Ｒesearch Progress On Plant-derived Anti-fatigue Peptides in Plant Extract

Abstract: Plant-derived anti-fatigue peptide was a bioactive peptide extracted from plant protein， which could resist or alleviate fatigue． The source， action mechanism， evaluation methods, and application products of plant-derived anti-fatigue peptides were summarized, and future development prospected． 

Keywords: plant; bioactive peptide; anti-fatigue; mechanism of action; evaluation method

Fatigue is a physiological phenomenon with a complex mechanism produced by the human body after a period of physical or mental exhaustion, usually resulting in temporary physiological dysfunction of muscles or organs, and specific phenomena include drowsiness, inattention, etc. [1-2 ]. With the development of modern society, fatigue has become a problem that affects the quality of life of most people. More than 35% of the world's population will feel fatigued, of which middle-aged men account for 60%. Anti-fatigue peptides that are absorbed and have low toxicity and side effects have become one of the research hotspots [3].

The anti-fatigue peptides isolated from plants have a wide range of sources, a wide variety, and high cost performance. Its absorption mechanism is better than that of individual proteins and amino acids, and has unique physiological functions that the two do not have, such as regulating human hormone secretion, promoting or inhibiting Enzymes in the organism, regulate and improve the transport and absorption of minerals in the human body [4-5]. Some medicinal plants with extremely high nutritional and medicinal value in traditional Chinese medicine can also be identified as having anti-fatigue active ingredients after research and testing [6]. In addition, high-yield crops such as wheat and soybean can also be used to prepare anti-fatigue active foods after processing and extraction [7-8]. Therefore, plants are a research High value anti-fatigue food base [9].

This paper aims to summarize the material sources, action mechanisms, evaluation methods, and applied products of plant-derived anti-fatigue peptides in recent years, in order to provide theoretical references for the research and development of such functional peptides in the field of food.

1 Sources of plant-derived anti-fatigue peptides

The sources of plant-derived anti-fatigue active peptides can be roughly divided into grains, beans, nuts and medicinal materials according to the types of plant raw materials. As shown in Table 1: At present, researches at home and abroad mainly focus on grains and legumes, while there are relatively few reports on anti-fatigue peptides derived from nuts and medicinal materials.

2 The mechanism of action of anti-fatigue peptides

2. 1 Inhibit oxidative stress in the body

Most studies have shown that free radicals in the body increase during physical activity in humans and animals. If the production of reactive oxygen species is excessive or the body's antioxidant capacity is reduced, the ROS metabolism in the body will be unbalanced, which will lead to oxidative stress or oxidative damage in the body, such as DNA hydroxylation, tissue damage and protein denaturation, etc., resulting in fatigue in the body [ 22-23]. Active peptides with antioxidant capacity in vivo can increase superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities, enhance the body's ability to scavenge free radicals while protecting the body from peroxides It can avoid lipid peroxidation to generate malondialdehyde (MDA), thereby reducing inflammation and peroxidative damage caused by excessive free radicals.

Therefore, supplementation of such peptides can be considered as an effective means to relieve fatigue and the damage caused by the body [24]. In addition, amino acids with antioxidant properties may play an important role in the above-mentioned mechanism of action. Aromatic amino acids (such as tyrosine, phenylalanine, and tryptophan) can readily donate protons to free radicals, and peptides containing such amino acids tend to have antioxidant capacity; hydrophobic amino acids (such as N-terminal valine or Leucine, and proline, histidine or tyrosine in the sequence) have been shown to enhance the antioxidant activity of peptides [25], and these two amino acids are usually present in anti-fatigue peptides. In addition, by activating the Nrf2/Keap2 signaling pathway in animals, the expression of Nrf2 and HO-1 proteins is up-regulated, and the expression of Keap1 is down-regulated, which has also been proved to be a mechanism to inhibit the body's oxidative stress [26].

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2. 2 Participate in and regulate energy metabolism

In the process of life activities, the main substances that provide energy for the body include adenosine triphosphate, creatine phosphate, glycogen and blood sugar, etc., and insufficient energy supply is one of the main factors leading to reduced exercise capacity and fatigue. Energy substances can cause different fatigue mechanisms and different degrees of fatigue [27]. Active peptides are easy to be digested, absorbed and utilized by the human body, and can quickly replenish the body's energy substances.

Intense exercise puts the body in a state of hypoxia. In this state, lactic acid is produced in the process of glycolysis, resulting in an increase in blood lactic acid, which in turn reduces tissue pH, disrupts acid-base balance, affects metabolism, and leads to decreased exercise capacity and muscle contraction. 28]. In addition, the increase in the level of some exercise by-products can also lead to the appearance or aggravation of fatigue [29-30]. In recent years, studies have shown that certain anti-fatigue peptides can relieve exercise-induced fatigue by increasing the glycogen levels in the liver and muscles and reducing the levels of metabolites in the blood [31]. In conclusion, increased glycogen production and elimination of produced metabolic byproducts may be potential mechanisms by which antifatigue peptides alleviate exercise-induced fatigue.

2. 3 Suppress inflammation

Some studies suggest that fatigue, as a physiological phenomenon, may be related to the body's spontaneous inflammatory response under strenuous exercise, which is characterized by an increase in pro-inflammatory cytokines, and its markers include interleukins (IL-1β, IL-6) , tumor necrosis factor-α (TNF-α) and positive acute phase proteins (such as CRP and elastase) [32-33]. NAM et al[34] studied the anti-fatigue mechanism of porcine placental dipeptide and found that two short peptides, Gly-Leu and Leu-Gly-OH, could inhibit LSP-stimulated RAW264. 7 TNF-α, IL-1β and IL-6 production in cells

To a certain extent, the link between anti-fatigue peptides inhibiting inflammatory responses and relieving fatigue has been proved.

