Comparative analysis of human milk and infant formula derived peptides following in vitro digestion

Peptidome profiling of human, bovine, and donkey colostrum through label-free quantitative analysis reveals proteolysis of milk proteins

Proteins and peptides play vital roles in different biological processes in vivo. As a dynamic hydrolysis system, milk is rich in proteins and proteases and provides a constant supply of endogenous bioactive peptides to newborn mammals. Previous studies have primarily focused on researching bioactive peptides by adding exogenous enzymes to milk samples. However, such an approach overlooks the significance of endogenous peptides and parent proteins that naturally exist in milk. Herein, we analyzed and compared parent proteins and their releasing peptides in human colostrum (HC), bovine colostrum (BC), and donkey colostrum (DC). The predominant proteins and hydrolyzed peptides in the three types of milk were identified. Among them, peptides were found to possess common bioactivities, including ACE inhibitory, antioxidant, antibacterial and immunomodulatory properties in HC, BC, and DC. Furthermore, the biological functions of these parent proteins were clarified using bioinformatics. These insights offer a novel perspective on natural bioactive peptides and the potential utilization of specific parent proteins and peptides to develop infant formulae derived from diverse milk sources.

Revealing the diversity of endogenous peptides and parent proteins in human colostrum and mature milk through peptidomics analysis

Endogenous peptides and their parent proteins are important nutritional components with diverse biological functions. The objective of this study was to analyze and compare endogenous peptides and parent proteins found in human colostrum (HC) and human mature milk (HM) using a 4D label-free technique. In total, 5162 and 940 endogenous peptides derived from 258 parent proteins were identified in human milk by database (DB) search and de novo, respectively. Among these peptides, 2446 differentially expressed endogenous peptides with various bioactivities were identified. The Gene Ontology analysis unveiled the cellular components, biological processes, and molecular functions associated with these parent proteins. Metabolic pathway analysis suggested that neutrophil extracellular trap formation had the greatest significance with 24 parent proteins. These findings will offer a fresh perspective on the development of infant formula powder, highlighting the potential for incorporating these changes to enhance its nutritional composition and benefits.

Characterization and biological function analysis of endogenous peptides derived from donkey colostrum proteins

Donkey colostrum, due to its abundance of active ingredients, including lysozyme, proteins, and peptides, is essential for the growth and immune defence of newborns. However, research on endogenous peptides in donkey colostrum is inadequate. This study analysed the profiles of endogenous peptides, their potential bioactivity, and the enzymes that generated these peptides using two different strategies. A total of 6202 endogenous peptides were characterised through a database search, while an additional 2997 peptides were identified de novo. Among the 1142 proteins identified, trypsin and plasmin demonstrated the highest bioactivities. Furthermore, a bioinformatics-based screening identified antioxidant peptides, angiotensin I-converting enzyme inhibitory peptides, and dipeptidyl peptidase IV inhibitory peptides as the three most active peptides. Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were conducted. These findings enhance our knowledge of endogenous peptides in donkey colostrum and provide crucial information regarding these peptides as nutritional factors for the future development of functional foods derived from donkey sources.

Protein ingredient quality of infant formulas impacts their structure and kinetics of proteolysis under in vitro dynamic digestion

