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Proceedings of the Fourth International Symposium on Edible Alliaceae
Ed. Liu Guangshu
Acta Hort. 688, 41-57 (2005)
Biological Activity of Allium Compounds: Recent Results
Eric Block
Department of Chemistry
State University of New York at Albany
Albany, New York 12222
USA
Keywords: Allium sativum, Allium cepa, organosulfur compounds, organoselenium
compounds, antibiotics, anticarcinogenic acitivity
Abstract
A brief overview of the chemistry of genus Allium plants, particularly garlic
(Allium sativum) and onion (Allium cepa), is followed by a discussion of the metabolism
of Allium compounds and their anticarcinogenic, antibiotic, anticoagulant, antiathero-
genic, antihyperlipidaemic, antihypertensive, immune system, heptapic system, bird
repellent and insecticidal activity. Health problems following ingestion of Allium plants
are also summarized. The emphasis is on work published during the past five years.
INTRODUCTION
Garlic (Allium sativum), onion (Allium cepa) and other genus Allium plants have
been used for thousands of years as foods and spices as well as popular remedies. In 1858
Louis Pasteur reported that garlic is antibacterial, while Albert Schweitzer in the early and
mid-1900s employed garlic in Africa for treatment of amoebic dysentary, cholera and
typhoid fever (Block, 1985). Garlic was prized for its antimicrobial effects long before
microbes were even discovered. French priests of the Middle Ages used garlic to protect
themselves against bubonic plague, now known to be a bacterial infection. During the World
Wars, physicians in Europe and Russia prevented infection by putting garlic poultices (moist
applications of crushed plant materials) directly on wounds and amputations. Garlic has also
been recommended as an insect repellent (Stjernberg and Berglund, 2000, 2001). Recent
human population studies suggest that regular consumption of Allium plants lowers the risk
of developing heart disease and cancer.
In view of the widespread consumption of Alliums as fresh plants, as well as supple-
ments, the biological activity of these plants is of great current interest . Through the year
2000 more than 2000 scientific papers and a monograph (Koch and Lawson, 1996) have
been published describing scientific research on the chemistry, pharmacology and clinical
use of garlic (Block, 1985, 1992; Kiple and Ornelas, 2000). For the time period 2000-2004
alone there have been more than 112 publications on the subject of biological activity of
Alliums and identification of Allium compounds. This paper reviews these recent papers on
the biological activity of Allium plants and their chemical components, along with a few
older publications. A brief discussion of the chemistry of genus Allium plants is followed by
a discussion of the metabolism of Allium compounds and anticarcinogenic, antibiotic,
anticoagulant, antiatherogenic, antihyperlipidaemic, and antihypertensive activity as well as
2
immune, heptapic system, bird repellent and insecticidal activity. Finally, health problems
following ingestion of Allium plants are summarized.
RESULTS AND DISCUSSION
Allium Chemistry
To understand the biological activity of Alliums and chemical compounds formed
from them it is important to understand the chemistry of these plants. Onions are rich in two
chemical groups that have perceived benefits to human health, the flavonoids and the alk-
(en)yl cysteine sulfoxides (ACSOs). Two different types of flavonoids are found in onion,
the anthocyanins, which impart a red/purple color, and the flavanols such as quercetin and
its derivatives, responsible for the yellow and brown skins. The ACSOs are important flavor
precursors which, when cleaved by the enzyme alliinase, generate the characteristic odor and
taste of onion as well as the unique lachrymatory properties. In addition to the above com-
pounds, proteins from garlic have been shown to possess antifungal activity (Wang and Ng,
2001) while a lectin isolated from garlic shows antitumor activity (Karasaki, et al., 2001).
In the case of garlic, the major sulfur containing ACSO is called alliin (S-allylcyste-
ine sulfoxide; CH2=CHCH2S(O)CH2CH(NH2)CO2H). This mixes with the alliinase enzymes
(alliinase characterization: Kuettner et al., 2002) to rapidly form allicin (CH2=CHCH2S(O)-
SCH2CH=CH2) and other thiosulfinates (allicin is classified as a thiosulfinate, RS(O)SR).
