Beneficial Effects of Garlic Components on Neurological Disorders

Garlic is a dietary supplement derived from the bulb of Allium sativum, which belongs to the family Liliaceae. It is widely used as a flavoring agent for food and a medicinal agent for the treatment of a variety of diseases (Essman 1984). The chemical composition of garlic is complex and the most important and unique feature of garlic chemical composition is its high content of organosulfur compounds. Two classes of organosulfur compounds, such as (a) γ-glutamylcysteines, and (b) cysteine sulfoxides are found in whole garlic cloves. The γ-glutamylcysteine is hydrolyzed and oxidized to alliin (+S-allyl-l-cysteine sulfoxide). This compound is then converted to allicin (thio-2-propene-1-sulfinic acid S-allyl ester) by alliinase, which is released upon cutting, crushing, or chewing the garlic (Fig. 9.1). Alliinase is a very stable enzyme. Although the exact molecular mechanism of alliinase-catalyzed reaction is not fully understood, but it is suggested that alliinase catalyzes the formation of sulfenic acids from cysteine sulfoxides. Sulfenic acids spontaneously react with each other to form unstable compounds called thiosulfinates. In the case of alliin, the resulting sulfenic acids react with each other to form a thiosulfinate known as allicin (half-life in crushed garlic at 23 °C is 2.5 days). The formation of thiosulfinates is completed within 10–60 s of crushing garlic. The storage of garlic powder preparation up to 5 years shows little loss in ability to generate allicin. Allicin not only interacts with thiol-containing proteins, but also decomposes into 2-propenesulfenic acid (Rabinkov et al. 1998). This compound has ability to bind the free radicals and may be responsible for antioxidant effects of garlic (Vaidya et al. 2009). Whole garlic typically has ∼1 % alliin, together with (+)-S-methyl-l-cysteine sulfoxide (methiin). The other components of garlic include (+)-S-(trans-1-propenyl)-l-cysteine sulfoxide, S-(2-Carboxypropyl)-glutathione, γ-glutamyl-S-allyl-l-cysteine, γ-glutamyl-S-(trans-1-propenyl)-l-cysteine, and γ-glutamyl-S-allyl-mercapto-l-cysteine (Fenwick and Hanley 1985). Storage of garlic bulbs at cool temperatures induces alliin to accumulate naturally. On average, a garlic bulb contains up to 0.9 % γ-glutamylcysteines and up to 1.8 % alliin. In addition to these main sulfur compounds, intact garlic bulbs also contain a small amount of S-allylcysteine (SAC), but no allicin. SAC is formed from γ-glutamyl cysteine catabolism. Allicin and related thiosulfinates are highly unstable and instantly decompose to yield various sulfur compounds including diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), dithiins, ajoene, methyl allyl disulfide, methyl allyl trisulfide, 2-vinyl-1,3-dithiin, 3-vinyl-1,2-dithiin (Figs. 9.1 and 9.2) (Block 1985; Fenwick and Hanley 1985; Lawson 1998; Rybak et al. 2004). These compounds provide garlic its characteristic odor and flavor as well as most of its biological properties. It is estimated that 1 g of fresh garlic contains up to 2.5 mg of allicin and 500 mg of DATS or DADS. The amount of organosulfurs required for biological responses and beneficial effects is generated through dietary intake of garlic (Lawson 1998; Rybak et al. 2004). In addition to organosulfur compounds, garlic contains carbohydrates and proteins, which are the major components of garlic powder accounting for more than 80 %. Garlic also contains antioxidant vitamins A, C, and E as well as selenium, a key element for the synthesis of the antioxidant enzyme glutathione peroxidase (GPx) (Gorinstein Leontowicz et al. 2006).