The Enzyme Alliinase

Members of the allium family (onions, garlic, and leeks) contain cysteine sulfoxide derivatives (allyl sulfur compounds), such as allicin and alliin (see Figure 14.9). When the plant cells are damaged, the enzyme alliinase is released from vacuoles and catalyzes the formation of thiosulfinates and thiols, including the lachrymator thiopropanal S-oxide. Their function in the plant is presumably to provide protection against attack by pests. 

Figure 2 When garlic is bruised or damaged, alliin and the enzyme alliinase produce allicin. When you taste fresh garlic, neurons embedded in your mouth cause an electrical signal to be sent to your brain. The brain interprets the electrical signal as a burning sensation.

The biochemistry of onion flavor is now well known and has been extensively reviewed by Shankaranarayana et al. [67]. The chemistry involved has been covered in detail in an earlier chapter of this text. The distinctive flavor of onion and other alliaceous vegetables is due to S-alk(en)yl derivatives of L-cysteine sulfoxide, which are rapidly hydrolyzed by the enzyme alliinase to give an unstable sulfenic acid derivative together with pyruvic acid and ammonia. The sulfenic acid compound breaks down and rearranges to form the relatively more stable thiopropanal S-oxide, which has lachrymatory properties, or it reacts with other compounds to produce a complex mixture of di- or trisulfides which ultimately characterize the product. 

Diallylthiosulfinate (allicin) and diallyldisulfide are formed from the main component by means of the enzyme alliinase. Both are character impact compounds of garlic. 

Garlic supplements - powder tablets or capsules, steam-distilled oil, vegetable oil macerate extract, or extract aged in dilute alcohol - are widely available and are taken by millions. Since the active principle, allicin, is not present in garlic bulb, the supplements rely on the presence of precursor alliin and enzyme alliinase. In tests on 24 commercial brands of enteric-coated tablets, all except one gave low dissolution allicin release 83% of the brands released less than 15% of their potential allicin.78,79 Relevant factors were impaired enzyme activity caused by excipients and slow tablet disintegration. Caveat emptor ... 

This enzyme [EC 4.4.1.4], also known as alliinase and cysteine sulfoxide lyase, catalyzes the conversion of an 5-alkyl-L-cysteine 5-oxide to an alkyl sulfenate and 2-aminoacrylate. The enzyme requires pyridoxal phosphate. 

Main components Alliin, a water soluble and odour-less substance, which is degraded by the plant enzyme alliinase to allicin. The latter constitutes an important aroma carrier. Except for alliin, several homologs are present, such as methyl-, ethyl-, n-butyl- and cycloalliin [110]. 

Hetnihydrate. odorless, hunched needles from dil acetone, mp 164-166° (effervescence), [ajg1 +63,5 (c = 2). Freely sol in water. Practically insol in abs ethanol, chloroform, acetone, ether, benzene, Upon cleavage by the specific enzyme alliinase, an odor of garlic develops, and the fission products show antibacterial action similar to allicin. q-v. 

Sulfinylpropene is the main tear-inducing compound (technical term lachrymator) released when you slice onions. The most interesting part of this story is that this chemical is not present in onions before you slice them. In fact, the generation of this tear-inducing molecule is not a mistake, but part of the defense system that plants have developed to deter animals from continuing to eat them. When you cut, or an animal takes a bite of, an onion, an enzyme, alliinase, that is normally safely stored within the plant s cells is released and starts to wreak havoc. Alliinase converts sulfoxides to sulfinyl groups, which makes you cry like a baby. 

The compound which causes tears (the lachrymatory factor) is (Z)-propanethial-S-oxide (II) which, once the onion bulb is sliced, is derived from trans-(-h)-S-(l-propenyl)-L-cysteine sulfoxide (I) by the action of the enzyme al-liinase. Alliinase has pyridoxalphosphate as its coenzyme (cf. reaction sequence 17.11). Chopping of onions releases 3-mercapto-2-methylpentan-l-ol, which, with its very low threshold of 0.0016 pg/1 (water), smells of meat broth and onions. Raw onions contain 8-32pg/kg, and onions which have been cut, stored for 30 minutes and then cooked contain... 

Traditional medicine suggests that onions and garlic are good for you and modem chemistry has v, revealed some of the reasons. These bulbs of the genus Allium exhibit some remarkable sulfur chemistry and we will end this chapter with a few examples. Both onions and garlic are almost odourless when whole but develop powerful smells and, in the case of onions, tear gas properties when they are cut. These all result from the action of alliinase enzymes released by cell damage on unsaturated sulfoxides in the bulb. 

Alliinase onion, garlic, leek, etc. Enzyme and flavor precursor react on tissue damage (cutting) to release the active flavors. 

S Alkylcysteine Sulfoxide Lyase (alliinase), the Allium Flavor Enzyme... 

Enzymes of Aldehyde Production. Lipoxygenases are discussed elsewhere in this volume. The aldehyde-lyase has only recently been obtained even in crude extracts 145), However, use of 13- and 9-hydroper-oxyoctadecanoic acids to form Cg and C9 aldehydes and corresponding w-oxoacids is evidence that the pathway is correct (145). As with alliinase and myrosinase these reactions occur only when leaves or fruit are cut or crushed (138,143,144,145). If they are heated before blending or blended under N2 (138), little carbonyl compound is formed (142). H2O2 inhibits (138) this reaction as expected for lipoxygenase (153). lrarw-2-Hexenal is formed nonenzymatically from m-S-hexenal (138). 

When garlic is mechanically disrupted, alliinase or alliin lyase (EC 4.4.1.4.) catalyzes the conversion of the cysteine sulfoxides to the biologically active diallyl thiosulfinates via sulfenic acid intermediates (Block, 1992). Alliinase is localized to a few vascular bundle sheath cells around the veins or phloem, whereas alliin and other cysteine sulfoxides are found in the clove mesophyll storage cells. This enzyme is approximately 10 times more abundant in the cloves than in the leaves and accounts for at least 10% of the total protein in the cloves (Ellmore and Feldberg, 1994). Alliinase is temperature and pH dependent optimal activity is between pH 5.0-10.0, but allinase can be irreversibly deactivated at pH 1.5-3.0 (Krest and Keusgen, 1999). 

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