Allicin in turn decompose to other compounds including ajoene (CH2=CHCH2S(O)CH2-
CH=CHSSCH2CH=CH2)(Block et al., 1984) as well as diallyl disulfide, trisulfide and tetra-
sulfide (these latter three compounds are called diallyl polysulfides, CH2=CHCH2Sn-
CH2CH=CH2, n = 2-4) and vinyl dithiins (Block, 1985). In the case of onion, the major
ACSO is isoalliin (S-1-propenylcysteine sulfoxide; CH3CH=CHS(O)CH2CH(NH2)CO2H).
This mixes with the onion alliinase and the lachrymatory factor (“LF”) synthase enzymes
(Imai et al., 2002) to rapidly form the lachrymatory factor (responsible for causing tearing,
or lachrymation; CH3CH2CH=S=O). In onion, isoalliin is also converted into thiosulfinates
related to allicin, as well as cepaenes (related to ajoene) and zwiebelanes (Block, 1992). As
in the case of allicin from garlic, the onion thiosulfinates are unstable compounds which de-
composes giving polysulfides.
When genus Allium plants are grown in selenium-rich soil, selenium is incorporated as
selenocysteine forming compounds such as Se-methyl selenocysteine (CH3SeCH2CH(NH2)-
CO2H), its gamma-glutamyl derivative and selenoproteins (Dong et al., 2001; Yang, 2002).
Much work has appeared on the extraction, chromatographic separation, isolation, spectro-
scopic identification, characterization, synthesis and chemical and kinetic study (Block et
al., 1996a,b, 1997, 2001a; Shen and Parkin, 2000; Shen et al., 2002a,b) of the diverse sulfur-
and selenium-containing compounds from genus Allium plants. This work uses techniques
such as microwave spectroscopy (Block et al., 1996c), GC-MS (Block et al., 1992a; Arnault
et al., 2000; Tsuge et al., 2002; Kubec et al. 2000, 2002; Mondy et al., 2002), GC-AED (Cai
et al., 1994, 1995b), HPLC (Block et al., 1992b; Ferary et al., 1998; Krest et al., 2000), LC-
MS (Calvey et al., 1997, 1998; Arnault et al., 2003), LC-ICP-MS (Bird et al., 1997; Ip et al.,
2000), synchrotron X-ray atomic spectroscopy (XAS; Sneeden et al., 2004), supercritical
fluid extraction (Calvey et al., 1994a,b; Rybak et al., 2004), and electochemical methods
(Keusgen et al., 2003). The subject of “Allium chemistry” has been reviewed (Block, 1992).
Commercial garlic preparations sold as dietary supplements differ in their method of
processing, which determines the types of sulfur compounds in the product. Thus the pro-
ducts differ in their chemical composition (Block, 1998). Major forms of dietary supple-
3
ments include: aged garlic extract, dehydrated preparations, distilled oil, and oil macerates
(chopped garlic in oil). Controversy exists regarding the health benefits of these
supplements, particularly as to their effect on blood lipid and cholesterol levels, which is
notably dependent on the ability of the garlic products to release allicin under simulated
gastrointestinal conditions (Lawson et al., 2001, Lawson and Wang, 2001).
Biological Activity
Antitumor (anticarcinogenic), antimicrobial, antiplatelet, antithrombotic, antihyper-
lipidaemic, antihypertensive and antiasthmatic effects as well as immune-stimulatory prop-
erties are attributed to garlic and other Alliums. Consumption of garlic and other Alliums is
said to reduce cancer risk including prostate cancer (Grant, 2004; Hsing et al., 2002). .
Extracts and components of garlic effectively block experimentally induced tumors. Bird
repellent effects of garlic have recently been identified (Hile et al., 2004). Information on
biological activity of genus Allium plants and Allium compounds can be easily obtained via
a web search using PubMed (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi), the free search
and retrieval system of the National Center for Biotechnology Information (NCBI) of the
U.S. National Library of Medicine (NLM). Using the terms “garlic”, “allium”, “allicin”,
“ajoene”, “diallyl trisulfide” and “vinyl dithiin” generates more than 3000 references.
