1. Introduction
Garlic is one of the edible plants which has generated a
lot of interest throughout human history as a medicinal
panacea. A wide range of microorganisms including bacteria,
fungi, protozoa and viruses have been shown to be
sensitive to crushed garlic preparations. Moreover, garlic
has been reported to reduce blood lipids and to have
anticancer effects. Chemical analyses of garlic cloves have
revealed an unusual concentration of sulfur-containing
compounds (1–3%) [1, 2].
Analysis of steam distillations of crushed garlic cloves
performed over a century ago showed a variety of allyl
sulfides. However, it was not until 1944 that Cavallito and
his colleagues [3] isolated and identified the component
responsible for the remarkable antibacterial activity of
crushed garlic cloves. The compound turned out to be an
oxygenated sulfur compound which they termed allicin,
from the Latin name of the garlic plant, Allium sativum.
Pure allicin is a volatile molecule that is poorly miscible in
aqueous solutions and which has the typical odor of
freshly crushed garlic [4]. Final proof of the chemical
structure of allicin (figure 1) came in 1947, when it was
shown that allicin could be synthesized by mild oxidation
of diallyl disulfide [2]. The debate on the presence of
allicin in crushed cloves versus its absence in odorless
intact cloves was resolved after Stoll and Seebeck [5]
isolated, identified, and synthesized an oxygenated sulfur
amino acid that is present in large quantities in garlic
cloves and which they named alliin (figure 1). Alliin was
found to be the stable precursor that is converted to allicin
by the action of an enzyme termed alliinase which is also
present in the cloves [6]. Only one isomer of alliin ((+)-Sallyl-
L-cysteine-sulfoxide) was found to be present, which
in itself had no antimicrobial activity. The amounts of alliin
and allicin present in different strains of garlic were studied
by numerous investigators. Considerable variations
have been reported, ranging from 2.8 to 7.7 mg/gram
found in Romanian red [2].
The transformation of alliin into the biologically active
allicin molecule upon crushing of a garlic clove is extremely
rapid, being complete in seconds. The enzyme
responsible for the lysis is alliinase, or alliin-lyase
(E.C.4.4.1.4), a pyridoxal 5-phosphate-dependent glycoprotein
consisting of two subunits [7, 8]. Alliinase is
present in unusually high amounts in garlic cloves: at least and reprints 10% of the total protein content (10 mg/g fresh weight)