Biochem. J. (2003) 376,1–14 (Printed in Great Britain)
1
REVIEW ARTICLE
A perspective of polyamine metabolism
Heather M. WALLACE*†
1
,Alison V. FRASER*† and Alun HUGHES*†
*Department of Medicine and Therapeutics, University of Aberdeen, Polwarth Building, Foresterhill, Aberdeen AB25 2ZD, Scotland, U.K., and †Department of Biomedical Sciences,
University of Aberdeen, Polwarth Building, Foresterhill, Aberdeen AB25 2ZD, Scotland, U.K.
Polyamines are essential for the growth and function of
normal cells. They interact with various macromolecules, both
electrostatically and covalently and, as a consequence, have a
variety of cellular effects. The complexity of polyamine meta-
bolism and the multitude of compensatory mechanisms that
are invoked to maintain polyamine homoeostasis argue that these
amines are critical to cell survival. The regulation of polyamine
content within cells occurs at several levels, including transcrip-
tion and translation. In addition, novel features such as the
+ 1 frameshift required for antizyme production and the rapid
turnover of several of the enzymes involved in the pathway
make the regulation of polyamine metabolism a fascinating
subject. The link between polyamine content and human disease
is unequivocal, and significant success has been obtained in
the treatment of a number of parasitic infections. Targeting the
polyamine pathway as a means of treating cancer has met with
limited success, although the development of drugs such asDFMO
(α-difluoromethylornithine), a rationally designed anticancer
agent, has revolutionized our understanding of polyamine
function in cell growth and provided ‘proof of concept’ that
influencing polyamine metabolism and content within tumour
cells will prevent tumour growth. The more recent development
of the polyamine analogues has been pivotal in advancing our
understanding of the necessity to deplete all three polyamines
to induce apoptosis in tumour cells. The current thinking is that
the polyamine inhibitors/analogues may also be useful agents
in the chemoprevention of cancer and, in this area, we may
yet see a revival of DFMO. The future will be in adopting a
functional genomics approach to identifying polyamine-regulated
genes linked to either carcinogenesis or apoptosis.
Keywords: apoptosis, cancer, cell growth, putrescine, spermidine,
spermine.
INTRODUCTION
The initial discovery of the polyamines dates back to 1678 when
Antonie van Leeuwenhoek isolated some ‘three-sided’ crystals
from human semen [1]. However, it was not until 1924 that
the empirical formula of the crystals was deduced [2], and it
wasafurther 2 years before the products were synthesized
chemically [3]. The names spermidine and spermine therefore
reflect the original discovery. Putrescine (1,4-diaminobutane) was
first isolated from Vibrio cholerae,butitderives its common name
from the large quantities found in putrefying flesh [4]. From these
inauspicious beginnings it is therefore perhaps surprising that,
today, polyamines should be considered critical regulators of cell
growth, differentiation and cell death. In the last 30 years there
has been a steady rise in the number of publications per annum
focussing on polyamines, with approx. 1600 papers published in
2000.
Polyamines are found in all living species, except two orders of
Archaea, Methanobacteriales and Halobacteriales [5]. This con-
servation across evolution is a positive feature in that it argues for
their importance in cell survival, but it may also be a drawback in
that it implies a lack of specific function [6].
POLYAMINES AS CATIONS
At physiological pH, polyamines carry a positive charge on each
nitrogen atom and it has been suggested that polyamines are sim-
Abbreviations used: AZ, antizyme; AZI, AZ inhibitor; cdk, cyclin-dependent kinase; CHENSpm, N
1
-ethyl-N
4
-[(cycloheptyl)methyl]-4,8-diazaundecane;
dcSAM, decarboxylated S-adenosylmethionine; DFMO, α-difluoromethylornithine; MDL 72527, N
1
,N
4
-bis(buta-2,3-dienyl)butane-1,4-diamine; MGBG,
methyglyoxal bis(guanylhydrazone); Nrf-2, nuclear factor-E2-related factor 2; ODC, ornithine decarboxylase; PAO, polyamine oxidase; PRE, polyamine
response element; SAMDC, S-adenosylmethionine decarboxylase; SMO, spermine oxidase; SSAT, spermidine/spermine N
1
-acetyltransferase; UTR,
untranslated region.
1
To whom correspondence should be addressed (e-mail h.m.wallace@abdn.ac.uk).
ply ‘supercations’, equivalent to one or two calciumormagnesium
molecules. However, the charge on the polyamines is distributed
along the entire length of the carbon chain, making them unique
and distinct from the point charges of the cellular bivalent cations.