1. Metabolism.
Presence of Garlic Compounds on Human Breath. The composition of the breath after con-
suming crushed fresh garlic provides important clues to the metabolism of its sulfur and sel-
enium compounds, particularly since none of the compounds found in the breath are found
in crushed garlic. When fresh garlic is eaten, the initial breath odor is due mainly to allyl
mercaptan (2-propenethiol) and methanethiol as well as much smaller amounts of diallyl
disulfide; however, this represents odor from the mouth and throat and disappears in about
one hour (Cai et al., 1995a; Suarez et al., 1999; Tamaki and Sonoki, 1999). The odor that
comes from the lungs rises slowly and lasts over 24 hours. It consists mainly of allyl methyl
sulfide (87% of the sulfur compounds present at 9 hours) and dimethyl sulfide (11%)(Cai et
al., 1995a; Ruiz et al., 1994; Suarez et al., 1999; Rosen et al., 2000, 2001). Allyl methyl
sulfide reaches maximum levels in 3-4 hours after consumption and requires 30-50 hours for
complete disappearance. It is a major metabolite of allicin and other allyl thiosulfinates
(Lawson 1998, Lawson and Wang 2001) and, in contrast to the disulfides and thiols, is only
slowly metabolized by the gut and liver (Suarez et al., 1999). The levels of sulfur com-
pounds in human breath after consumption of heat-treated garlic are lower than those after
consumption of untreated raw garlic (Tamaki and Sonok,i 1999), suggesting that allicin is
the major source of breath sulfur compounds. Dimethyl selenide is the main selenium
compound found in the breath after consuming garlic and may be sufficiently abundant to
contribute to the breath odor (Cai et al, 1995a).
Diallyl Disulfide. In the rat, diallyl disulfide is absorbed and transformed into allyl mercap-
tan, allyl methyl sulfide, allyl methyl sulfoxide, and allyl methyl sulfone, which are detected
throughout the excretion period. Overall, the highest amounts of metabolites were measured
48-72 hours after the diallyl disulfide administration. Allyl methyl sulfone is the most
abundant and persistent of these compounds. The levels of all the sulfur compounds rapidly
decline within the first week after administration and disappear during the second week.
Only allyl methyl sulfoxide and allyl methyl sulfone are significantly excreted in urine
(Germain et al., 2002). In earlier work only allyl mercaptan and allyl methyl sulfide were
reported as metabolites of diallyl disulfide in primary rat hepatocytes (Sheen et al., 1999).
4
2. Anticarcinogenic Activity.
General. A meta-analysis (Fleischauer et al., 2000) systematically reviewed human epidemi-
ological studies of the relationship of garlic consumption and cancer. The mean intake of
raw or cooked garlic among the highest consumers was 18.3 g/week (ca. 6 cloves). An aver-
age reduction by 30% in colorectal and by 50% in stomach cancers, with a trend towards re-
duction in prostate, breast, and laryngeal cancers was reported. The possibility of publication
bias was noted due to potential lack of publication of negative studies. In a follow-up critical
assessment of much of these data (Fleischauer and Arab, 2001) it was suggested that caution
in interpreting study results is warranted due to methodological flaws, small patient popula-
tions in some, and in others, confounding variates. Additionally, consumption of garlic is us-
ually correlated with a relatively high consumption of vegetables, which may contribute to
the cancer protective effects reported in garlic studies. Laboratory and animal studies show
that compounds in garlic inhibit tumor cell metabolism, as well as the initiation and promo-
tion of tumors, and alter immune responses. For example, laboratory studies involving cell
cultures suggest that pure samples of some compounds found in crushed garlic, garlic distill-
ed oil, or garlic macerates inhibit cell division and have antimutagenic properties (prevents
potentially damaging changes in deoxyribonucleic acid (DNA)). Thus, the garlic distilled oil
component diallyl disulfide at concentrations of 263-2630 ppm inhibits the growth of human
breast cancer cells (Nakagawa et al., 2001). Animal studies show that selenium enriched
garlic given in the diet to female rats at the level of 3 ppm for a period of six weeks reduced
the incidence of carcinogen-induced mammary tumors to the extent of 66-70% compared to
control rats, who were given 0.1 ppm dietary selenium for the same time period. These
experiments demonstrate that selenium enriched garlic has cancer preventive properties (Ip
et al., 2000; Dong et al., 2001).
