Antioxidant Peptides Change Chemically While Fighting Oxidation, Review Finds

Food-derived antioxidant peptides, widely promoted as natural alternatives to synthetic additives in functional foods and nutraceuticals, do not simply neutralize reactive molecules and remain unchanged. According to a review published in Discover Molecules, these short amino acid sequences are themselves transformed by the very oxidizing environments they are meant to counter, yielding hydroperoxides, carbonyls, backbone fragments, covalent dimers, and adducts with oxidized lipids and sugars. The authors, Matthew D. Ezema, Harthika Mylvaganam, and Apollinaire Tsopmo, argue that while the chemistry of these transformations is increasingly well mapped, the functional and toxicological consequences for bioactive peptides remain largely unexplored.

The review, published on 3 July 2026, integrates current knowledge on the mechanisms of antioxidant activity of food-derived peptides with an emerging safety dimension. Bioactive peptides are short sequences encrypted within plant and animal proteins and released through enzymatic hydrolysis, microbial fermentation, or, increasingly, computational discovery pipelines that mine protein databases for promising motifs. Their antioxidant activity operates through several complementary mechanisms: direct radical scavenging via electron transfer, hydrogen atom transfer, or proton-coupled electron transfer; chelation of transition metals such as iron and copper that would otherwise catalyze the formation of highly reactive hydroxyl radicals through Fenton chemistry; and, in cellular and animal models, modulation of endogenous antioxidant enzymes and redox-responsive signalling pathways.

The structural basis of this activity is sequence-dependent. Histidine-containing peptides show high activity through the imidazole ring’s participation in proton-coupled electron transfer and metal chelation, while tyrosine and tryptophan donate hydrogen from phenolic and indole groups that stabilize the resulting radicals through resonance. Cysteine and methionine act as potent electron donors, and acidic residues such as aspartate and glutamate bind Fe2+ and Cu2+, suppressing Fenton chemistry. The review cites numerous examples: the pea-derived peptide YLVN shows exceptionally high ABTS radical scavenging with an IC50 of 0.002 mg/mL and an ORAC value of 1.12 micromoles of Trolox equivalent per micromole, outperforming glutathione, while the whey peptide LDQW reaches 3.79 micromoles Trolox equivalent per micromole in the ORAC assay. Hemp seed peptides WVYY and PSLPA bind 94 and 96 percent of ferrous iron at 0.5 mg/mL, respectively.

In cellular and animal models, the protective effects extend beyond simple radical chemistry. A yak bone collagen-derived peptide reduced ultraviolet A-induced damage in human fibroblasts by lowering malondialdehyde levels and restoring catalase and superoxide dismutase activities through activation of the Keap1/Nrf2 pathway. The microalgal RuBisCO-derived dipeptide Phe-Cys reduced hydrogen peroxide-induced reactive oxygen species in Chang liver cells by 20 to 45 percent and lipid peroxidation by up to 61 percent. In the roundworm Caenorhabditis elegans, peptides from Arca subcrenata extended mean lifespan under paraquat-induced oxidative stress and downregulated the aging-associated gene age-1. Casein-derived peptides alleviated chemically induced liver injury in mice and showed osteoprotective effects in ovariectomized rats.

Yet the same residues that make peptides effective antioxidants are precisely those most vulnerable to oxidative transformation. Hydroxyl radicals, generated from hydrogen peroxide through metal-catalyzed Fenton chemistry, initiate site-specific oxidation that produces peptide hydroperoxides with a mass shift of plus 32 daltons, alcohols at plus 16, carbonyls at plus 14, backbone scission products, and residue-specific dimers. In a model system, the peptide Val-Gly-Val-Ala-Pro-Gly yielded a mixture of oxidation products in which every amino acid was oxidized. Barley peptides exposed to hydroxyl radicals from a copper/hydrogen peroxide system formed hydroperoxides, carbonyls, and a covalent dityrosine dimer. Tryptophan-containing di- and tripeptides yielded Trp-derived hydroperoxides that decompose to N-formylkynurenine, kynurenine, and ring-opened species, and oat peptides lost more than half of their concentrations under peroxyl radical attack.

A second layer of modification arises from secondary lipid oxidation products. Malondialdehyde forms Schiff bases with lysine, histidine, arginine, glutamine, and asparagine side chains, while the alpha,beta-unsaturated aldehyde 4-hydroxy-2-nonenal predominantly forms Michael-type adducts with cysteine, histidine, and lysine. In soy protein peptides, researchers identified 53 peptides reacting with malondialdehyde through Schiff base formation, more than half occurring at peptide N-termini, alongside dihydropyridine-lysine adducts and both Schiff and Michael adducts with hydroxynonenal. Similar products were found for beta-lactoglobulin peptides. In proteins, such modifications are associated with aggregation, reduced digestibility, altered allergenicity, and cytotoxicity; malondialdehyde-derived pyrroles can intercalate into and disrupt membranes, and dihydropyridine-lysine shows photoreactivity and cytotoxicity in human skin cells. Whether antioxidant peptides modified by these aldehydes behave similarly remains untested.

