Season-dependent variation in the contents of glucosinolates and S-methyl-l-cysteine sulfoxide and their hydrolysis in Brassica oleracea

Brassica vegetables contain glucosinolates and S-methyl-l-cysteine sulfoxide, which can be enzymatically hydrolyzed to form bioactive compounds. Glucosinolate hydrolysis can result in formation of health-promoting isothiocyanates, however, often less desirable nitriles and epithionitriles are formed due to presence of specifier proteins. Also, S-methyl-l-cysteine sulfoxide yields beneficial volatile organosulfur compounds (VOSC), such as S-methyl methanethiosulfinate. To optimize the nutritional value of Brassica oleracea vegetables, the outcome of these hydrolysis pathways was monitored over the harvest seasons of different cultivars in three consecutive years. Strong seasonal shifts in glucosinolate hydrolysis were observed only in red cabbages. They formed up to 40-fold more isothiocyanates in summer than in autumn, when nitriles and epithionitriles increased due to the induction of epithiospecifier proteins. Further, higher VOSC levels were found in autumn red cabbage. By elucidating the impact of abiotic growth factors on the regulation of the hydrolyses, the health value of Brassica vegetables can be improved.