Glucosinolates side chains

Figure 13.1 Degradation of glucosinolates. Hydrolysis is catalyzed by myrosinases and gives rise to different degradation products dependent on the structure of the glucosinolate side chain and the hydrolysis conditions. (I) isothiocyanates, the major product at pH >7 (II) nitriles, the major product at pH <4 (ID) thiocyanates, produced from 2-propenyl-, benzyl-, and 4-methylthiobutylglucosinolates (IV) oxazolidine-2-thiones, produced from glucosinolates with P-hydroxylated side chains, (V) epithionitriles, produced in the presence of epithiospecifier proteins.

A major proportion of the glucosinolate hydrolysis products formed upon myrosinase cleavage in some plants are nitriles. In vitro, nitrile formation associated with myrosinase-catalyzed hydrolysis is enhanced at low pH (pH<3) and in the presence of ferrous ions. In vivo, protein factors in addition to myrosinase may be responsible for nitrile formation. If the glucosinolate side chain has a terminal double bond, the sulfur released from the thioglucosidic bond may be captured by the double bond and an epithionitrile is formed.9 This reaction takes place only in plants that possess a protein factor known as epithiospecifier protein (ESP). ESP activities have been identified in several species of the Brassicaceae and shown to influence the outcome of the myrosinase-catalvzed hydrolysis reaction although they have no hydrolytic activity by themselves.10 12 The mechanism by which ESPs promote epithionitrile formation is not known. 

The reaction products depend on pH and other factors such as the presence of ferrous ions, epithiospecifier protein, and the nature of the glucosinolate side chain. Epithiospecifier protein has recently been independently purified and characterized7,8 and appears to require ferrous ions indicating the formation of an organometallic (FeN - aglycone) intermediate that leads to die formation of a thiirane ring. 

Figure 2.2 Glucosinolate side chains (for details of R, see Figure 2.1), found within frequently consumed cruciferous vegetable and salad crops. Most crops contain a mixture of a small number of these, combined with glucosinolates with indolyl side chains (Figure 2.3).

Glucosinolates such as sinigrin are molecules that consist of a /3-thioglucose moiety, a sulfonated oxime, and a variable side chain derived from various amino acids (Figure 5).43 44 Glucosinolates themselves are not toxic to herbivores and are widely distributed in plant tissues. On the contrary, the enzyme called myrosinase or thioglucosidase is distributed in myrosin cells that do not contain glucosinolates. In the flower stalk of... 

Elongation of amino acid side chains prior to glucosinolate biosynthesis has been studied in several plants. The mechanisms involved are believed to be similar to the formation of leucine from valine and acetate (Fig. 3.13). Through transamination, the amino acid is converted to the corresponding... 

Figure 3.13 Side-chain elongation of amino acids. In analogy to the conversion of valine to leucine, the methene group is introduced to various other amino acids, which subsequently serve as precursors of glucosinolates.

Giamoustaris, A. and Mithen, R. (1996) Genetics of aliphatic glucosinolates. IV. Side-chain modification in Brassica oleracea. Theor. Appl. Genet., 93,1006-10. 

Mi then, R., Clarke, J., Lister, C. and Dean, C. (1995) Genetics of aliphatic glucosinolates. III. Side chain structure of aliphatic glucosinolates in Arabidopsis thaliana. Heredity, 74, 210-5. 

MAGRATH, R., BANG, F., MORGNER, M., PARKIN, L, SHARPE, A., LISTER, C., DEAN, C., TURNER, J., LYDIATE, D., MITHEN, R., Genetics of aliphatic glucosinolates I. Side chain elongation in Brassica napus and Arabidopsis thaliana.. Heredity, 1994, 72, 290-299. 

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