Mechanism of Action. The anticancer effects of garlic are likely due to the induction (en-
hancing action) of garlic organosulfur compounds on phase II carcinogen detoxification en-
zymes, such as glutathione S-transferase and quinone reductase, in the liver and other organs
(Guyonnet et al., 2001; Cohen et al., 1999, Munday and Munday, 2001, 2004; Munday et
al., 2003). Lectins, which are non-sulfur compounds found in garlic strongly inhibit the
growth of human cancer cell lines in laboratory studies (Karasaki et al., 2001). The ability of
allicin to react rapidly with free radicals (reactive oxygen molecules that can damage
important parts of cells including DNA) together with its remarkable ability to cross through
membranes (artificial and biological), including those in human red blood cells may also
contribute to its antioxidant and anticancer activity (Rabinkov et al., 1998, 2000). Heliobac-
ter pylori infection is a major stomach cancer risk factor and garlic has been shown to kill
this organism. (Ernst, 1999). Allicin may combine with glutathione to form an adduct which
itself can function as an antioxidant and protective agent (Rabinkov et al., 2000). Ajoene is
reported to possess anti-inflammatory activity similar to that of the non-steroidal anti-in-
flammatory drug indomethacin (Dirsch et al., 2001). The anticancer activity of selenium en-
riched garlic can be attributed to the presence of analogs of the various garlic sulfur com-
pounds, such as the cysteine derivatives, in which the essential minerial selenium replaces
sulfur (Ip et al., 2000; Dong et al., 2001).
Allicin. Allicin induces apoptosis of cancer cells and inhibits the growth of cancer cells of
murine and human origin. Allicin induced the formation of apoptotic bodies, nuclear con-
densation and a DNA ladder in cancer cells. Furthermore, activation of caspases-3, -8 and -9
and cleavage of poly(ADP-ribose) polymerase were induced by allicin. These results pro-
vide a mechanistic basis for the antiproliferative effects of allicin and partly account for the
known chemopreventive action of garlic extracts (Oommen et al., 2004). Thus, allicin, but
5
not its precursor alliin, inhibited proliferation of human mammary (MCF-7), endometrial,
and colon (HT-29) cancer cells (50% inhibitory concentration = 10-25 microM). This effect
may be attributed to the ability of allicin to transiently deplete the intracellular glutathione
level (Hirsch et al., 2000). Alliinase chemically conjugated to a tumor cell could be used for
site-directed generation of allicin upon addition of alliin. The allicin generated from the tu-
mor-localized alliinase had a high antitumor activity leading to tumor growth arrest which
became significant 2 weeks after its onset, and it continued to rise, reaching highly signifi-
cant inhibition a week later (Miron et al., 2003).
Ajoene. The pharmacologic properties of ajoene and its possible role in the prevention and
treatment of cancer has received increasing attention. Ajoene inhibits skin-tumor promotion
in mice (Nishikawa et al., 2002). The antitumor activity of ajoene (particularly the Z-isomer
toward human leukemia cells) is thought to be associated with its apoptosis-inducing effect
and activation of caspase-3 and -8 (Ahmed et al., 2001; Ledezma et al., 2004; Antlsperger et
al., 2003; Tilli et al., 2003; Li et al., 2002a,b; Dirsch, 2002). The antitumor activities of Z-
ajoene in vitro and in vivo was demonstrated against a panel of human tumor cell lines with
IC(50) values varying from 5.2 mM to 26.1 mM and at a lower extent in normal marsupial
kidney cells (PtK2). In PtK2 cells, exposure to 20 microM Z-ajoene for 6 h induced a com-
plete disassembly of the microtubule network so that an increased number of cells were
blocked in early mitotic stages. In vivo, Z-ajoene inhibited tumor growth by 38% and 42%
in mice grafted with sarcoma 180 and hepatocarcinoma 22, respectively (Li et al., 2002a,b).
A study of the effects of ajoene on the in vitro activities of 20S proteasome purified from
human erythrocytes revealed that ajoene inhibited the trypsin-like activity in a dose- and
time-dependent manner (Xu et al., 2004). Further, the ability of 20S proteasome to degrade
the OVA(51-71) peptide, a model proteasomal substrate, was partially but significantly
inhibited by ajoene. In addition, when human leukemia cell line HL60 was treated with ajo-
ene, both trypsin- and chymotrypsin-like activities were affected, cells arrested in G(2)/M
phase and total amount of cytosolic proteasome increased. All these data clearly indicate that
ajoene may affect proteasome function and activity both in vitro and in the living cell.