Glycation with reducing sugars, often deliberately used to enhance peptide functionality, adds further complexity. The review highlights animal and cell studies suggesting that in vivo behaviour can diverge sharply from in vitro antioxidant readouts: glucose-glycated silver carp peptides, despite improved antioxidant activity in vitro, increased oxidative stress in diabetic mice, and glucose-glycated anchovy peptides raised body weight in male mice while lowering it in females. Galactose-glycated gelatin peptides inhibited Caco-2 cell proliferation, whereas glycation with whey peptides enhanced proliferation by roughly 30 percent. Under high-temperature, prolonged-heating conditions, glycation can also generate heterocyclic aromatic amines and advanced glycoxidation products with recognized toxicological concerns, leading the authors to recommend that quantifying such products become a standard component of safety assessment for glycated peptide ingredients. They note, however, that dietary acrylamide, a probable human carcinogen formed mainly from free asparagine and reducing sugars, receives minimal contribution from bioactive peptides.

On the safety side, the review finds that systematic toxicological evaluation of antioxidant peptides remains limited. Most studies use cytotoxicity assays only to confirm that chosen bioactivity-testing concentrations do not harm cell viability, reporting toxicity incidentally rather than as a primary objective. Available data nonetheless show dose-dependent effects: the tilapia peptide piscidin-4 reduced viability of lung cancer cells by 30 to 35 percent and normal lung fibroblasts by 15 percent at 15 micrograms per milliliter; a milk-derived peptide caused 23.54 percent hemolysis of human red blood cells at 512 micromolar; and several wheat-germ peptides showed IC50 values between 2.3 and 11.2 micromolar in lung carcinoma cells. In mice, whey protein hydrolysates produced genotoxic effects at elevated doses, with DNA strand breaks detected at 50 to 100 mg/kg body weight and increased micronucleated erythrocytes at 200 mg/kg. In zebrafish embryos, egg-lysozyme peptides were non-toxic below 50 micrograms per milliliter, but one peptide caused 50 percent embryo mortality at 800 micrograms per milliliter despite showing the strongest antioxidant activity, illustrating that antioxidant potency does not preclude toxicity.

The evidence on human exposure, drawn from recent reviews of intervention studies, is comparatively reassuring. Purified peptides have been assessed at 1 to 50 mg per day and peptide-rich hydrolysates at 0.2 to 150 g per day, with most studies below 35 g per day, and milk-derived peptides Val-Pro-Pro and Ile-Pro-Pro showed no adverse effects at 3 to 50 mg per day, with gram-scale hydrolysate intake producing no toxicity. Bovine hair hydrolysates showed no acute toxicity, mutagenicity, or genotoxicity in rats at 10 g/kg body weight. The authors caution, however, that these figures do not cover concentrated supplements or oxidatively modified peptide products, and that most toxicological data rely on in vitro assays and short-term animal studies that cannot capture long-term exposure, cumulative metabolic effects, or interactions with the human microbiome and immune system.

The review closes with a research agenda: systematic mapping of oxidative peptide modifications using mass spectrometry-based oxidative peptidomics, which can localize modifications to specific residues through tandem fragmentation; development of quantitative structure-toxicity relationship databases; improved quantification of human exposure and dose-response relationships; and deeper study of how gut microbiota transform both native and modified peptides that escape host digestion. Only di- and tripeptides are efficiently absorbed through intestinal peptide transporters, so larger peptides may act locally in the gut or be metabolized by microbes, and oxidative, glycated, or lipoxidized modifications can alter stability, absorption, and metabolic fate. The authors’ central message is that antioxidant peptides should not be viewed as universally safe molecules but as chemically dynamic entities whose safety depends on structure, dose, transformation, and biological context.

Subject of Research: Oxidative transformations of food-derived antioxidant peptides and their implications for function and safety

Article Title: Food-derived antioxidant peptides undergo oxidative transformations that may alter their function and safety

Article References: Ezema, M. D., Mylvaganam, H., & Tsopmo, A. (2026). Food-derived antioxidant peptides undergo oxidative transformations that may alter their function and safety. Discover Molecules, 3(1), Article 20. https://doi.org/10.1007/s44345-026-00063-x

Image Credits: AI Generated

DOI: 10.1007/s44345-026-00063-x

Keywords: antioxidant peptides, bioactive peptides, oxidative modification, food chemistry, reactive oxygen species, lipid oxidation, glycation, food safety, toxicology, mass spectrometry, functional foods, nutraceuticals