Similar to Cys Met can undergo ROS
Similar to Cys, Met can undergo ROS-mediated oxidation to Met sulfoxide, which can result in changes in protein conformation and activity . In our results, the Met content in wild ginseng is higher compared to cultivated ginseng (1.3 times, Table 1). Met is a substrate for the synthesis of various polyamines with important roles in stress tolerance . This biosynthetic pathway involves the intermediate SAM as a primary methyl donor. SAM is also a source for ethylene synthesis , which reinforces the pivotal role of Met in the plant stress response. The content of SAM in wild ginseng was higher compared to cultivated ginseng (1.23 times, Fig. 3A). These results could indicate that Met metabolism is more active in wild ginseng compared to cultivated ginseng, which could be the result of the different growth conditions. Considering that Met is also an essential amino ARCA EGFP for humans, Met metabolism of ginseng during growth and development could play a partial role in its medicinal functions.
In plants, aspartate provides carbon skeletons for purine and pyrimidine synthesis. The content of this amino acid in wild ginseng was higher compared to cultivated ginseng (1.22-fold, Table 1). Moreover, asparagine synthetase (Q5 in iTRAQ results), an important enzyme in aspartate synthesis, was upregulated in wild ginseng as assessed using iTRAQ. In humans, aspartate has been proposed to have functions in liver- and cardiac muscle-protective functions. Thus, these observations suggest that a higher level of aspartate synthesis could play a partial role in the medicinal functions of wild ginseng.
Our iTRAQ results revealed a higher accumulation of tryptophan synthetase (Q4 in iTRAQ results) in wild ginseng compared to cultivated ginseng. This result was consistent with a high level of tryptophan in wild ginseng (Table 1). Tryptophan has the capacity to serve as a precursor to auxin, IAA . The phytohormone auxin plays a central role in plant growth and development as a regulator of numerous biological processes, ranging from cell division, elongation, and differentiation to tropic responses, fruit development, and senescence . The content of IAA in wild ginseng was higher compared to cultivated ginseng (Fig. 3B). Importantly, similar to methionine, tryptophan is one of the essential amino acids for the human body, and it is the precursor of serotonin synthesis. Serotonin is an important neurotransmitter in the cerebrum, and perturbations in this neurotransmitter can produce humoral and behavioral disorders. Thus, different levels of tryptophan between wild and cultivated ginseng could be one of the reasons underlying the more beneficial medicinal effects of wild ginseng compared to cultivated ginseng.
In plants, glutamate decarboxylase (Q1 in iTRAQ results) catalyzes the synthesis of GABA in the presence of calmodulin (Spot 14 in 2DE results) . We observed that the accumulation of these two proteins in wild ginseng was higher compared to cultivated ginseng. In addition, further studies revealed that GABA had a higher level of accumulation in wild ginseng (2.04-fold greater compared to cultivated ginseng, Fig. 3C). As a four-carbon nonprotein amino acid, GABA is present at high levels in plants. It is also involved in several physiological processes, such as nitrogen metabolism, cytosolic pH regulation, and carbon flux into the TCA cycle . The basic effects of GABA have been characterized as reducing blood pressure and protecting the liver . Thus, a high level of GABA could contribute to the enhanced medicinal functions of wild ginseng compared to cultivated ginseng.
Conclusion The mechanisms underlying the difference in amino acid metabolism between wild and cultivated ginseng were revealed in this study using proteomic techniques. Based on the results of this study, the following conclusions could be drawn (Fig. 4): in wild ginseng, the contents of medicinal amino acids, such as sulfur-containing amino acids (methionine and cysteine) and tryptophan, and their derivatives were higher compared with those of cultivated ginseng. In addition, the expression and contents of enzymes and intermediate products related to glycolysis and TCA, which support amino acid biosynthesis with material and energy, were higher in wild ginseng compared to cultivated ginseng. This study elucidated the differences in amino acids between wild and cultivated ginseng. Our results provide a reference for further studies on the medicinal functions of wild ginseng.