Particularly popular in Asian cuisine, mung bean sprouts are not difficult to grow and are produced year-round. Seeking to better understand metabolism during germination, Jin et. al treated sprouting mung beans with organic acid salts. Following this, they used an iTRAQ-based approach to look for changes in the proteome.
After surface-sterilizing mung bean seeds and then washing and soaking them for four hours, they sprouted the seeds in a sprouter with an automatic spraying system. They used freshly prepared solutions of sodium acetate (SA), sodium citrate (SC) and sodium tartrate (ST) for the spraying treatments. The automatic system sprayed seeds for two minutes every hour. After four days, the team analyzed the new sprouts.
For the iTRAQ analysis, Jin and colleagues extracted proteins using the P-PERTM Plant Protein Extraction Kit (Thermo Scientific). They alkylated and digested proteins prior to labeling with an 8-plex iTRAQ kit. Next, they performed High pH reverse phase chromatography on a Dionex UltiMate 3000 HPLC Pump system (Thermo Scientific) and nano electrospray ionization followed by tandem mass spectrometry on an LTQ Orbitrap XL (Thermo Scientific). They used Proteome Discoverer software (v. 1.3) (Thermo Scientific) to generate peak lists.
In addition to the protein analysis, Jin et al also analyzed the:
- Phytic acid and phytase activity.
- Sprout length and respiration.
- Antioxidant enzyme activities.
- Lipid peroxidation and relative electrolyte leakage.
- Reducing sugar, soluble protein, and free amino acid contents.
In the four day old sprouts, they identified 81 (SC treated), 101 (SA treated) and 90 (ST treated) differentially expressed proteins. Many of these proteins played a role in carbohydrate and energy metabolism. As a whole, they found the salt treatments significantly reduced the phytic acid content and increased the antioxidant enzyme activities.
The treated mung bean sprouts grew significantly (p < 0.05) longer and had an increased respiratory rate, but by the fourth day, the SA-treated sprouts inhibited growth. They saw the highest free amino acid content after SA treatment, followed by ST, SC, and the control, with no significant difference in SC and ST. Looking at electrolytes, the researchers noted that SA treatment produced the highest relative electrolyte leakage rate, whereas ST and SC treated sprouts showed significantly reduced relative electrolyte leakage rates compared with the control (p < 0.05). They concluded that SA treatment may cause membrane injury to some extent.
Analyzing the soluble protein content with all three treatments, the team saw an increase for the first three days showing the maximum on the third day. On the fourth day, this value decreased. They saw a consistent increase in the reducing sugar content during the 4-day germination. They saw an increase in the reducing sugar content compared with the control with the exception of the 4th day. They observed a significant decrease in the reducing sugar content after SA spraying, but no significant difference was observed between ST, SC and the control on the 4th day.
The team maintains that these treatments might be a good option for stimulating phytic acid degradation and antioxidant enzyme activities. They suggest further studies investigate the molecular mechanisms involved in these protein changes.
Jin, X. et al. (2016) “iTRAQ analysis of low-phytate mung bean sprouts treated with sodium citrate, sodium acetate and sodium tartrate.”, Food Chemistry 218, 1 March 2017, (pp. 285–293)