Calcium affects glucoraphanin metabolism in broccoli sprouts under ZnSO₄ stress

CaCl₂, Ca²⁺ chelator (EGTA) and Ca²⁺ channel blocker (verapamil) were used to investigate mechanism of glucoraphanin metabolism in broccoli sprouts under ZnSO₄ stress. CaCl₂ treatment promoted sprout growth, reduced MDA (malonaldehyde) content and electrolyte leakage in sprouts under ZnSO₄ stress. The highest MDA content and electrolyte leakage were obtained in ZnSO₄ plus verapamil-treated sprouts. In addition, ZnSO₄ plus CaCl₂ treatment significantly enhanced glucoraphanin content and sulforaphane formation, while an opposite result was observed after ZnSO₄ plus EGTA treatment; which were further supported by expression of glucoraphanin biosynthetic and hydrolytic genes as well as myrosinase (MYR) and epithiospecifier protein (ESP) activities. These results indicated that exogenous and endogenous calcium promoted glucoraphanin biosynthesis and the conversion rate of glucoraphanin into sulforaphane. Verapamil treatment also stimulated glucoraphanin biosynthesis, but exerted an adverse influence on sulforaphane formation from the hydrolysis of glucoraphanin because of much higher ESP expression and ESP activity than ZnSO₄ treatment.