Further research showed that the inhibitory effect of anti-fatigue peptides on chronic inflammation may be related to the regulation of NF-κB and MAPK, the two main signaling pathways of inflammatory factors [35]. For example, YE et al. [36] found that the expression of NF-κB and pro-inflammatory cytokines (TNF-α, IL1β and IL-6) decreased after administration of anti-fatigue peptides isolated from sea cucumber to forced swimming rats. The level of anti-inflammatory cytokine IL10 increased in gastrocnemius muscle; MAJUMDER et al. [37] also confirmed that peptides can affect the expression of pro-inflammatory cytokines and anti-inflammatory cytokines by regulating signaling pathways involved in anti-inflammatory effects. These studies strongly suggest that some anti-fatigue peptides may exert anti-fatigue effects by blocking NF-κB and MAPK signaling pathways.

2. 4 Regulates neurotransmitters

There are many studies on the relationship between exercise-induced fatigue and neurotransmitters, but the specific mechanism is still unclear. A common theory is that exercise-induced serotonin (5-HT), dopamine (DA) and normethylation Changes in epinephrine (NE) levels lead to fatigue during exercise [38]. Studies have shown that some anti-fatigue peptides may achieve anti-fatigue effects by regulating movement-related neurotransmitters in the central nervous system and cholinergic system. For example, long-term intake of glutamine-rich wheat peptides can improve the central nervous system of mice. The serotonergic and cholinergic systems of the system [39]; HAN et al. [40] found that oral administration of Leu-Gly and Gly-Leu in mice could increase DA levels and improve endurance exercise performance of the test animals. In addition, due to the acceleration of fat metabolism during exercise, the level of free fatty acids increases, which in turn triggers the competition between FFA and tryptophan for binding sites on proteins, which ultimately leads to an increase in the level of serotonin in the brain [36, 41].

3 Evaluation methods of anti-fatigue peptides

Because exercise-induced fatigue is a complex systemic syndrome, it is difficult to characterize various changes in the body during the fatigue process by only relying on some in vitro indicators. Therefore, studies using animal models as platforms have been widely used to evaluate the effect of anti-fatigue peptides. Effect.

3. 1 Mouse swimming experiment

The forced swimming test in mice is a classic behavioral test that can reflect the body's ability to exercise, and is widely used in the functional evaluation of anti-fatigue peptides. The nature of swimming experiments is usually determined according to the degree of fatigue or situation. For example, the most widely used exhaustive swimming test is to let mice or rats swim continuously until they sink into the water due to exhaustion. The experimental steps of exhaustive swimming are as follows [42]: (1) Mice/rats were given a certain dose of

anti-fatigue peptide; (2) place the animal in a tank with an appropriate amount of water and let it swim with lead pieces of appropriate quality tied to the legs/tails; (3) record the mice falling down due to complete exhaustion The forced swimming experiment can also be terminated at a quantitative time point (usually 20-30 min), this design can reflect the changes of various physiological indicators in mice/rat after exercise of the same intensity and duration. 43].

3. 2 Mice climbing rod experiment

The rod climbing test is also an experiment that is often used to evaluate the effect of anti-fatigue peptides. On a glass rod (fixed at the upper end and suspended at the lower end), their muscles were put in a state of tension, and when they were exhausted from fatigue, they would slip off the glass rod, and finally the time to climb the rod was used as the main evaluation index.

3. 3 Rotarod experiments

The rotarod experiment was originally designed to screen the effects of drugs on nerves and has also been used to measure fatigue levels and physical performance [45]. The general procedure for rotarod testing is as follows: (1) Before the test, all mice were subjected to several days of adaptive training; (2) On the day of the test, the mice were placed on the rotarod of the rotarod to start the test. Statistical analysis of quantitative indicators of mice fall time to assess their fatigue status [46].

3. 3 Mouse tail suspension experiment

The mouse tail suspension test is usually used in mice rather than in heavier rats (to avoid damage to the rat's own tail due to excessive weight). The experiment is based on creating an unavoidably stressful environment where the animals go through a process from struggle to exhaustion and then to cessation. Therefore, the anti-fatigue activity of various test substances can be assessed by measuring the difference in the length of time that the test animals try to persist in the escape behavior [47]. The tail suspension experiment is usually carried out according to the following steps

Row [48]: (1) Place the subject animal in a soundproof behavior box and hang its tail on a suspension rod or rack; (2) Use a video recording device to record the animal behavior during the experiment (the experimental time is usually 6 min); (3) Statistics of the immobility time of the tested animals through the animal behavior analysis file.

4 Applications and Prospects of anti-fatigue peptides

In recent years, plant-derived peptides are often used as the core raw materials of functional ingredients in anti-fatigue healthcare products due to the advantages of low price, easy availability, and good dispersibility, the raw material of plant-derived peptides.

At present, my country's research on plant-derived anti-fatigue peptides has achieved certain results. In the future, it can be promoted in the following two directions: (1) Increase the development of plant-derived products with anti-fatigue effects, and make them more compatible with commercial production. (2) In-depth exploration of the mechanism of action of anti-fatigue peptides, the establishment of clear and effective evaluation methods for anti-fatigue activity, and dosage of active anti-fatigue peptides, in order to promote the development of new anti-fatigue food and drug products.