Table Of ContentForeword
Plant and animal organisms offer a wide diversity of compounds which can
serve as exciting new pharmacophores and which can reveal new mechanisms
of action for controlling disease processes. Vol. 22 of Studies ni Natural
Products Chemistry comprises articles written on bioactive natural products by
leading authorities ni their respective fields.
Robinson has reviewed the metabolism and pharmacology of alkaloids found ni
animals. Chemical ecology can be an attractive tool for identifying antifungal
natural products and this area has been reviewed by Graham et .la The chapter
by R ios and co-workers presents studies carried out on triterpenes which have
shown anti-inflammatory activity. Malaria continues to be a major health problem
ni developing countries and there are a large number of deaths each year
caused by .ti The review by Kawanishi et .la presents the current status of work
done ni this field as well as on natural anti-diabetic compounds. Synthetic
approaches involving intramolecular diyl trapping reaction are described by Little
et .la for the synthesis of linearly fused tricyclopentanoids. The use of classical
and biocombinatorial approaches to bioactive fungal natural products si
discussed by Jiang et .la F010p has presented the chemistry of 2-
aminocyclopentanecarboxylic acid. Antioxidants continue to attract attention ni
medicine and many interesting flavonoids have been found ni nature which
possess antioxidant and pro-oxidant properties. These studies have been
reviewed by Vanden Berghe and co-workers, while antioxidant activity found ni
South American plants si reviewed by Desmarchelier. Other interesting reviews
include those on insect juvenile hormones ni plants by Tobe, antiulcer and
gastroprotective activity of flavones by Martin et ,la biological activity of simple
flavones by Tahara et .la Anti-convulsant plants by Raza and biological activity
of anthracenones of the Karwinskia genus by PiSeyro-Lopez. A number of
different species of plants of munogyloP species possess interesting biological
activities. The bioactive compounds ni such plants are reviewed by Adamczeski.
mucirepyH mutarofrep (St. Johns wort) si one of the extensively studied plants
because of its wide range of biological activities, specially its use for the
treatment of mild to moderate depression. The review by Erdelmeier describes
the research carried out on this plant. Finally tropane alkaloids are reviewed by
Christen ni respect of their chemistry and biological activity.
I would like to express my thanks to .rD Shakil Ahmad and .rD Durre Shahwar
for their assistance ni the preparation of the index. I am also grateful to .rM
Waseem Ahmad for typing and to .rM Mahmood Alam for secretarial assistance.
It is hoped that this volume, which represents the third volume of this series
devoted to bioactive natural products, will be of great interest to organic
chemists, medicinal chemists and pharmacologists.
Atta.ur.Rahman
Ph.D. (Cantab), Sc.D. (Cantab)
February, 2000
Preface
nI his fascinating and beautifully written autobiography (For eht Love of
Enzymes, Harvard University Press, Cambridge, MA, 1989), Arthur Kornberg
repeatedly emphasizes the central role of chemistry ni understanding life
processes. Nowhere si this connection clearer and more direct than ni the field
of "Natural Products" chemistry. From the early stereochemical studies on
tartaric acid carried out by Louis Pasteur one-and-a-half centuries ago, through
the subsequent pioneering work of chemists such as Emil Fischer, Otto Wallach,
Robert Robinson, Vlado Prelog, R.B. Woodward, D.H.R. Barton .... (the list could
go on and on), we have seen again and again how the careful study of naturally-
occurring compounds has, on the one hand, enriched our understanding of the
science of organic chemistry itself, and on the other hand, provided deep insights
into biological phenomena. Despite wide swings ni the popularity or even
"trendiness" of natural products research, the field continues to advance around
the world.