Diallyl Sulfide, Disulfide and Trisulfide. Garlic oil, diallyl disulfide and trisulfide, and bis(1-
propenyl)sulfide are potent inducers of the phase II enzymes quinone reductase, and gluta-
thione reductase and S-transferase in rat tissues (Munday and Munday, 2001; Wu et al.,
2001). Diallyl disulfide and trisulfide modulate cytochrome P4501-mediated bioactivation
of benzo[a]pyrene in human hepatoma cells, providing a possible explanation for the known
protective effect of allyl sulfides on benzo[a]pyrene-induced carcinogenesis (Chun et al.,
2001). Diallyl sulfide, sulfoxide and sulfone reduce the incidence of a multitude of chemi-
cally induced tumors in animal models. The impediment of phase I activation of these car-
cinogens is hypothesized to explain the reduction in tumor incidence. Diallyl sulfide, sulfox-
ide and sulfone are competitive inhibitors of cytochrome P(450)2E1; diallyl sulfone is also a
suicide inhibitor of cytochrome P(450)2E1 (Yang et al., 2001). It is perhaps significant that
allyl methyl sulfone is the ultimate metabolite of diallyl disulfide so the possibility exists
that the former compound is also an inhibitor of cytochrome P(450)2E1.
3. Antibiotic Activity.
General. The antibiotic activity of garlic (antibacterial, antiviral, antifungal, antiprotozoal)
has been reviewed (Harris et al., 2001). Allicin and related compounds (thiosulfinates)
formed on crushing garlic, as well as ajoene and similar compounds in oil-macerated garlic,
have a variety of antimicrobial activity (Ankri et al., 1999; Yoshida et al., 1999, 1998). In la-
boratory cell cultures garlic oil and its component diallyl polysulfides show antibacterial
activity against multidrug-resistent strains of E. Coli and possess antifungal activity against
6
Candida albicans with the magnitude of activity of the diallyl polysulfides following the
order diallyl tetrasulfide > diallyl trisulfide > diallyl disulfide > diallyl monosulfide (Tsao
and Yin, 2001). Allicin has antiparasitic activity against some major human intestinal proto-
zoan parasites including Entamoeba histolytica and Giardia (Harris et al., 2000) and shows
antiviral activity. In experiments involving cell cultures, ajoene at concentrations of 82 ppm
prevents replication of the human immunodeficiency virus (HIV-1)(Walder et al., 1997).
Garlic oil and individual garlic oil components (such as diallyl polysulfides and
dimethyl trisulfide) show antimicrobial effects after 24-48 hours at concentrations ranging
from 0.002 to 2.5% toward a variety of human intestinal bacteria, although the effect is
generally smaller than that of garlic powder (Ross et al., 2001). Garlic compounds all show
substantial activity against the bacteria H. pylori which causes chronic gastritis and ulcers in
the stomach and upper part of the intestine. Thus, the lowest concentrations showing com-
plete inhibition of growth (MIC or “minimal inhibitory concentration”) after 24 hours for H.
pylori strains is 16-32 ppm of mixed garlic oil and 250-500 ppm for a commercial garlic
powder (500 ppm of garlic powder, from sliced, dried, pulverized fresh garlic cloves, is
equivalent to 4 ppm of allicin). Allicin is more active than its breakdown products diallyl
disulfide, garlic oil or garlic powder but similar in activity to diallyl tetrasulfide, a minor
component of distilled oil of garlic (O’Gara, 2000). Oil-macerated garlic constituents are also
active against H. pylori (Ohta, 1999).
Mechanism of Action. The mechanism of action is most likely the binding of several com-
ponents (allicin, ajoene, polysulfides) to –SH groups in enzymes essential to microbial pro-
liferation, thus inhibiting growth (Naganawa et al., 1996; Ankri and Mirelman 1999). The
notable ability of allicin to rapidly cross through membranes (artificial and biological), in-
cluding those in human red blood cells, may greatly enhance the intracellular interaction of
allicin with sulfur-containing enzymes in microbes (Miron et al,, 2000; Ankri et al,, 1997)
and thereby kill microbes by shutting down their metabolism (Ankri and Mirelman, 1999).
