Alliin reactions

This enzyme [EC 4.4.1.6] catalyzes the pyridoxal-phos-phate-dependent hydrolysis of an 5-alkyl-L-cysteine to generate an alkyl thiol, ammonia, and pyruvate. The reaction is an a,/3-elimination. In yeast, the enzyme may be identical to cystathionine /3-lyase. See also Alliin Lyase... 

The recommended dose is about 4g of fresh garlic daily, which is equivalent to approximately 8 mg garlic oil or 600 to 900 mg garlic powder preparations standardized to 1.3% alliin content. Adverse effects of garlic are usually mild and transient they include breath and body odor, allergic reactions, nausea, heartburn, and flatulence. 

The active constituents are usually secondary metabolites, derived from biosynthetie pathways present within the plant tissue. Allicin, a sulphur eompound, present in garlie (Allium sativum) is considered to be the active constituent. It is produced from alliin by an enzymatic reaction in response to injury. Allicin due to its noxious smell proteets garlic from attack of pests also. Thus an active constituent has therapeutic as well as protective activity. 

Onions may not make us Hve forever, but some of their components may indeed have medical benefits — lowering cholesterol and blood pressure and perhaps even reducing the risk of cancer. This is why their chemistry has drawn a great deal of attention. We now know that sHcing an onion sets off a cascade of reactions that begins with alliin, a compotmd that occurs naturally in onions and garlic. When the tissues of the onion are disturbed, allicinase — an enZ)one that converts alliin to allicin — is released. This, in turn, breaks down to syn-propanethial-s-oxide, the stuff that makes your eyes water. 

It is also feasible to monitor enzymatic reactions taking place inside biological tissues by bringing the specimens very close to the ion source [84]. Biotransformation of alliin to allicin by allinase in raw garlic cloves could be monitored in vivo by internal EESI-MS. The reactants were extracted by an infused solution running throughout the tissue. The extract was ionized on the edge of the specimen [84]. 

The final pH, final appearance and flavor description of the thermal reaction products of IMP and alliin, IMP and deoxyalliin, as well as thermal decomposition products of IMP, are listed in Table I. The flavor of the model system of IMP and alliin can be described as slightly fried garlic-like with a roasted meaty character. On the other hand, the flavor of the model system of IMP and deoxyalliin had a more pungent garlic note with roasted character. 

Final pH, Final Appearance and Flavor Description of IMP + Alliin, IMP + Debxyalliin, and IMP Model Reaction Systems... 

The gas chromatographic profiles of the volatile compounds generated from the model reaction systems are shown in Fig. 1. The identification and quantification of the volatile compounds generated from the model systems of IMP and alliin as well as IMP and deoxyalliin are listed in Tables II and III, respectively. As shown in Fig. 1 (C), in the absence of alliin or deoxyalliin, thermal degradation of IMP produced only a few trace components. 

The abundance of thiazoles, especially 2-propylthiazole and 2-ethyl-4-propylthiazole, in the IMP and alliin systems was thought to be the reason that the roasted meaty character in the IMP and alliin model system is stronger than that in IMP and deoxyalliin systems. 2-Propylthiazole was reported to possess green, herbal and nutty odors (24). These thiazole compounds, together with other volatile compounds generated in IMP and deoxyalliin, could then contribute to the roasted meaty character of the reaction products. 

Table IV. Comparison of the Yields of Volatile Compounds Generated from IMP + Alliin, and BMP + Deoxyalliin Model Reaction Systems...

As shown in Table IV, many pyrazine compounds were generated in IMP and deoxyalliin as well as in the IMP and alliin model systems. Pyrazines are widely distributed in thermally processed foods, such as roasted beef, roasted peanut, and roasted barley. The Maillard reaction, which involves the interactions of reducing carbonyl compounds and amino-containing compounds, has been shown to be the major mechanism for the formation of pyrazines (26). 

The flavor precursors, 5 -alk(en)yl-L-cysteine sulfoxides, themselves do not have any specific flavor. However, when garlic or onion cells are damaged by cutting or crushing and the 5 -alk(en)yl-L-cysteine sulfoxides meet the C-S lyase (alliinase), they are converted into various volatile sulfuric compounds. Stoll and Seebeck (1951) first proposed the production of diallyl thiosulfinate (allicin) from the 5 -allyl-L-cysteine sulfoxide (alliin) by the C-S lyase. Through the reaction catalyzed by the C-S lyase, 5 -alk(en)yl-L-cysteine sulfoxides yield alk(en)yl sulfenic acid and aminoacrylic acid, the latter being spontaneously degraded... 

The main products of alliin transformation in polar media are sulfides. At room temperature or on heating, allicin is converted by water into diaUyl disulfide, diaUyl trisulfide and diallyl polysulfides, which are the principal components of garlic essential oil and aged garlic (Figure 8.62). Other reaction products are allyl alcohol, sulfur dioxide and propene. Disproportionation of thiosulfinates in aqueous (polar) solutions also leads to disulfides and thiosuhonates of general formula R-SO2-S-R. For example. 

Figure 7.8 The characteristic aroma of garlic is due to allicin which is formed from alliin, in a reaction catalysed by alliinase which is released when the cloves arecut or crushed. When garlic is cooked, allicin breaks down to a number of products including sulphides and disulphides which are responsible for bad breath after eating garlic flavoured foods.

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