Classification and Application Research Progress of Donkey-Derived Bioactive Peptides

Bioactive peptides (BAPs) are polypeptide compounds composed of 20 amino acids that exert beneficial effects on organisms. Most are short peptides containing 2–5 amino acids, while some are polypeptides with 10–50 amino acids. They possess antioxidant, antibacterial, antihypertensive, immunomodulatory and other biological activities [1]. Bioactive peptides originate from a wide range of sources, including animals, plants and food materials. Animal-derived bioactive peptides can be isolated from blood, muscle, bone and other tissues of deer, cattle, donkeys, pigs and other livestock [2].

Among various animal-derived bioactive peptides, donkey-derived bioactive peptides have attracted extensive research attention and found wide applications. They are mainly prepared from donkey blood, donkey milk and donkey bones. Donkey blood, milk and bones are rich in protein and nutrients. After a series of processing treatments, bioactive peptides with antioxidant, antihypertensive and anti-fatigue functions can be obtained. These peptides feature strong biological activity, high edible safety and few side effects, showing promising market potential. The development of donkey-derived bioactive peptides can effectively improve the utilization rate of donkey blood, milk and bones and bring additional economic benefits to the donkey breeding industry.

Classification of Donkey-Derived Bioactive Peptides

There are numerous types of donkey-derived bioactive peptides. According to physiological functions, they can be classified into antioxidant peptides, antibacterial peptides, anti-fatigue peptides, antihypertensive peptides, hypoglycemic peptides and others, each exerting distinct physiological effects in vivo.

Antioxidant Peptides

Oxidation triggers the generation of free radicals. When free radical production exceeds the scavenging capacity of the organism, imbalance between the oxidation and antioxidant systems occurs, leading to oxidative stress [3]. Antioxidant peptides react with and eliminate free radicals, restore the balance of redox homeostasis and protect bodily health. Donkey-derived antioxidant peptides are mainly sourced from donkey milk [4]. Liang Xing [5] isolated bioactive peptides with excellent antioxidant activity from donkey whey protein.

Antibacterial Peptides

The overuse of antibiotics has aggravated bacterial resistance, creating an urgent demand for antibiotic alternatives. As natural substances, antibacterial peptides are promising substitutes for antibiotics. They have diverse sources, and the Antimicrobial Peptide Database (APD) contains a large number of animal- and plant-derived antibacterial peptides [6]. China has gradually banned antibiotic supplementation in feed since 2020, so antibacterial peptides have been increasingly applied as feed additives in animal husbandry. Donkey-derived antibacterial peptides are mainly obtained from donkey milk [7]. F. Tidona et al. [8] separated antibacterial peptides from β-casein, αs1-casein and β-lactoglobulin in donkey milk.

Anti-Fatigue Peptides

Approximately 30% of the global population suffers from fatigue [9], which severely impairs work efficiency and quality of life. Anti-fatigue peptides supply essential amino acids, maintain stable internal environment, blood glucose and blood lipid levels during physical activity, and effectively alleviate or eliminate fatigue [10]. Miao Fujun et al. [11] isolated anti-fatigue peptides from donkey bone protein.

Antihypertensive Peptides

Hypertension induces multiple diseases including cardiovascular blockage and stroke. Two blood pressure regulatory systems exist in organisms: the renin-angiotensin system (RAS) and the kallikrein-kinin system (KKS) [12]. Within the RAS, angiotensin-converting enzyme (ACE) cleaves angiotensin I into active angiotensin II, which causes vasoconstriction and elevates blood pressure [13]. Antihypertensive peptides suppress ACE activity and thereby lower blood pressure [14]. Donkey-derived antihypertensive peptides are mainly extracted from donkey blood [15]. Li Hao et al. [16] separated antihypertensive peptides from donkey blood with potent ACE inhibitory activity.

Hypoglycemic Peptides

Diabetes mellitus is a common metabolic disorder, with type 2 diabetes mellitus (T2DM) being the most prevalent type. Hypoglycemic peptides reduce blood glucose mainly by inhibiting dipeptidyl peptidase-4 (DPP-4) and α-glucosidase activities, lowering blood glucose concentrations and maintaining glucose within healthy ranges for diabetic patients [17]. Donkey-derived hypoglycemic peptides are primarily sourced from donkey blood [18]. Liu Dan [19] isolated hypoglycemic peptides from donkey blood that effectively inhibit DPP-4 activity.

Antitumor Peptides

Antitumor peptides target tumor cells, inhibit tumor cell proliferation or induce tumor cell apoptosis, and exhibit favorable targeting ability and low immunogenicity [20]. Donkey-derived antitumor peptides exert antitumor effects by accelerating tumor cell apoptosis and enhancing host immunity [21]. Li Qingye [22] isolated antitumor peptides from donkey blood that can suppress the growth of hepatoma cells.

Anti-Osteoporotic Peptides

Osteoporosis is a metabolic disease characterized by decreased bone mass, deteriorated bone quality, increased bone fragility and high fracture risk, which seriously threatens the health of middle-aged and elderly populations [23]. Chen Chen et al. [24] confirmed that anti-osteoporotic peptides play positive roles in osteoporosis prevention and bone growth promotion. Currently, donkey-derived anti-osteoporotic peptides have only been isolated from donkey bones. Liu Chuyi et al. [25] prepared anti-osteoporotic peptides with different molecular weights from donkey bones.

Other Peptides

Apart from the above categories, many other donkey-derived bioactive peptides have been successfully developed, including immunomodulatory peptides, anti-colitis peptides, anti-aging peptides and antiparasitic peptides.

