Effect of sulfur supply on alliinase, the flavor generating enzyme in onions

The characteristic flavor of onions occurs when the enzyme alliinase (EC 4.4 1.4) hydrolyzes S-alk(en)yl-L-cysteine sulfoxides (ACSOs) to form sulfur-containing volatiles, pyruvate and ammonia. Two onion cultivars, Pukekohe Longkeeper and Grano, were grown to maturity at a high (4.0 meg S/L) and low (0.25 meg S/L) sulfur supply and assayed for alliinase specific activity and enzymatically produced pyruvate. Alliinase specific activity was greater (1.3-1.7 fold) at low sulfur supply in bulbs of both cultivars. Levels of enzymatically produced pyruvate were lower at low sulfur supply. Low sulfur supply has been previously shown to decrease levels of ACSOs and their biosynthetic intermediates but increase levels of proteins involved in sulfate uptake and mobilization. Alliinase may have an endogenous role in remobilizing ACSOs during conditions of sulfur deprivation.