Abstract: The relationship between activity of malic acid metabolism-related key enzymes and accumulation of malic acid aswell as the soluble sugars was investigated by the hybrid from the crossing between apple cultivar‘Dongguang’and ‘Fuji’. The results showed that cytosolic phosphoenolpyruvate carboxylase ( PEPC) and NAD-dependentmalate dehydrogenase (cyMDH) , implicated in malic acid (MA) synthesis in the cytosol, and vacuolar H+-pyrophosphatase (VHP) , implicated in MA storage in the vacuole, exhibited no significant difference between the high- and low-acid apple fruits. Whereas, vacuolar H+-ATPase (VHA) , involved in MA storage in the vacuole, had much higher enzyme activity in the high-acid fruit, while MA degradation-related cytosolic NADP-dependent malic enzyme ( cyME) and ATP-dependent phosphoenolpyruvate carboxylase kinase ( PEPCK) exhibited high enzyme activities in the low-acid fruit. In addition, the effects of these key enzymes on the accumulation of soluble sugarswere discussed in terms of the transformation of malic acid into sugars.

Key words: apple, key enzymes involved in malic acid metabolism, accumulations of malic acid and the soluble sugars