Corn peptides are a general term for low-molecular-weight peptides extracted from corn proteins via enzymatic hydrolysis or microbial fermentation. They are mainly composed of amino acids that meet human nutritional requirements and possess high nutritional value, along with diverse biological activities including anti-tumor [1] and antioxidant effects [2]. Peptides can coordinate with metal ions to form peptide-metal chelates, which improve the bioavailability of metal ions in organisms and promote human absorption of trace elements [3]. Both corn peptides and zein can be modified by enzymatic glycosylation to enhance their biological activities [4]. At present, corn peptide products are available on the domestic market. Based on the principle of complementary nutritional composition and efficacy, functional foods, anti-fatigue foods, antihypertensive foods and other products are manufactured by compounding corn peptides with other bioactive substances. Therefore, research on the biological activities of corn peptides is of great significance. This paper reviews three biochemical modification methods of corn peptides, namely metal ion chelation, glycosylation modification, and compounding with other substances, as well as the biological activities of modified corn peptides, so as to provide references for related research.

一．Biochemical Modification Methods of Corn Peptides 

Metal Ion Chelation of Corn Peptides

The chelation reaction between corn peptides and metal elements is a coordination chemical process. After enzymatic hydrolysis of zein, side-chain or main-chain functional groups of amino acid residues in corn peptide molecules undergo coordination reactions with metal ions under specific conditions. The biological activities of corn peptide-trace element chelates prepared under different process conditions vary greatly. Accordingly, research on the preparation technology of corn peptide-trace element chelates is of vital importance.

The protease hydrolysis process exerts a significant influence on the synthesis and biological activity of polypeptide-trace element chelates. Common preparation methods for polypeptides include enzymatic hydrolysis, chemical synthesis and extraction. Extraction and chemical synthesis feature complicated operations, high costs and abundant chemical reagent residues. In contrast, enzymatic hydrolysis has the advantages of convenient operation and high polypeptide yield, and thus is widely adopted. Frequently used hydrolases include pepsin, trypsin and flavor protease. Due to different enzyme cleavage sites of various hydrolases, zein hydrolyzed by different enzymes produces bioactive corn peptides with distinct molecular weights and structures, which exhibit various biological activities such as antioxidant activity [5] and anti-fatigue activity [6]. In recent years, researchers have innovated and improved enzymatic hydrolysis technologies.

Gao et al. [7] adopted protease produced by Armillaria mellea fermentation as a novel enzyme source. Different from traditional microbial proteases, the neutral protease produced exhibits excellent stability at 40–60 °C and pH 6–8, with the optimum temperature of 60 °C and optimum pH of 8. It also shows high hydrolytic activity toward hydrophobic proteins, providing a new direction and potential advantages for zein hydrolysis. Li et al. [8] sequentially hydrolyzed crude corn peptides with α-chymotrypsin and carboxypeptidase A, which accurately and efficiently altered peptide molecular structures. Combined with activated carbon adsorption to remove aromatic amino acids, low-bitter corn oligopeptides with a Fischer ratio of 41.87 were successfully prepared. This study offers novel ideas and methods for preparing functional specific corn peptides via enzymatic hydrolysis, and provides references for subsequent related research and applications.

Glycosylation Modification of Corn Peptides

Glycosylation modification is a common post-translational modification of proteins, referring to the precise covalent linkage of glycosyl groups (monosaccharides, oligosaccharides or polysaccharides) to specific amino acid residues on protein or peptide chains. Widely existing in eukaryotic cells, this modification is crucial for protein functions and normal cellular physiological activities, and can endow proteins or peptides with novel functions.

At present, glycosylation modification of food proteins is mainly achieved through two approaches: transglutaminase-catalyzed enzymatic glycosylation and Maillard reaction. Ren et al. [13] prepared corn peptide-maltodextrin glycosylation products via Maillard reaction using corn peptides and maltodextrin as raw materials. Li [14] adopted ultrasound-assisted Maillard reaction to effectively modify the structure of goat whey protein and improve its functional properties. Nevertheless, the Maillard reaction is difficult to control accurately, easily generates various by-products, and is greatly affected by temperature, pH and other factors with harsh reaction conditions. In addition, it may produce harmful substances to human body such as advanced glycation end products (AGEs), which limit its application in products with high safety requirements.

In comparison, enzymatic glycosylation possesses high specificity. It can accurately attach glycosyl groups to specific amino acid residues of corn peptides, produce single products favorable for subsequent separation and purification, and proceed under mild conditions to well retain the original functional properties of corn peptides. Wang et al. [15] used zein as the starting material and hydrolyzed it with Alcalase alkaline protease to prepare corn peptides. Meanwhile, transglutaminase was employed as a biocatalyst and glucosamine as an acyl acceptor to modify corn peptides via glycosylation reaction.

