# Profiling Taste and Aroma Compound Metabolism during Apricot Fruit Development and Ripening
## 2 Results
### Enzymes Involved in Flavor Compound Metabolism during Development and Ripening  
Nine enzymes involved in sugar metabolism were analyzed in apricot fruit during the whole development process (Figure 7a). With regard to enzymes involved in sucrose accumulation, significant increase of activities in sucrose synthase (SS) responsible for synthesis direction (SSthy) and sucrose phosphates(SPS) were found throughout the whole development process. A significant increase of activity in SS responsible for degradation direction (SSca) was also observed during the development process, although to a lesser extent than SSthy and SPS. Significantly higher levels of SSthy, SPS and SSca were found in LT than those in other cultivars. In addition to synthesis, sucrose can also be cleaved by neutral invertase (NI) and acid invertase (AI), no significant changes were found for these enzymes. Sorbitol can be inverted into fructose by sorbitol dehydrogenase (SDH), and a significant increase SDH enzyme activity was observed during the whole development period (Figure 7b). Fructose can be cleaved by fructokinase (FK), nevertheless, no significant change was found during ripening process (Figure 7b). In addition, sorbitol can be also oxidized into glucose by sorbitol oxidase (SO), and a significant increase was found during whole development period (Figure 7c). In contrast, glucose can be cleaved by glucokinase (GK), however, no significant change was observed in the enzyme activity. For all cultivars tested, no significant differences in the nine enzymes for sugar metabolism were observed between peels and pulps. Figure 7: The flavor compound-related enzyme activities (U/mg protein) of apricot fruit during development and ripening. (**a**) SSthy, sucrose synthase for synthesis direction, SPS, sucrose phosphates(s), SSca, sucrose synthase for degradation direction, NI, neutral invertase, AI, acid invertase; (**b**) SDH, sorbitol dehydrogenase, FK, fructokinase; (**c**) SO, sorbitol oxidase, GK, glucokinase; (**d**) DH, quinate dehydrogenase; (**e**) MS, malate synthase, NADP-ME, NADP-malic enzyme, NAD-ME, NAD-malic enzyme; (**f**) CS, citrate synthase, GAD, glutamate decarboxylase; (**g**) LOX, lipoxygenases, HPL, hydroperoxide lyase; (**h**) ADH, alcohol dehydrogenase; (**i**) AAT, alcohol acyl-transferases; (**j**) ACX, acyl-CoA Oxidase; (**k**) CCD, carotenoid cleavage dioxygenases; (**l**) TPS, terpene synthase. Two capital letters in the top are the cultivars shown in Table 1. All data are expressed as means \(\pm\) standard error of triplicate samples. Six enzymes involved in organic acid metabolism were analyzed in apricot during the fruit development and ripening process. Quinate dehydrogenase (QH) is responsible for converting 3-dehydroquinate into guinea, and significant decrease was found in apricot during fruit development process (Figure 7d). With regard to malate, malate synthase (MS) is the key enzyme for synthesis, and a significant decrease in its activity was observed throughout the whole development process, which was consistent with the decrease of malate content (Figure 7e). Significantly higher levels of MS enzyme activity was found in YC fruit than in other cultivars. Malate can also be metabolized by NADP-malic enzyme (NADP-ME), and NAD-malic enzyme (NAD-ME), and a significant increase in these enzymes were found in cultivars tested in both peels and pulps, which leads to the decease of malate content during the development and ripening process (Figure 7e). Citrate synthase (CS) mainly is responsible for the citrate biosynthesis, and the activity was induced significantly during fruit development and ripening (Figure 7f). It is reported that glutamate decarboxylase (GAD) activity participates in regulating cytosolic pH, while no significant change in GAD activity was found in apricot throughout whole development process (Figure 7f). Similar to the enzymes of sugar metabolism, no significant differences in the six enzymes for organic acid metabolism were observed between peels and pulps.