Infant formula (IF) is a complex matrix requiring numerous ingredients and processing steps. The objective was to understand how the quality of protein ingredients impacts IF structure and, in turn, their kinetics of digestion. Four powdered IFs (A/B/C/D), based on commercial whey protein (WP) ingredients, with different protein denaturation levels and composition (A/B/C), and on caseins with different supramolecular organisations (C/D), were produced at a semi-industrial level after homogenization and spray-drying. Once reconstituted in water (13 %, wt/wt), the IF microstructure was analysed with asymmetrical flow field-flow fractionation coupled with multi-angle light scattering and differential refractometer, transmission electron microscopy and electrophoresis. The rehydrated IFs were subjected to simulated infant in vitro dynamic digestion (DIDGI®). Digesta were regularly sampled to follow structural changes (confocal microscopy, laser-light scattering) and proteolysis (OPA, SDS-PAGE, LC-MS/MS, cation-exchange chromatography). Before digestion, different microstructures were observed among IFs. IF-A, characterized by more denatured WPs, presented star-shaped mixed aggregates, with protein aggregates bounded to casein micelles, themselves adsorbed at the fat droplet interface. Non-micellar caseins, brought by non-micellar casein powder (IF-D) underwent rearrangement and aggregation at the interface of flocculated fat droplets, leading to a largely different microstructure of IF emulsion, with large aggregates of lipids and proteins. During digestion, IF-A more digested (degree of proteolysis + 16 %) at 180 min of intestinal phase than IF-C/D. The modification of the supramolecular organisation of caseins implied different kinetics of peptide release derived from caseins during the gastric phase (more abundant at G80 for IF-D). Bioactive peptide release kinetics were also different during digestion with IF-C presenting a maximal abundance for a large proportion of them. Overall, the present study highlights the importance of the structure and composition of the protein ingredients (WPs and caseins) selected for IF formulation on the final IF structure and, in turn, on proteolysis. Whether it has some physiological consequences remains to be investigated.

Differences in gut microbial fructoselysine degradation activity between breast-fed and formula-fed infants

The Amadori product fructoselysine is formed upon heating of food products and is abundantly present in infant formula while being almost absent in breast milk. The human gut microbiota can degrade fructoselysine for which interindividual differences have been described for adults. The aim of this study is to compare functional differences in microbial fructoselysine degradation between breast-fed and formula-fed infants, in view of their different diets and resulting different fructoselysine exposures. First, a publicly available metagenomic dataset with metagenome-assembled genomes (MAGs) from infant fecal samples was analyzed and showed that query genes involved in fructoselysine degradation (frlD/yhfQ) were abundantly present in multiple bacterial taxa in the fecal samples, with a higher prevalence in the formula-fed infants. Next, fecal samples collected from exclusively breast-fed and formula-fed infants were anaerobically incubated with fructoselysine. Both groups degraded fructoselysine, however the fructoselysine degradation activity was significantly higher by fecal samples from formula-fed infants. Overall, this study provides evidence that infant formula feeding, leading to increased dietary fructoselysine exposure, seems to result in an increased fructoselysine degradation activity in the gut microbiota of infants. This indicates that the infant gut microbiota adapts towards dietary fructoselysine exposure.

In vitro Digestion Characteristics of Hydrolyzed Infant Formula and Its Effects on the Growth and Development in Mice

Infant formula, an important food for babies, is convenient and nutritious, and hydrolyzed formulas have attracted much attention due to their non-allergicity. However, it is uncertain whether hydrolyzed formulars cause obesity and other side effects in infants. Herein, three infant formulas, standard (sIF), partially hydrolyzed (pHIF), and extensively hydrolyzed (eHIF), were analyzed in an in vitro gastrointestinal digestion model. With increasing degree of hydrolysis, the protein moleculars, and allergenicity of the proteins decreased and the long-chain polyunsaturated fatty acid content increased. Moreover, the digestion model solutions quickly digested the small fat globules and proteins in the hydrolyzed formula, allowing it to become electrostatically stable sooner. The eHIF-fed mice presented larger body sizes, and exhibited excellent exploratory and spatial memory abilities in the maze test. Based on villus height and crypt depth histological characterizations and amplicon sequencing, eHIF promoted mouse small intestine development and changed the gut microbiota composition, eventually favoring weight gain. The mouse spleen index showed that long-term infant formula consumption might be detrimental to immune system development, and the weight-bearing swimming test showed that eHIF could cause severe physical strength decline. Therefore, long-term consumption of infant formula, especially eHIF, may have both positive and negative effects on mouse growth and development, and our results might shed light on feeding formula to infants.

Combining in silico and in vitro approaches to identify endogenous hypoglycemic peptides from human milk

Potential endogenous hypoglycemic peptides derived from breast milk were screened by in silico approaches against intestinal glucose absorption- and metabolism-related membrane proteins (i.e., SGLT1, ATPase, and GPR40), and their inhibitory...