Protein-polysaccharide complexes generated by glycosylation can act as delivery carriers for active ingredients and enhancers to improve the functional properties of food proteins. Numerous studies have demonstrated excellent functional properties of proteins after glycosylation modification. Wang et al. [16] modified corn peptides with D-glucosamine. In vitro simulated gastrointestinal digestion, rat nitrogen metabolism and growth experiments revealed that corn glycopeptides promote animal growth and development more effectively than original corn peptides, and are high-quality protein sources.

Compounding Corn Peptides with Other Substances

Compounding corn peptides with other substances is a biochemical modification technology that combines corn peptides with one or more different bioactive substances through physical, chemical or biological methods to form complexes. As an effective modification technology, this compounding method significantly improves the physicochemical properties and biological activities of corn peptides by virtue of synergistic effects among different substances, and possesses broad application prospects.

At present, single raw materials are commonly used in the preparation of bioactive peptides, while comprehensive utilization of multiple raw materials is relatively rare. Single raw materials can produce bioactive peptides with characteristic physiological functions, whereas comprehensive utilization of multiple raw materials can enhance and expand physiological functions and realize complementary advantages of raw material properties. Wang et al. [17] compounded soy protein isolate and zein at a specific ratio, and hydrolyzed the mixture with alkaline protease to obtain corn-soybean composite peptides. The resulting peptides exhibited superior antihypertensive capacity and nutritional value compared with single corn peptides and soybean peptides. Wang et al. [18] mixed soy protein isolate and zein at a mass ratio of 1:1, and conducted two-step hydrolysis with malt powder and Alcalase alkaline protease successively. The optimum conditions for preparing composite antioxidant peptides were determined, laying a foundation for subsequent research on the stability of corn-soybean composite antioxidant peptides.

Some polyphenols are unstable and susceptible to environmental factors. However, compounding with corn peptides can significantly improve their stability and biological activities. Zhu et al. [19] prepared curcumin-corn peptide complexes via alkali dissolution-acid neutralization method, which markedly enhanced the solubility and stability of curcumin. Lei [20] compounded corn peptides with microcrystalline chitin to form stable novel Pickering emulsion gels, which effectively improved the stability and antioxidant activity of curcumin. Therefore, compounding corn peptides with other bioactive peptides and polyphenols can improve the stability and biological activities of products. Compounding corn peptides with certain bioactive substances significantly increases their economic and practical values, showing potential applications in health food and pharmaceutical fields.

二．Research Progress on Biological Activities of Modified Corn Peptides

Enhanced Activity in Promoting Absorption of Nutrient Elements

Nutrient elements play vital roles in promoting organism growth and development, regulating physiological functions and maintaining human metabolism. Compared with trace elements in inorganic salt form, polypeptide-trace element chelates are much easier to be absorbed by human body.

Iron is an essential trace element and a key component of hemoglobin, participating in important physiological processes in human body. Iron deficiency can cause iron deficiency anemia and impair human immune function. Xu [21] found in in vitro simulated digestion experiments that chelation between whey protein isolate peptides and ferrous ions improves iron stability in the digestive tract, reduces adverse effects of free iron ions on the gastrointestinal tract and overall health, optimizes iron absorption mechanism, and significantly enhances iron bioavailability. Specific amino acid residues in partial peptide sequences can stably and efficiently chelate metal ions, improving iron absorption and utilization in organisms [22]. Based on the iron absorption-promoting property of polypeptides, corn angiotensin-converting enzyme (ACE) inhibitory peptides can be chelated with iron through synergistic effects to develop superior iron supplements. Feng [23] prepared peptide-iron chelates with dual functions of ACE inhibition and iron absorption promotion via ultrasound-assisted gradient feeding enzyme-membrane coupling chelation between corn ACE inhibitory peptides and ferrous ions.

Calcium is an essential nutrient element existing mainly in compounds, and is the most abundant inorganic element in human body. Calcium deficiency may result from insufficient intake, internal environment imbalance or insufficient hormone secretion, all of which reduce calcium absorption efficiency. Thus, calcium ions are critical for regulating physiological functions. Jiao et al. [24] prepared nano-sized peptide-chelated calcium using corn peptides and inorganic calcium chloride via organic solvent precipitation technology, and preliminarily explored its mechanism, providing references for developing novel calcium supplements. Qu et al. [25] fabricated corn peptide-chelated calcium microcapsules using synchronous dual-frequency ultrasound enhancement method, which greatly improved the solubility of calcium chelates. Moreover, ultrasound-treated and encapsulated microcapsules showed obvious sustained-release effects both during gastrointestinal digestion and in vitro. In conclusion, chelation of corn peptides with nutrient elements can supplement essential nutrients for human body, thereby improving immunity and preventing certain diseases.

