The structure of myrosinase shows features which illustrate the adaptation of the plant enzyme to the dehydrated environment of the seed. The catalytic mechanism of myrosinase is explained by the excellent leaving group properties of the substrate aglycons, which do not require the assistance of an enzymatic acid catalyst. The replacement of the general acid/base glutamate of O-glycosidases by a glutamine residue in myrosinase suggests that for hydrolysis of the glycosyl-enzyme, the role of this residue is to ensure a precise positioning of a water molecule rather than to provide general base assistance.

The enzymatic hydrolysis of glucosinolates (GLs), typical compounds of Cruciferae, produces molecules with fungitoxic activity. Eleven GLs and their enzymatic hydrolysis products obtained by myrosinase were tested in vitro against Fusarium culmorum. Toxicity of hydrolysis products from glucoiberin, glucotropaeolin, sinigrin, and epiprogoitrin were assayed on eight plant pathogenic fungi. The results showed (i) the native GLs showed no fungitoxic activity, whereas their hydrolysis products inhibited fungal growth depending on their chemical structure; (ii) the hydrolysis products from glucoiberin, glucoerucin, glucocheirolin, and glucotropaeolin were the most effective, with 50% inhibition of fungal growth at 0.1 mg/mL; (iii) the fungitoxic activity of hydrolysis products obtained from glucoiberin, glucotropaeolin, sinigrin, and epiprogoitrin was confirmed on eight pathogenic fungi, with different responses depending on their chemical structure; (iv) the most effective hydrolysis products were those from glucoiberin, showing EC50 values of 0.05 mg/mL on Rhizoctonia solani, Sclerotinia sclerotiorum, Diaporthe phaseolorum, and Pythium irregulare and a minimum inhibitory concentration varying from 0.1 to 1.2 mg/mL. Keywords: Cruciferae; isothiocyanates; Fusarium culmorum; myrosinase

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