Their positive charge enables polyamines to interact electro-
statically with polyanionic macromolecules within the cell.
Spermidine and spermine can bridge the major and minor grooves
of DNA, acting as a clamp holding together either two different
molecules or two distant parts of the samemolecule [7]. Structural
studies indicate that the polyamines interact with individual
rather than multiple DNAmolecules [8]. Selectivity of polyamine
binding to secondary structures of DNA has been suggested from
crystallographic studies with polyamines having a preference
for pyrimidine residues, particularly thymidine, although this
may be influenced by the neighbouring nucleotides and the
nature of the secondary structure [9]. Polyamine analogues such
as bis(ethyl)homospermine (‘BEHSpm’; ‘BE-4-4-4’) have been
shown to alter the DNA–nuclear matrix interaction, suggesting
that not only do polyamines alter the structure of DNA, but they
also influence its function [10]. In the nucleosome, polyamine
depletion results in partial unwinding of DNA and unmasking
of sequences previously buried in the particle. These newly re-
vealed sequences are potential binding sites for factors regulating
transcription [11]. This, together with the fact that polyamines
favour the formation of triplex DNA at neutral pH, may provide
amechanism whereby polyamines regulate the transcription of
growth regulatory genes such as c-myc [12–14].
c©
2003 Biochemical Society

2 H. M. Wallace, A. V. Fraser and A. Hughes
Scheme 1 Pathways of polyamine metabolism
Further abbreviation: MAT,methionine adenosyltransferase.
In addition to interracting with DNA and RNA, polyamines
can also interact with acidic phospholipids in membranes [15].
In general, spermidine and spermine increase the rigidity of the
membrane by forming complexes with phospholipids and pro-
teins. They may also have an antioxidant role, preventing lipid
peroxidation [16]. Polyamines have been implicated in the regu-
lation of several membrane-bound enzymes, including adenylate
cyclase [17], tissue transglutaminase [18] and some ion channels
such as NMDA (N-methyl-D-aspartate), KIR (inwardly rectifying
K
+
)andvoltage-activated Ca
2+
channels [19–21].
If, however, charge is the defining feature of the polyamines,
then surely one polycation would be sufficient? Themost obvious
choice would be spermine, as it has the greatest charge, longest
length andmost flexibility. The sheer complexity of the regulation
and metabolism used by the polyamines argues that they, or their
associated enzyme activities, have other critical functions within
the cell not based solely on direct charge–charge interactions.
POLYAMINE METABOLISM
In eukaryotic cells, the three polyamines are synthesized from
L-arginine (via L-ornithine) and L-methionine by a series of
six interdependent enzyme reactions (Scheme 1). Putrescine
is formed from the decarboxylation of ornithine, by ODC
(ornithine decarboxylase; EC 4.1.1.17), and this combines
with dcSAM (decarboxylated S-adenosylmethionine) formed by
SAMDC (S-adenosylmethionine decarboxylase; EC 4.1.1.50), to
produce spermidine via spermidine synthase (EC 2.5.1.16), and
spermine through a second aminopropyltransferase reaction
involving spermine synthase (EC 2.5.1.22). The synthases are
stable enzymes that are expressed constitutively with little re-
corded inducibility [22]. Both enzymes are active as homo-
dimers: spermidine synthase has a subunit molecular mass of
36 kDa, whereas spermine synthase consists of two subunits
of 44 kDa. Unlike the decarboxylases, both enzymes are regulated
by the availability of their substrates, with the K
m
values resembl-
ing closely the tissue concentrations for dcSAM and putrescine
or spermidine.
SSAT (spermidine/spermineN
1
-acetyltransferase; EC2.3.1.57)
is the first step in the retroconversion process, using acetyl-
CoA to form N
1
-acetylspermidine and spermine. The N
1
-acetyl
derivatives are then the preferred substrates of FAD-dependent
PAO(polyamine oxidase; EC1.5.3.11), producing spermidine and
putrescine respectively [23]. The intermediate products of poly-
amine catabolism, N
1
-acetylspermidine and N
1
-acetylspermine,
are found only rarely in normal cells, mainly because these are the
major polyamines exported from the cell [24]. Acetylpolyamines
are, however, found in high concentrations in cancer cells, provi-
ding a link between alterations in polyamine metabolism and
c©
2003 Biochemical Society