Digestibility of proteins in camel milk in comparison to bovine and human milk using an in vitro infant gastrointestinal digestion system

The absence of β-lactoglobulin, high β-/α_s-casein ratio and protective proteins make camel milk a promising alternative protein base for making human infant formulae. In this study, protein digestibility of camel milk was compared with that of bovine and human milk using an in vitro infant gastrointestinal digestion system. A low degree of gastric proteolysis was observed in all three kinds of milk, and a single clot was formed in camel milk. The soluble milk proteins remaining in the gastric digesta were digested rapidly and extensively in the intestinal phase, while the proteins in the camel milk clot were hydrolysed gradually. Despite several similarities, bioactive peptides unique to individual milk were identified in the three intestinal milk digesta. The results suggest that camel milk proteins are equally digestible as bovine and human milk proteins under infant gastrointestinal digestion conditions, and it may be a prospective substitute for infant formula base.

Analysis of the Endogenous Peptidomes of Different Infant Formula Types and Human Milk

Infant formula (IF) is a commonly used replacement whenever mother’s own milk is not available. Most IFs are based on cow milk (powders, liquids). Alternatives, based on other sources such as goat milk or plants, exist. Independent of the source, IF production and composition are strictly regulated. Besides proteins, minerals, and lipids, milk contains a variety of endogenous peptides. Whereas the human milk peptidome has been studied intensively, the peptidomes of IFs have been mostly neglected. This study investigated the peptidomes of different types of first stage IF, including cow milk-based powders and liquids, and powdered goat milk-based IF, highlighting major similarities and differences to human milk. Extracted native peptidomes were analyzed by nanoRPC-ESI-MS/MS using two different fragmentation techniques allowing the confident identification of 1587 peptides. β-Casein peptides dominated in all samples. Interestingly, powdered and liquid cow milk-based IFs differed in the numbers of β- and α_S1-casein peptides, indicating processing-derived variations. However, the peptidomes of cow and goat milk-based IF appeared to be more comparable to each other than to human milk. Despite an overlap in the major source proteins, many peptide sequences were different, i.e., species-specific. Remarkably, the data indicate that the human milk peptidome might be donor-specific as well.

Bioactive peptide release and the absorption tracking of casein in the gastrointestinal digestion of rats

Bovine casein is considered as an important source of many bioactive peptides (BAPs), which can also be produced via in vitro simulated gastrointestinal hydrolysis. To perform their physiological functions, some active peptides need to pass through the intestinal epithelial barrier and keep their structural integrity after oral administration. Owing to the complexity of in vivo digestion and absorption, there have been few studies in this area. In this study, casein was labeled with FITC to trace its digestion and absorption in Sprague Dawley (SD) rats. Gastric juice, intestinal fluid, blood, and intestinal tissue samples were collected at different time-points for preservation and analysis after intragastric administration. The results showed that CN-FITC exhibited good labeling stability in the gastrointestinal digestive juice both in vivo and in vitro, suggesting its potential to be used for the detection and tracking of casein hydrolysate. After the intra-gastric administration of FITC, the diffusion rates of fluorescent substances in serum were much higher than in the CN-FITC group. The maximum peptide content in the CN-FITC group during intestinal digestion was achieved 2 h after administration, and electrophoretic analysis of the hydrolysate composition suggested that the molecular weights of the peptides were mainly concentrated in the range of 3.4-10 kDa. The hydrolyzed peptides from CN-FITC could be absorbed into the blood just 1 h after administration. Frozen sections of rat duodenal tissue were observed under a confocal laser scanning microscope, and they showed that the CN-FITC digested products were absorbed from villi to mucosa in the rat intestines, and the casein-hydrolyzed polypeptides were accumulated significantly in tissue samples 2 h after administration. The peptides were mainly absorbed in the duodenum on the basis of absorption experiments using an everted gut sac. After intestinal digestion for 2 h, peptides with weights less than 5 kDa were enriched and identified using LC-MS-MS, and they were found to be mainly derived from β-casein, containing potential angiotensin-I-converting enzyme inhibitory, antioxidant, dipeptidyl peptidase IV inhibitory, and morphine-like peptides. The peptides from casein hydrolysate were tracked entering the blood through the intestinal epithelial barrier in the form of complete fragments, and they might exert potential physiological activity in vivo.