Allicin and Related Allyl Methyl Thiosulfinates. These compounds inhibit the in vitro growth
of E. coli (Seo et al., 2001) and H. pylori (Canizares et al., 2004). The in vivo efficacy of
allicin was tested in mice infected intravenously (iv) with A. fumigatus. Allicin (5 mg/kg
body weight) was administered iv once daily for 5 days post-infection or orally (po) (9
mg/kg body weight) for 5 days pre-infection and 10 days post-infection. No ill effects were
observed in allicin-treated uninfected mice. The in vitro MICs (minimum inhibitory concen-
trations) of allicin were 8-32 mg/L. Thus, allicin may be an effective fungicide in vitro; it
exerts its fungicidal activity within 2-12 h of administration in vitro. Allicin treatment signi-
fIcantly prolonged survival of infected mice from 7.7 days in untreated mice to 21.3 and
13.9 days for allicin iv and po treated mice, respectively. Allicin iv treatment led to a
significant 10-fold reduction in fungal burden in A. fumigatus infected mice (Shadkchan et
al., 2004). Garlic derivatives such as ajoene and diallyl trisulfide as well as raw garlic extract
have in vitro activity against Scedosporium prolificans (Davis et al., 2003).
Garlic and Onion Distilled Oils and Diallyl Trisulfide. The distilled oils of garlic and onion
and their constituent sulfides with three or more sulfur atoms are potent inhibitors of yeast
growth. The minimum inhibitory concentrations of garlic oil, onion oil, diallyl trisulfide,
diallyl tetrasulfide, and dimethyl trisulfide for all the yeasts tested ranged between 2 and 45
ppm. The oils and their constituent sulfides, however, were only very weakly antibacterial,
showing MICs of greater than 300 ppm for most of the bacteria tested (Kim et al., 2004;
Avato et al., 2000). For dimethyl trisulfide and garlic oil MICs of ca. 20-40 ppm are found
for the human enteric pathogens Shigella sonnei, Yersinia enterocolitica and Listeria mono-
cytogenes (Ross et al., 2001). The toxicity of garlic oil, allicin, and diallyl trisulfide to H.
7
pylori measured as MICs is 16-32, 6, and 3-6 microgram/mL, respectively (O’Gara et al.,
2000). The therapeutic efficacy of diallyl trisulfide (25-75 mg/kg per day) in mouse models
with murine cytomegalovirus hepatitis has been demonstrated (Liu et al., 2004).
Ajoene. Ajoene was applied topically to localized lesions of Cladophialophora carrionii
chromoblastomycosis once a day for 12-16 weeks. Complete clinical and mycological re-
mission was achieved in 14/19 patients (74%)(Perez-Blanco et al., 2003). Ajoene has an an-
tiproliferative and leishmanicidal effect on various Leishmania species (Ledezma et al.,
2002) and is useful in the treatment of tinea pedis (Ledezma et al., 2000).
4. Anticoagulant (Antiplatelet) and Anti-atherogenic Activity
Ajoene, which inhibits baboon platelet aggregation in vitro and in vivo (Teranishi et al.,
2003) and prevents the development of microangiopathy and thrombotic disorders seen in
primates infused with pig cells (Benatuil et al., 2003), is thought to prevent blood clotting by
selectively blocking the binding of a protein involved in blood clotting called fibrinogen to
platelets, which are cells that circulate in the blood and participate in blood clotting This is
the crucial step in platelet-platelet interactions leading to blood clotting (Apitz-Castro et al.,
1986). Allicin and related sulfur containing compounds (thiosulfinates) are implicated as a
principle source of the antiplatelet property (inhibiting the effect of platelets) of garlic juice
and are significantly more potent platelet inhibitor than aspirin at nearly equivalent concen-
trations (Briggs et al., 2000). They inhibit platelet aggregation and microparticle shedding at
a calpain-dependent step (Rendu et al., 2001). The anti-atherogenic action of garlic may be
related to its inhibitory influence on geranylgeranyltransferase I (GGTase I)(Golovchenko et
al., 2003). Diallyl trisulfide, a breakdown product of allicin, shows antiplatelet activity and
inhibits formation of thromboxane (Bordia et al., 1998). Allyl methyl trisulfide inhibits the
arachidonic acid cascade at the reaction site with PGH synthase and specifically inhibits the
conversion of PGG2 to PGH2 (Ariga et al., 2000). Onion juice inhibits platelet-mediated
thrombosis in dogs (Briggs et al., 2001). Extracts of Welsh onion inhibit platelet function in
vitro while boiled extracts activate platelets (Chen et al., 2000).Aggregation inhibition by
freshly-juiced onion extracts varied with time, showing induction of aggregation in the initi-
al time period following juicing, which was dissapated after 30 minutes, as well as platelet
inhibition which increased as the onion extracts aged (Osmont et al., 2003).