tI si not a coincidence that natural products have played, and will continue to
play, a seminal role ni the discovery and development of pharmaceutically and
agrochemically important agents. Three billion years of biological evolution have
resulted ni the development of metabolic pathways leading to the synthesis of
hormones, pheromones, antibacterial, anti-fungal, anti-protozoan, and anti-
insectan agents, as well as many other bioactive compounds that are of adaptive
value ni the lives of the organisms that produce them. The natural products
chemists of the world (in some respects indistinguishable from "chemical
ecologists") isolate these molecular entities, focussing chiefly on compounds with
particularly interesting biological activities either from the human point of view or
from that of the producing organism. They establish their structures and define
their biosynthetic pathways. They study their mechanisms of action and their
metabolic pathways. They devise synthetic methods which make novel target
structures accessible for further research and for application. While much of this
research si driven by the entirely worthy desire to obtain "useful knowledge," it si
clear that scientists entering the field of natural products chemistry are often
deeply motivated by their love of nature ni general, of chemistry ni particular, and
by their fascination with understanding as much of life as possible at the
molecular level.
The "Studies in Natural Products Chemistry' series, now in its twenty-second
volume, documents an incredible diversity of research. If we bear ni mind the
fact that for some of the most important groups of organisms (i.e. soil dwelling
microbes; insects and other arthropods), most species have not yet been
described, let alone subjected to chemical investigation, we can look forward
eagerly to many future volumes ni this series. With respect to the present
volume, the reader can expect a veritable chemical feast.
Jerrold Meinwald
Cornell University
Ithaca, NY 14853 USA
CONTRIBUTORS
Madeline Adamczeski Department of Chemistry, American University,
Washington, D.C. 20016-8014, USA
Zhiqiang An Millennium Pharmaceutical Inc., One Kendall Square
Building 300, Cambridge, MA 02139-1562, ASU
Atta-ur-Rahman International Center for Chemical Sciences, H.E.J.
Research Institute of Chemistry, University of Karachi,
Karachi-75270, Pakistan
Jacqueline .C Bede Department of Zoology, University of Toronto, 25
Harbord St., Toronto, Ontario, M5S 3G5, Canada
.D Vanden Berghe Department of Pharmaceutical Sciences, University of
Antwerp (U.I.A.), Universiteitsplein ,1 B-2610 Antwerp,
Belgium
.M Calomme Department of Pharmaceutical Sciences, University of
Antwerp (U.I.A.), Universiteitsplein ,1 B-2610 Antwerp,
Belgium
.M Iqbal Choudhary International Center for Chemical Sciences, H.E.J.
Research Institute of Chemistry, University of Karachi,
Karachi-75270, Pakistan
.P Christen University of Geneva, Laboratory of Pharmaceutical
Analytical Chemistry, 20, Boulevard d'Yvoy, CH-1211
Geneva 4, Switzerland
.G Ciccia C6tedra de Microbiologia Industrial y Biotecnologia,
Facultad de Farmacia y Bioqulmica, Universidad ed
Buenos Aires, Junin 659 1113 Buenos Aires, Argentina
.P Cos Department of Pharmaceutical Sciences, University of
Antwerp (U.I.A.), Universiteitsplein ,1 B-2610 Antwerp,
Belgium
.J Coussio ardet~C de Farmacognosia, IQUIMEFA-CONICET,
Facultad de Farmacia y Bioquimica, Universidad de
Buenos Aires, Jun|n 956 1113 Buenos Aires, Argentina
.C Alarc6n de la Lastra Department of Pharmacology, Faculty of Pharmacy,
University of Seville, Prf. Garcfa Gonz~lez s/n, 41012-
Seville, Spain
xii
.C Desmarchelier C6tedra de Microbiologta Industrial y Biotecnologta,
Facultad de Farmacia y Bioquimica, Universidad ed
Buenos Aires, Junin 659 1113 Buenos Aires, Argentina
Stephanie .J Eckerman Chemistry Department College of .tS Benedict/St. John's
University 73 .S College Avenue, .tS Joseph, NM 56374,
USA
C.A.J. Erdelmeier .rD Willmar Schwabe GmbH & Co., Research dna
Development, Karlsruhe, Germany
N.R. Famsworth Program for Collaborative Research ni the Pharmaceutical
Sciences, College of Pharmacy, University of Illinois ta
Chicago, Illinois, ASU
Ferenc pOl~tF Institute of Pharmaceutical Chemistry, Albert Szent-
Gy0rgyi Medical University, H-6701, Szeged, BOP ,121
Hungary
R.M. Giner Departament de Farmacologia, Facultat ed Farmacia,
Universitat de Valencia, Avda. Vicent s~/rdnA Estell~s s/n.,
46100 Burjassot (Valencia), Spain
Kate .J Graham Chemistry Department College of .tS Benedict/St. John's
University 73 .S College Avenue, .tS Joseph, NM 56374,
USA
.R Hoerr .rD Willmar Schwabe GmbH & Co., Research dna
Development, Karlsruhe, Germany
J.L. Ingham Department of Food Science dna Technology, University
of Reading, Whiteknights, P.O. Box 226, Reading 6GR
2AP, England, U.K.