Comprehensive Profiling of the Native and Modified Peptidomes of Raw Bovine Milk and Processed Milk Products

Bovine milk contains a variety of endogenous peptides, partially formed by milk proteases that may exert diverse bioactive functions. Milk storage allows further protease activities altering the milk peptidome, while processing, e.g., heat treatment can trigger diverse chemical reactions, such as Maillard reactions and oxidations, leading to different posttranslational modifications (PTMs). The influence of processing on the native and modified peptidome was studied by analyzing peptides extracted from raw milk (RM), ultra-high temperature (UHT) milk, and powdered infant formula (IF) by nano reversed-phase liquid chromatography coupled online to electrospray ionization (ESI) tandem mass spectrometry. Only unmodified peptides proposed by two independent software tools were considered as identified. Thus, 801 identified peptides mainly originated from α_S- and β-caseins, but also from milk fat globular membrane proteins, such as glycosylation-dependent cell adhesion molecule 1. RM and UHT milk showed comparable unmodified peptide profiles, whereas IF differed mainly due to a higher number of β-casein peptides. When 26 non-enzymatic posttranslational modifications (PTMs) were targeted in the milk peptidomes, 175 modified peptides were identified, i.e., mostly lactosylated and a few hexosylated or oxidized peptides. Most modified peptides originated from α_S-caseins. The numbers of lactosylated peptides increased with harsher processing.

Peptidome comparison following gastrointestinal digesta of bovine versus caprine milk serum

Infant formula is used as a supplement for newborns. Although bovine milk-based infant formulas dominate the market, caprine milk-based infant formula has attracted increasing attention because of its lower allergenicity. This study compared the digestive peptidome of bovine and caprine milk serum proteins by using in vitro infant simulating conditions. The result showed that the degradation pattern of milk proteins was similar, whereas the digestive rates of milk proteins differed between bovine and caprine milks. Several proteins, such as α-lactalbumin (LALBA), β-lactoglobulin (LGB), serum amyloid A protein (SAA1), glycosylation-dependent cell adhesion molecule 1 (GLYCAM1), and lactotransferrin (LTF), released more peptides during digestion of caprine milk serum than during digestion of bovine milk serum; however, more peptides derived from α_S1-casein (CSN1S1) were found in bovine digesta. In addition, antimicrobial-related peptides were mostly only found in caprine intestinal digesta. The results of this study may be useful in understanding the digestion characteristics of milk serum proteins and providing guidance on the improvement of infant formula.

Human milk and infant formulae: Peptide differences and the opportunity to address the functional gap

Bovine-derived formula milk (FM) is a common substitute to human milk (HM), but lacks key functional benefits associated with HM. Accordingly, there have been significant efforts to humanise FM. Recent research has demonstrated that HM-derived peptides convey an array of beneficial bioactivities. Given that peptides serve as important signalling molecules offering high specificity and potency, they represent a prime opportunity to humanise FM. To further understand how HM-derived peptides contribute to infant health, we used peptidomics and bioinformatics to compare the peptide profile of HM to commercially available FM. We found clear and substantial differences between HM and FM in terms of peptide physicochemical properties, protein coverage and abundance. We additionally identified 618 peptides specific to HM that represent an important untapped source to be explored for novel bioactivities. While further study is required, our findings highlight the potential of a peptide-based approach to address the functional gap in FM.