5. Antihyperlipidaemic and Antihypertensive Activity.
General. Cardiovascular disease prevention by garlic has been reviewed (Gardner et al.,
2003; Banerjee and Maulik, 2002). Garlic has the ability to relax vascular smooth muscle
(Koch and Lawson, 1996) and ameliorate hypoxic pulmonary vasoconstriction, underlying
high-altitude pulmonary edema (“high-altitude sickness”)(Fallon et al. 1998). An in vitro
biosensor model of isolated blood vessels demonstrated that aqueous garlic extract inhibits
calcium binding to proteoheparan sulfate receptor, which in turn blocks the interaction of
calcium with LDL Thus, garlic inhibits calcium-dependent generation of artherosclerotic
plaque. Furthermore, the addition of HDL and garlic extract caused freshly-formed
nanoplaques to dissolve (Siegel et al., 2004).
Mechanism of Action. The ability of garlic to reduce blood cholesterol levels is attributed to
the effect of allicin a key enzyme, HMGCoA reductase, involved in cholesterol synthesis in
the liver (Teyssier et al., 1999; Cho and Xu , 2000). Allicin decreases formation of fatty
streaks (atherosclerosis) in mice fed a cholesterol-rich diet (Abramovitz et al., 1999). The
effect of garlic, and allicin, to elicit a NO-dependent relaxation in rat isolated pulmonary
arteries is proposed to be mediated via garlic activation of NO formation rather than its sta-
bilization (Ku, et al. 2002; Schwartz et al., 2002). A similar effect is proposed for the
antihypertensive effect of onion in spontaneously hypertensive rats (Sakai et al., 2003).
8
Ajoene and thiosulfinates. Ajoene inhibits cholesterol biosynthesis by affecting HMG-CoA
reductase and late enzymatic steps of the mevalonate (MVA) pathway. MVA constitutes the
precursor not only of cholesterol, but also of a number of non-sterol isoprenoids, such as
farnesyl and geranylgeranyl groups. Covalent attachment of these MVA-derived isoprenoid
groups (prenylation) is a required function of several proteins that regulate cell proliferation.
Ajoene has been shown to interfere with the protein prenylation reaction, an effect that may
contribute to its inhibition of smooth muscle cell proliferation (Ferri et al., 2003). Allicin,
methyl allyl thiosulfinate, and allyl methyl thiosulfinate are more effective inhibitors against
geranylgeranyl protein transferase (GGPT) than farnesyl protein transferase.
Garlic Tablets. Short-term garlic therapy in adults with mild to moderate hypercholesterole-
mia was shown to affect neither lipid levels nor various psychopathologic parameters
(Preleg et al., 2003). In a third study, in hypercholesterolemic patients using an enteric-coat-
ed garlic supplement standardized for 9.6 mg allicin-releasing potential, a 4-6% cholesterol-
lowering effect was observed after 12 weeks (Kannar et al., 2001).
Antioxidant Activity. The antioxidant activity of garlic has been reviewed (Banerjee et al.,
2003). The antioxidant activity of garlic, and in particular its active principle allicin, is attri-
buted in part to the inhibition of nitric oxide (NO) production by cytokine-induced NO syn-
thase (Schwartz et al., 2002). Various Allium-derived sulfur compounds showed antioxida-
tive activity inhibiting lipid hydroperoxide (LOOH) formation in human LDL (Higuchi et
al., 2003). Pure Allium thiosulfinates at levels up to 4 mM are incapable of scavenging H2O2
or superoxide anion. They were less efficient relative to standard antioxidants at quenching
singlet oxygen or reducing 1,1-diphenyl-2-picrylhydrazyl but were about equally effecive at
scavenging hydroxyl radical. Allicin was the most effective and PrS(O)SPr the least effec-
tive thiosulfinate in these assays. These thiosulfinates were incapable of in vitro induction of
quinone reductase in murine hepatoma cells (Xiao and Parkin, 2003).
6. Immune System.
How allicin affects the immune system is not well known. Allicin (20-100 microM) down-
regulates T cell functions, explaining its beneficial biological effects in processes where T
cells play an important role. These observations suggest that allicin may be used ther-
apeutically with chronic inflammatory diseases (Sela et al., 2004). The immune stimulatory
effect of allicin is mediated by redox-sensitive signaling such as activation of p21(ras). The
antitumor effect of allicin is likely related to its immune-stimulatory properties (Patya et al.,
2004). Ajoene shows anti-inflammatory activity similar to that of the non-steroidal anti-
inflammatory drug indomethacin (Dirsch and Vollmer, 2001). Ajoene inhibits the activation
of human endothelial cells induced by porcine cells (Benatuil et al., 2003).