Zhi-Dong Jiang Millennium Pharmaceutical Inc., One Kendall Square
Building 300, Cambridge, AM 02139-1562, ASU
.K Kawanishi Kobe Pharmaceutical University, Kobe, Japan
.E Koch .rD Willmar Schwabe GmbH & Co., Research dna
Development, Karlsruhe, Germany
.C La Casa Department of Pharmacology, Faculty of Pharmacy,
University of Seville, Prf. Garcia Gonzfilez s/n, 41012-
Seville, Spain
XIU ~176176
.R Daniel Little Department of Chemistry, University of California, Santa
Barbara, Santa Barbara, AC 93106, ASU
.S zelfi~M Departament ed Farmacologia, Facultat ed Farmacia,
Universitat de Valencia, Avda. Vicent Andr6s s~/lletsE s/n.,
46100 Burjassot (Val/mcia), Spain
M.J. Martin Department of Pharmacology, Faculty of Pharmacy,
University of Seville, Prf. Garcia zeli~znoG s/n, 41012-
Seville, Spain
.V Motilva Department of Pharmacology, Faculty of Pharmacy,
University of Seville, Prf. Garcia zeli~znoG s/n, 41012-
Seville, Spain
Nwaka Ogwuru Department of Chemistry, American University,
Washington, D.C. 20016-8014, ASU
Michael .M Ott Department of Chemistry, University of California, Santa
Barbara, Santa Barbara, AC 93106, ASU
.L Pieters Department of Pharmaceutical Sciences, University of
Antwerp (U.I.A.), Universiteitsplein ,1 B-2610 Antwerp,
muigleB
.A Pifleyro-Lopez Departmento de Farmacologia, Y Toxicologia, Apdo.
Postal 146, Col. led Valle, 66220, Garza Garcia, N.L.
Mexico
Mohsin Raza International Center for Chemical Sciences, H.E.J.
Research Institute of Chemistry, University of Karachi,
Karachi-75270, Pakistan
M.C. Recio Departament de Farmacologia, Facultat ed Farmhcia,
Universitat ed ,blaV ncia, Avda. Vicent sOrdnA Estell6s s/n.,
46100 Burjassot (Valencia), Spain
J.L. Rios Departament ed Farmacologia, Facultat ed ,aic.bmraF
Universitat de Valencia, Avda. Vicent sOrdnA Estell~s s/n.,
46100 Burjassot (Valencia), Spain
.T Robinson Lederle Graduate Research Center, Department of
Biochemistry and Molecular Biology, University of
Massachusetts, Amherst xoB 34505, Amherst, AM 01003-
4505, U.S.A.
xiv
.S Tahara Department of Applied Bioscience, Faculty of Agriculture,
Hokkaido University, Kita-ku, Sapporo, 060-8589, Japan
Stephen .S Tobe Department of Zoology, University of Toronto, 52
Harbord St., Toronto, Ontario, M5S 3G5, Canada
A.J. Vlietinck Department of Pharmaceutical Sciences, University of
Antwerp (U.I.A.), Universiteitsplein ,1 B-2610 Antwerp,
Belgium
N. Waksman Departmento de Farmacologia, Y Toxicologia, Apdo.
Postal 146, Col. del Valle, 66220, Garza Garcta, N.L.
