic chemicals are toxic and that all
natural compounds are harmless, an obvious fallacy when one considers the
great number and variety of natural toxic substances which are known, such
as snake venoms, bacterial and fungal toxins,(325) plant alkaloids,(297)
natural carcinogens (e.g. safrole and aflatoxin)(232a* 293) and teratogens
(e.g. Veratrum alkaloids).(20) Both synthetic and natural foreign compounds
are similarly detoxicated by enzyme reactions which are dependent on the
6 BIOCHEMICAL MECHANISMS
chemical structure of the compound, and not on its origin. A real danger is
that synthetic toxic compounds may be designed which do not fit any of the
enzyme mechanisms and therefore cannot be detoxicated. Such compounds
may even have natural counterparts. For example, natural analogues of
fluoroacetic acid such as ω-fluoro-oleic acid occur in the toxic South African
plants Dichapetalum cymosum and D. toxicarium.(258)
Synthetic chemicals often confer great benefits by way of improved
health, food supplies and living standards, but it must be remembered that
they are usually toxic substances which may also be potential carcinogens(28)
and teratogens.(281) They must therefore be used with the highest regard
for safety and a knowledge of their biochemistry is an essential contribution
to this end.
Since the administration of one compound may affect the metabolism of
another(128) it is also desirable to ascertain the effects of synergism, particu
larly when compounds are chronically ingested, as for example, the effects
of common food additives, such as benzoic acid or butylated hydroxytoluene,
on the metabolism of drugs exhibiting capricious toxicity, such as phenacetin.
In the following chapters the foreign compounds are classified according
to usage, namely "natural foreign compounds", "food additives", "drugs",
"pesticides", and "industrial chemicals". However, from the point of view
of their biochemistry there is no difference between these various categories,
and the animal body treats them all the same, that is as foreign compounds.
Moreover, some compounds can be classified in two or more of these cate
gories, for example, quinine, a natural compound, once used as a medicine
in the treatment of malaria, is now used mostly for its bitter flavour in the
manufacture of soft drinks and confectionery.
DETOXICATION OR INTOXICATION
The conjugation of foreign compounds leads to the masking of functional
groups (e.g. —COOH, —OH, —NH , —SH, etc.) and to deactivation of
2
the molecule, so that usually a decrease in toxicity (detoxication) results.
These conjugation mechanisms are responsible for the deactivation of the
natural hormones and also for the deactivation of drugs which would other
wise remain in the body to exert their pharmacological activity for such
long periods that chemotherapy would be impracticable.
Detoxication by conjugation:
Phenol -+ Phenyl sulphate
Benzoic acid -> Benzoylglucuronide
Cyanide -» Thiocyanate
In contrast, metabolic transformations, such as oxidation, reduction,
hydrolysis, etc., usually result in the introduction of new, polar, functional
INTRODUCTION 7
groups, which may produce an increase or decrease in toxicity. In this way,
drugs and other foreign compounds may be deactivated (detoxication) or
activated (intoxication). Inactive "pro-drugs" may be converted into thera-
peutically-active compounds (activation, e.g. prontosil is reduced to sul-
phanilamide) and systemic insecticides may be converted into active, toxic
principles (intoxication, e.g. parathion is desulphurated to paraoxon).
These new functional groups may also subsequently undergo conjugation
resulting in an ultimate detoxication.
Detoxication and intoxication by metabolic transformations
Detoxication : Phenobarbitone hydroxylation p-Hydroxyphenobarbitone
(active drug) (inactive metabolite)
reductive scission
Activation : Prontosil Sulphanilamide
(inactive "pro-drug") (active drug)
Parathion desulphuration Paraoxon
Intoxication: (inactive insecticide) —— ■»- (active insecticide)
It has been suggested(299) that the enzymes involved in these mechanisms
are not primarily concerned with detoxication of foreign compounds but
have a physiological role as yet unknown and are "enzymes in search of a
substrate". Without doubt, the conjugation mechanisms are also involved
in the formation of conjugates of many endogenous compounds such as the
hormones, bilirubin, vitamin A,(108) etc., bringing about their deactivation
and facilitating their excretion. But the known role of detoxication of foreign
compounds is as vital as any other biochemical function, since on it may
depend the survival of the animal organism. Obviously these mechanisms
were not developed to deal with synthetic foreign chemicals such as modern
drugs and pesticides, but were probably evolved to protect the animal from
the toxic effects which would result from ingestion and accumulation of the
naturally occurring foreign compounds present in foodstuffs.
Some foreign compounds are sufficiently similar to normal endogenous
compounds to participate in the pathways of intermediary metabolism or
to become incorporated into the tissues, a process which is usually toxic
and is known as lethal synthesis. Other foreign compounds interact with
tissues resulting in alkylation and arylation of proteins and nucleic acids,
a process thought to be a cause of chemical allergy and carcinogenesis.
HISTORICAL SURVEY
Probably the earliest observation of the metabolism of a foreign compound
was made by Gmelin (1824) who noticed the garlic-like odour in the viscera