Enhancement Mechanism of Antioxidant Activity

Antioxidants can donate electrons to pair with unpaired electrons of DPPH radicals and neutralize radicals. In addition, hydroxyl radicals with strong oxidizability react with lipids, proteins, DNA and other substances in organisms, inducing cell damage and oxidative stress. Accordingly, antioxidant capacity of substances can be evaluated by measuring DPPH and hydroxyl radical scavenging rates.

The enhanced antioxidant activity of corn peptides after metal ion chelation originates from altered electron cloud distribution in chelate structures. In corn peptide-Ca chelates, calcium ions bind to amino acid residues of corn peptides via coordination bonds, inducing electron cloud rearrangement of corn peptide molecules, enhancing electron-donating capacity, and facilitating pairing with unpaired electrons of DPPH radicals to scavenge radicals. Lu et al. [26] compared DPPH radical scavenging rates of ascorbic acid, corn peptides and corn peptide-calcium chelates, and calculated half maximal inhibitory concentration (IC₅₀). The results showed that corn peptide-calcium chelates had lower IC₅₀ values than original corn peptides. At a concentration of 10 mg·mL⁻¹, chelates exhibited higher scavenging activity and superior performance at high concentrations. However, no significant difference in scavenging rates was observed between the two when the concentration exceeded 1 mg·mL⁻¹.

Glycosylation modification improves antioxidant activity of corn peptides mainly by changing molecular spatial conformations through glycosyl-peptide binding. When chitosan oligosaccharide glycosylation modification of corn peptides is catalyzed by transglutaminase, glycosyl groups attach to specific amino acid residues of corn peptides, increasing molecular hydrophilicity and steric hindrance. Enhanced hydrophilicity facilitates reactions between corn peptides and radicals, while altered steric hindrance prevents radicals from attacking peptide chains and reduces oxidation reactions, significantly improving DPPH and hydroxyl radical scavenging rates. Tong et al. [27] reported that chitosan oligosaccharide glycosylation modification catalyzed by transglutaminase markedly elevated DPPH and hydroxyl radical scavenging rates and greatly enhanced antioxidant activity of corn peptides.

Improved antioxidant activity of corn peptides after compounding with other substances derives from synergistic effects between different molecules. Corn peptides are rich in branched-chain amino acids such as alanine and leucine, while soybean peptides have distinct amino acid compositions and structures. Their combination forms more complex and stable structures with additional active sites, enabling efficient radical scavenging. Zhu [28] found that aqueous solutions of soybean-corn composite peptides eluted by ethanol at different concentrations exhibited different antioxidant activities. The highest DPPH and hydroxyl radical scavenging rates were achieved at 50% ethanol concentration, with the hydroxyl radical scavenging rate reaching 50.02%, indicating strong antioxidant activity. As natural antioxidants, corn peptides exert better antioxidant effects after metal ion chelation, glycosylation modification and compounding with other peptides, showing great potential in the food industry.

Research on Alcohol Detoxification Activity

In recent years, increasing alcohol-related diseases have made developing efficient and low-toxic alcohol detoxification methods a research hotspot. Relevant studies have confirmed that corn peptides possess favorable alcohol detoxification and liver-protective activities [29]. Jiang [30] analyzed scanning electron microscopy images and secondary structures, and found that glycosylation altered the microstructure and secondary structure of corn peptides with enhanced disorderliness of secondary structures, thus improving alcohol detoxification activity. In addition, combination of corn peptides with curcumin further enhances alcohol detoxification activity.

Zhu et al. [19] conducted research on alcohol-induced liver injury mouse models, and found that curcumin-corn peptide complexes significantly alleviated drunkenness in mice, effectively prolonged wakefulness time under alcohol exposure, and exhibited excellent alcohol detoxification capacity. Meanwhile, the complexes regulate biochemical indicators in mice, reduce serum aspartate transaminase and alanine transaminase levels, decrease triglyceride and malondialdehyde contents in liver tissues, and increase hepatic reduced glutathione concentration, realizing liver protection while detoxifying alcohol. In summary, corn peptides exert unique alcohol detoxification effects in complexes, which are of great significance for alleviating drunkenness and protecting the liver, providing novel ideas and approaches for solving alcohol-related problems.

三.Conclusion

Corn peptides are natural products with wide sources and no side effects. In recent years, with technological progress and continuous improvement of living standards, public attention to health has increased significantly. Further research on biological activities of corn peptides has promoted their popularization and application in developing health foods. Nevertheless, reports on compound modification of corn peptides are relatively scarce. Further in-depth research on corn peptides using advanced technologies is expected to develop multi-functional and high-value-added products. Exploration of their applications in novel food systems, improvement of bioavailability and pharmaceutical stability will promote the development of corn peptides in food and pharmaceutical industries, generating greater economic and social benefits.

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