Applications of Donkey-Derived Bioactive Peptides

China boasts a long history of donkey breeding with abundant donkey breeds. The total circulating blood volume of a donkey accounts for approximately 7.5% of its live weight, and 5–12 kg blood can be collected from each donkey after slaughter. Annually, 110,000–260,000 tons of donkey blood are available in China. Donkey blood is rich in protein and trace elements; the protein content in dry matter of donkey blood exceeds 90%, among which hemoglobin accounts for 60%–65% [37]. Only a small portion of donkey blood is processed into donkey blood curd, while most is discarded without sufficient utilization. Against this background, numerous studies have explored the preparation of bioactive peptides from donkey blood. At present, applied donkey blood-derived bioactive peptides include antibacterial peptides, antihypertensive peptides and hypoglycemic peptides. Buri’e et al. [38] found that donkey blood-derived antibacterial peptides display strong antibacterial activity against Escherichia coli, Staphylococcus aureus and Streptococcus agalactiae. Li Hao et al. [16,29] isolated an antihypertensive bioactive peptide LH6 from donkey blood and investigated its hypotensive effect as well as its in vitro and in vivo proliferative activity on bone marrow hematopoietic cells. The results revealed that peptide LH6 can effectively reduce blood pressure and promote proliferation of bone marrow hematopoietic cells. When exploring the application effects of donkey blood-derived hypoglycemic peptides, Liu Dan [19] found that these peptides reduce fasting blood glucose in type 2 diabetic mice, raise lactic acid levels by improving intestinal flora diversity and the abundance of beneficial bacteria, and optimize intestinal microecology in diabetic mice. Li Qingye [22] separated antitumor peptides from donkey blood and evaluated their activity via the MTT colorimetric assay, verifying that the peptides significantly inhibit the proliferation of HepG2 hepatocellular carcinoma cell line.

Applications of Donkey Milk-Derived Bioactive Peptides

Milk-derived bioactive peptides are functional peptides generated by proteolysis of milk proteins, generally consisting of 2–20 amino acids [39]. In ancient times, donkey milk was used to treat bronchitis, asthma, arthralgia, stomachache and other ailments [40]. The protein and lactose contents of donkey milk are close to those of human milk, and it features low allergenicity, making it suitable for infants and individuals allergic to cow milk [41]. Proteins and lactose in donkey milk facilitate intestinal calcium absorption and provide favorable substrates for intestinal flora development. Donkey milk contains abundant lysozyme to combat pathogenic microorganisms, along with abundant minerals and fat-soluble vitamins [42]. Owing to its rich nutrients and multiple immunomodulatory and bioactive substances, donkey milk has been widely used to prepare bioactive peptides in recent years. Liang Xing [5] isolated antioxidant peptides from donkey milk, and DPPH radical, hydroxyl radical, ABTS radical and superoxide anion scavenging assays confirmed their potent free radical scavenging capacity. Yin Yanling [43] obtained anti-colitis peptides from donkey milk and verified their anti-colitis efficacy via mouse colitis models. E. Akan [44] separated hypoglycemic peptides from donkey milk, whose hypoglycemic activity was validated by DPP-4 and α-glucosidase inhibition tests. S. Cirrincione et al. [30] isolated antibacterial peptides from donkey milk, and bacterial growth inhibition tests demonstrated their suppressive effects on E. coli and S. aureus.

Applications of Donkey Bone-Derived Bioactive Peptides

Donkey bones are by-products of donkey meat processing, containing collagen, fat, minerals and other nutrients. Research by Fan Yumei et al. [45] indicated that donkey bone collagen contains 16 amino acids and their derivatives, including five essential amino acids as well as bioactive amino acids such as γ-aminobutyric acid, taurine, pyroglutamic acid and leucine. Li Lanjie et al. [46] identified 52 fatty acids in donkey bones, including specific fatty acids such as caprylic acid, pentadecanoic acid and 11,14-eicosadienoic acid with relatively high contents. Donkey bones are widely used in cosmetics, health products, food and bioactive peptide production. Compared with donkey blood- and milk-derived peptides, research on donkey bone-derived bioactive peptides remains limited. Liu Chuyi et al. [25] prepared anti-osteoporotic peptides from donkey bones and examined their effects on alkaline phosphatase activity in MG-63 osteoblasts to confirm anti-osteoporotic activity. Miao Fujun et al. [11] isolated anti-fatigue peptides from donkey bones. Anti-fatigue efficacy was evaluated by measuring mouse body weight, exhaustive swimming time under load, hepatic glycogen and blood lactic acid content. The peptide showed no obvious influence on mouse weight gain, but significantly prolonged loaded swimming time, reduced blood lactic acid and increased hepatic glycogen, indicating effective anti-fatigue activity.

Conclusion

Donkey-derived bioactive peptides cover multiple categories and possess broad application prospects. Their development can greatly improve the utilization rate of donkey tissues and organs after slaughter and generate economic benefits for the donkey industry. At present, bioactive peptides are mainly prepared from donkey blood, milk and bones, while reports on peptide production from donkey skin, donkey placenta and other tissues remain scarce. Meanwhile, most existing studies only separate functional bioactive peptides from donkey blood, milk and bones using enzymatic hydrolysis, ultrafiltration, gel chromatography and other technologies, without in-depth exploration of their action mechanisms. Future research can attempt to isolate and prepare more donkey-derived bioactive peptides from donkey skin, placenta and other raw materials, and further clarify their molecular mechanisms, so as to provide more scientific evidence for the application of donkey-derived bioactive peptides.

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