Enzymes myrosinase

Glucobrassicin is a mustard oil glucoside found in various Bras-sica spp. (50). 3-Indoleacetonitrile (IAN) is released from glucobrassicin by the action of the enzyme myrosinase at pH 4.0. How-... 

It is very likely that the inhibitors are breakdown products of glucosinolates, which are considered nontoxic in themselves (23) but can yield physiologically active products upon hydrolysis by the enzyme myrosinase. Through hydrolysis glucosinolates can yield isothiocyanates (24-28). Ju et al. (29) demonstrated the capacity of thiocyanates as allelopathic agents, and isothiocyanates also have allelopathic activity (30). 

The myrosinase-glucosinolate system occurs in 16 plant families.17 It is likely that this system evolved from the ancient (and near ubiquitous) cyanogenic glycosides. This hypothesis is supported by the similarity, in sequence and structure, of glucosinolate biosynthetic enzymes and the hydrolytic enzymes (myrosinase or p-glucosidase).18 The biochemistry of these metabolites also indicates a convincing evolutionary relationship.19 Furthermore, on the basis of robust phylogenetic trees,... 

Myrosinases are the only known S-glucosidases. Whereas some myrosinases exhibit a pronounced substrate specificity towards glucosinolates (e.g. MacLeod and Rossiter, 1986 Bernardi et al, 2003), most of these hydrolytic enzymes, myrosinases, accept multiple glucosinolate substrates e.g., Chen and Halkier, (1999). However, the hydrolysis of other S- or 0-glucosides is only poorly catalysed by these enzymes (Lein, 1972 Durham and Poulton, 1990 Bernardi et al, 2003). 

Xue, J., Lenman, M., Falk, A. and Rask, L. (1992) The glucosinolate-degrading enzyme myrosinase in Brassicaceae is encoded by a gene family. Plant Mol. Biol, 18,387-98. 

Usage For sauces and salads, in fish and meat dishes. The glycoside glucocapparin, which is degraded by the plant enzyme myrosinase to the sharp tasting methyl isothiocyanate, is the most significant constituent. 

Main components The glucosinolates sinigrin (0.3% in the fresh plant) and glucona-sturtiin are hydrolysed by the plant enzyme myrosinase to allyl and a-phenylethyl isothiocyanate, the main constituents of the essential oil, which is present in amounts of 0.1 to 1.4%. For further components of the oil see [119, 120]. Furthermore, carbohydrates, ascorbic acid, vitamin B1 and a number of enzymes (lipases, inver-tases, amylases, proteases, peroxidases) have been found in the root. 

Figure 1. Structure of the thioglycoside progoitrin present in rapeseed and the products produced by the action of the enzyme myrosinase...

Glucosinolates are p-S-glucosides of isothiocyanates (ITC). They are non-volatile, water-soluble compounds, cleaved by the enzyme myrosinase, a P-thioglucosidase, when plant tissues are damaged to form isothiocyanates (mustard oils). 

Glucosinolates in the presence of water and a suitable catalyst and at an appropriate temperature may undergo transformation. The indigenous enzyme myrosinase unfortunately serves the catalytic function only too well once the seed is crushed. The architectural destruction of the seed required to produce good oil release inevitably results in intimate contact of the glucosinolates, myrosinase and water, and the reaction then needs only an appropriate temperature and period of time to occur. The reaction rate starts to... 

FIGURE 24.1. Glucosinolate hydrolysis by the endogenous enzyme myrosinase and formation of their hydrolysis products nitriles, isothiocyanates. and thiocyanates, lndol-3-ylmethylglucosinolate ( ucobrassicin) is further converted to other bioactive products, such as indole-3-carbinol and ascorbigen (Holst and Williamson 2004). 

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