Degradation of Proteins From Colostrum and Mature Milk From Chinese Mothers Using an in vitro Infant Digestion Model

This study provided insights into the degradation of human milk proteins in an in vitro infant digestion model by comparing colostrum (week 1) and mature milk (week 4) of 7 Chinese mothers individually. In this study, we adapted the exiting INFOGEST in vitro model, to conditions representative to infants (0 to 3 month-old). The level of undigested proteins was analyzed by LC-MS/MS after gel-electrophoretic separation and in-gel digestion. The BCA protein assay showed that the total undigested milk protein content decreased from the start to the end of digestion with variations between mothers, especially in the gastric phase (25–80%). Undigested proteins could also still be found after the intestinal phase, ranging from 0.5 to 4.2% of initial protein content. Based on LC-MS/MS analysis, milk protein digestion varied between the mothers individually, especially during the gastric phase. No differences could be observed between protein digestion from colostrum and mature milk after the intestinal phase. The highest levels of proteins remaining after intestinal digestion can be linked to the group immune-active proteins, for all mothers. The level of protease inhibitors and total protein content in the milk did not correlate with the overall proteolysis during digestion. The results also showed that milk serum proteins partly remained after the gastric phase, with 33% remaining from colostrum and 37% remaining from mature milk. More than 40 milk serum proteins were detected after the intestinal phase. Some of the highly abundant milk serum proteins (lactoferrin, serum albumin, bile salt-activated lipase, immunoglobulins, α₁-antichymotrypsin) were still partially present intact after the intestinal phase, for all mothers. Caseins were also not completely digested in the gastric phase, with 35% remaining from colostrum and 13% remaining from mature milk. Caseins, on the other hand, were almost completely digested after the intestinal phase. The complete degradation of caseins into peptides might be related to their structural features. Overall, this study showed that digestion differed for the various human milk proteins by adapting an in vitro digestion model to infant physiological conditions, with the main differences between digestion of the milk from individual mothers being observed after gastric digestion.

Current Evidence on the Bioavailability of Food Bioactive Peptides

Food protein-derived bioactive peptides are recognized as valuable ingredients of functional foods and/or nutraceuticals to promote health and reduce the risk of chronic diseases. However, although peptides have been demonstrated to exert multiple benefits by biochemical assays, cell culture, and animal models, the ability to translate the new findings into practical or commercial uses remains delayed. This fact is mainly due to the lack of correlation of in vitro findings with in vivo functions of peptides because of their low bioavailability. Once ingested, peptides need to resist the action of digestive enzymes during their transit through the gastrointestinal tract and cross the intestinal epithelial barrier to reach the target organs in an intact and active form to exert their health-promoting properties. Thus, for a better understanding of the in vivo physiological effects of food bioactive peptides, extensive research studies on their gastrointestinal stability and transport are needed. This review summarizes the most current evidence on those factors affecting the digestive and absorptive processes of food bioactive peptides, the recently designed models mimicking the gastrointestinal environment, as well as the novel strategies developed and currently applied to enhance the absorption and bioavailability of peptides.

Differences in peptide generation following in vitro gastrointestinal digestion of yogurt and milk from cow, sheep and goat

Fermentation of milk is commonly used throughout the world to produce a variety of foods with different health benefits. We hypothesised that due to differences in physicochemical properties and protein sequences among milk from different species and their fermented yogurt samples, their protein digestion and resulting peptide profiles would differ. Cow, goat and sheep milk and yogurt were compared at designated timepoints throughout in vitro gastric and intestinal digestion for differences in peptide profiles and peptide bioactivities. The results showed that most proteins in all milk and yogurt samples were digested within the early phase of gastric digestion. β-Lg and β-CN were digested faster in yogurt than milk, which was most evident for sheep products. Regardless of species, in vitro gastric and intestinal digestion released a higher concentration of specific peptides, particularly anti-hypertensives, from yogurt compared with their milk counterparts.

Effects of Protein-Derived Amino Acid Modification Products Present in Infant Formula on Metabolic Function, Oxidative Stress, and Intestinal Permeability in Cell Models