ocix~M
namhaR-ru-attA ).dE( Studies in Natural Pror Chemistry Vol. 22
(cid:14)9 0002 reiveslE ecneicS .V.B llA sthgir devreser
THE METABOLISM AND BIOCHEMICAL ACTIONS OF
ALKALOIDS IN ANIMALS
T. ROBINSON
Lederle Graduate Research Center Department of yrtsimehcoiB and
Molecular Biology ytisrevinU of ,sttesuhcassaM Amherst xoB ,50543
,tsrehmA AM 01003-4505 A.S.U
ABSTRACT: The metabolism and pharmacology of naturally-occurring alkaloids are
reviewed, with emphasis on work of the last ten years during which there have been
many important advances. It is recognized that the final effects of alkaloids on animals
may result from activity of their metabolic products rather than of the original substance.
Therefore one section discusses how alkaloids are metabolized, first in terms of the
general processes and then with many examples of the metabolism of particular types of
alkaloid structures. The second section on pharmacology deals with detailed
biochemistry of alkaloid action at the level of molecule-to-molecule, rather than
describing behavioral or gross physiological effects. Modern molecular biological
methods have revealed the intimate structures of neural receptors and other cellular
molecules with which the alkaloids interact. In this section of the review the material is
organized according to the processes being affected rather than according to types of
alkaloid. Thus there are subsections for the various neural receptors, structural
components of cells, enzymes, etc.
GENERAL INTRODUCTION
The toxic and stimulatory effects of alkaloid-containing plants 11o the
behavior of animals have been observed for many centuries; and since the
early 19th Century it has been possible to attribute many of these effects
to specific, chemically characterized substances, which became known as
"alkaloids" because many of them formed salts with acids. Late in the 19th
Century the action of certain alkaloids on specific physiological systems
became clear. Since the middle of the 20th Century the availability of
isolated and well-characterized biochemical systems has made possible
still further refinement in our knowledge of how alkaloids act as they do in
terms of membrane structures, enzymes, receptors, transporters, and so
on. It is such systems of molecule vs. molecule that are the focus of this
review. Behavioral effects, and even effects on gross physiological
systems, have a vast literature that will be mostly passed over here.
"Animals" is used here in a broad sense, to include insects, molluscs,
and other lower life forms, as well as mammals. Microorganisms are,
however, excluded.
With the growing realization that some effects of alkaloids are, in fact,
effects of their metabolic products rather than of the originally
4 T. ROBINSON
administered compound, it became essential to include information about
how alkaloids are metabolized.
METABOLISM
Introduction, Common Processes
Animals have processes for dealing with foreign compounds, some of
which are assimilated because of their nutritional importance, while others
are of no value or even detrimental. Alkaloids fall into the latter group, but
they are treated by processes that are not unique to them but have general
roles in metabolism. Such processes as hydrolysis, oxidation, reduction,
and conjugation are applied to alkaloids just as they are to such
nutritionally important molecules as carbohydrates or proteins. The
following sections review these processes, first in general and then as they
apply to alkaloids. As a general rule, it appears that the metabolism of
alkaloids in animals does not proceed to complete breakdown yielding
carbon dioxide but that a few small modifications of the structure are
produced.
A. Hydrolysis
Several alkaloids contain ester groups, and an early step in their
metabolism is the hydrolysis of the ester bond. Additional reactions may
then occur to complete the metabolism. As will be shown in specific
examples, esterases both in blood serum and the liver are active in different
cases.
B. Oxidation
Oxidative processes are well-known in animal metabolism; and they
include pocesses that abstract hydrogen atoms from the substrate as well
as those that add oxygen atoms to the substrate. For alkaloid molecules,
though, oxygenation is more usual than dehydrogenation. This may be
because the dehydrogenase enzymes are closely matched with the
structures of their usual substrates, while oxygenases are less specific.