7. Heptatic Detoxification Systems.
The modulatory effect of garlic oil on hepatic drug-metabolizing enzymes can be attributed
to diallyl sulfide, disulfide and trisulfide. These three allyl sulfides vary in modulatory
activity, and this variation is related to the number of sulfur atoms in the molecule. The
polysulfides also increased glutathione S-transferase (GST) activity and at the same time
lowered N-nitrosodimethylamine demethylase activity (Wu et al., 2002). Allyl sulfides have
been found to protect against liver injury caused by carbon tetrachloride (Fukao et al, 2004)
while ajoene protects against injury due to acetaminophen (Hattori et al., 2001).
8. Bird Repellent Activity.
European starlings significantly reduced their consumption of a food mixture that was 50%
food-grade garlic oil (GO)-impregnated granules, even after overnight food deprivation.
Food consumption during 3 hours following overnight food deprivation was reduced by 61-
65% compared to controls. By testing the same subjects with 25%, 10%, and 1% mixtures of
9
granules in feed, it was shown that commercial GO granules were repellent to birds in lower
concentrations, with more than a 50% decrease in feeding for birds presented with a 10%
mixture of GO granules in food and a 17% decrease for the 1% treatment. Products contain-
ing GO show considerable promise as inexpensive, environmentally-benign, non-lethal bird
repellents. In comparing various GO preparations used in this work, NMR methods are
particularly useful for rapid quantitation of major and minor components without requiring
fractionation or isolation procedures, which could adversely effect the less stable compo-
nents (Hile et al., 2004).
9. Insecticidal Activity.
Diallyl trisulfide is a more potent contact toxicant, fumigant and feeding deterrent than
methyl allyl disulfide to two species of stored-product pests (Huang et al., 2000). Allicin
with a 24 h LD50 value of 0.02-0.25 mg/L was more active than disulfides against six test
insects (Auger et al., 1999). Topical application of garlic was found to reduce northern fowl
mite infestation in laying hens (Birrenkott, et al., 2000).
10. Health Problems Following Ingestions of Alliums.
Garlic can cause breath and skin odor as well as heartburn and flatulence (Block, 1992; Cai
et al., 1995a). Consumption of raw garlic in quantities greater than one or two cloves at a
time can irritate the lining of the digestive tract (Koch and Lawson, 1996). Taking garlic
with food may reduce the chance of stomach irritation. Other garlic-related adverse reactions
include contact dermatitis, allergic asthma and rhinitis (inflammation of the mucous mem-
brane in the nose)(Ali et al., 2000). Heartburn from garlic is attributed to the action of allicin
on the valve separating the esophogus from the stomach, which opens and closes more fre-
quently when acted on by Allium compounds, causing “gastric reflux” (Block et al., 1992).
Use of garlic preparations increases the risk of bleeding during surgical procedures
and with the use of anticoagulants (Ang-Lee et al., 2001; Pribitkin and Boger, 2001) such as
warfarin (Heck et al., 2000; Argento, 2000; Evans, 2000). Individuals taking the protease
inhibitor saquinavir should use caution in taking garlic supplements since these supplements
reduce saquinavir blood levels by 50% (Piscitelli et al., 2002). Garlic is a very rare cause of
food allergy (Jappe, 1999; Perez-Pimiento et al., 1999). Alliin lyase was found to be a major
garlic allergen. Skin tests showed that the purified protein elicited IgE-mediated hypersensi-
tive responses in patients with garlic allergy. Carbohydrate groups are involved in the anti-
genicity, allergenicity, and cross-reactivity. Garlic alliin lyase showed strong cross-reactivity
with alliin lyases from other Allium species, namely leek, shallot, and onion (Kao et al.,
2004). Toxic effects associated with oxidative damage to canine erythrocytes cause garlic-
induced hemolysis in dogs (Yang, 2003; Yamato et al., 2003, 1999; Hu et al., 2002).
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation (NSF CHE-9906566),
the Petroleum Research Fund, administered by the American Chemical Society and the
Berryman Institute.
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