Proteins present in infant formulas are modified by oxidation and glycation during processing. Modified amino acid residues released from proteins may be absorbed in the gastrointestinal tract, and pose a health risk to infants. In this study, the markers of glycation furosine (1.7-3.5 μg per milligram of protein) and N^ε-(carboxymethyl)lysine (28-81 ng per milligram of protein) were quantitated in infant formulas. The effects of these species, and other amino acid modifications, at the levels detected in infant formulas, on 3T3-L1 (murine preadipocyte) and Caco-2 (human intestinal epithelial) cells were assessed. Incubation of 3T3-L1 cells for 48 h with amino acid side chain oxidation and glycation products (1 and 10 μM) resulted in a loss (up to 40%, p < 0.05) of cell thiols and decreased metabolic activity compared with those of the controls. In contrast, Caco-2 cells showed a stimulation (10-50%, p < 0.05) of cellular metabolism on exposure to these products for 24 or 48 h. A 28% ( p < 0.05) increase in protein carbonyls was detected upon incubation with 200 μM modified amino acids for 48 h, although no alteration in transepithelial electrical resistance was detected. Oxidation products were detected in the basolateral compartments of Caco-2 monolayers when modified amino acids were applied to the apical side, consistent with limited permeability (up to 3.4%) across the monolayer. These data indicate that modified amino acids present in infant formulas can induce effects on different cell types, with evidence of bioavailability and induction of cellular stress. This may lead to potential health risks for infants consistently exposed to high levels of infant formulas.

Variability of Serum Proteins in Chinese and Dutch Human Milk during Lactation

To better understand the variability of the type and level of serum proteins in human milk, the milk serum proteome of Chinese mothers during lactation was investigated using proteomic techniques and compared to the milk serum proteome of Dutch mothers. This showed that total milk serum protein concentrations in Chinese human milk decreased over a 20-week lactation period, although with variation between mothers in the rate of decrease. Variation was also found in the composition of serum proteins in both colostrum and mature milk, although immune-active proteins, enzymes, and transport proteins were the most abundant for all mothers. These three protein groups account for many of the 15 most abundant proteins, with these 15 proteins covering more than 95% of the total protein concentrations, in both the Chinese and Dutch milk serum proteome. The Dutch and Chinese milk serum proteome were also compared based on 166 common milk serum proteins, which showed that 22% of the 166 serum proteins differed in level. These differences were observed mainly in colostrum and concern several highly abundant proteins. This study also showed that protease inhibitors, which are highly correlated to immune-active proteins, are present in variable amounts in human milk and could be relevant during digestion.

Identification and availability of peptides from lactoferrin in the gastrointestinal tract of mice

The metabolic fate of lactoferrin in vivo.

Peptidomic profiling of human milk with LC-MS/MS reveals pH-specific proteolysis of milk proteins

Human milk is a dynamic protein-protease system that delivers bioactive peptides to infants. The pH of milk changes from the mother's mammary gland to the infant's digestive tract. Although the release of human milk peptides has been studied during in vivo or in vitro digestion, these models did not explicitly vary nor observe the effect of pH. The objective of this research was to determine the effect of pH on the proteolysis of human milk. Using high-resolution accurate-mass Orbitrap mass spectrometry, profiles of endogenous human milk peptides before and after incubation at various pH levels have been mapped. Over 5000 peptides were identified. Comparative analyses classified 74 peptides that were consistently found independent of pH alterations, and 8 peptides that were released only at pH 4 or 5 (typical infant gastric pH). Results documented that the proteolysis of milk proteins, particularly β-casein, polymeric immunoglobulin receptor, and α-lactalbumin, is pH-dependent.

Protein Digestion of Baby Foods: Study Approaches and Implications for Infant Health

Protein digestion is critical for infants. Dissimilarities between infants and adults in food intake and digestive physiology lead to distinct patterns of proteolysis between individuals. However, such differences are not well represented in many studies on protein digestion of baby foods. The complex biological structures of baby foods and the physiology of the infant digestive system are key factors affecting proteolysis during the first two years of life. Well-controlled in vitro studies have demonstrated that varying digestion conditions alter the specificity, rate, and extent of proteolysis of baby foods. Nonetheless, these models do not completely replicate in vivo proteolysis or the complex biogeography of the gastrointestinal tract. Animal and clinical studies have revealed the fate of dietary proteins along the digestive tract and the overall health impact on subjects. Building comprehensive and annotated datasets from human infants will require innovative and standardized measurement. Now, more systematic evaluations of digestion are emerging to advance the knowledge and its translation as food design for effective diet and health management in infants.