Cytochrome P-450 Enzymes occur in all classes of organisms, and a
single species may have several dozen different types, acting on hundreds
of different substrates as dioxygenases. Some use flavoprotein as a
reductant and some use cytochrome Bs. They have roles both in normal
metabolism of steroids, eicosanoids, nitric oxide, etc. and also in oxidizing
exogenous compounds, including certain alkaloids. Some exogenous
compounds induce the formation of P-450 enzymes through a complex
ALKALOIDS IN ANIMALS 5
series of molecular events 1 . As well as introduction of oxygen atoms,
the P-450 systems are also responsible for removal of N-methyl groups,
converting them to carbon dioxide.
Not all oxidations of alkaloids can be ascribed to microsomal P-450
systems. Mammalian liver contains a flavoprotein oxidase, first discovered
in rabit liver and called "quinine oxidase". It is also an aldehyde oxidase,
and its mechanism of action on heterocyclic nitrogen compounds probably
involves addition of hydroxide ion to a suitable ring position, followed by
dehydrogenation. Thus, the introduced oxygen atom comes from water
rather than from molecular oxygen, in contrast to the P-450 oxygenases.
2.
.C Conjugation
Conjugation describes a process in which some exogenous molecule
becomes joined to a common metabolite. Examples are the addition of acyl
groups, amino acid residues, carbohydrate groups or sulfate groups.
Conjugation may follow preliminary oxidative or hydrolytic processes
that release hydroxyl groups suitable for the derivatization, there are also
some cases of addition of methyl groups 3.
Besides metabolic process that degrade alkaloids, there are also process
that produce alkaloids in animals. Some of these are straightforward
condensations of amines with aldehydes or ketones as in the formation of
isoquinolines or harman derivatives by condensation of an aldehyde or
ketone with, respectively, dopamine or tryptamine 4, 5, 6. More
complex reactions must also occur to account for the formation of such
complex structures as morphine, which is now well-established as an
endogenous compound in animals, although at very low concentration 7,
8.
Amaryllidaceae Alkaloids
Galanthamine products found in human plasma and urine result from
epimerization of the hydroxyl group and then dehydration to a ketone 9.
Caffeine and other Purines
Methylated xanthines like caffeine are degraded in humans and rats by
oxidative removal of methyl groups 10. Thus caffeine goes to 1,7-
dimethylxanthine and 1-methylxanthine 11. After the first demethylation
there is an alternate pathway producing 5-acetylamino-6-formylamino-3-
methyluracil. This pathway is more active in people with a more active
acetylation system ,21 13. There are individual, quantitative differences
in the activity of this pathway in humans. In rats a major metabolite is
6 T. ROBINSON
1,3,8-trimethylallantoin, in which all three methyl groups are retained .41
In rabbits the major urinary products are, in order, 1-methylxanthine, -1
methyluric acid, 7-methylxanthine, and 1,7-dimethylxanthine 15.
Pretreatment of rats with caffeine increased the activities of the P-450
enzymes; so that demethylation and C-8 oxidation were doubled as
compared with untreated rats 16. Pretreatment with polycyclic aromatic
compounds causes a similar increase in this P-450 activity .71
Experiments with liver slices and cultured cell lines have corroborated
this pathway of purine degradation. In human liver slices 61 xanthine
derivatives were produced from caffeine by action of P-450 system.
Demethylation at N-3 was the most prominent process 18, 19.
Comparison of cell lines from humans, hamsters, mice, and rats show
some interspecies differences but all of them demethylated and oxidized
caffeine 20. Human liver cells give 1,3,7-trimethylurate as the major
metabolite of caffeine, but also made were the intermediate products
theobromine, theophylline, and paraxanthine 21. Human liver
microsomes convert theophylline to 1-methylxanthine, 3-methylxanthine,
and 1,3-dimethyluric acid 22, 23. Human kidney microsomes produced
each of the three possible demethylated products as well as 1,3,7-
0 3HC 0
3H~( Caffeine 3H~( enillyhpoehT
1 1
O 3HCi O H
CH3....,J~
1,7-Dimethylxanthine G
1 enihtnaxlyhteM-
O
CH3~ ~ H;C)C~"I~ 3
5-Acetylamino-6-formylamino-3-methyluracil
H
Caffeine Metabolism
