Predicting the important enzymes in human breast milk digestion

Comparison of protein digestibility of human milk and infant formula using the INFOGEST method under infant digestion conditions

Many improvements have been made to bring infant formula (IF) closer to human milk (HM) regarding its nutritional and biological properties. Nevertheless, the protein components of HM and IF are still different, which may affect their digestibility. This study aimed to evaluate and compare the protein digestibility of HM and IF using the infant INFOGEST digestion method. Pooled HM and a commercial IF were subjected to the infant INFOGEST method, which simulates the physiological digestion conditions of infants, with multiple directions, i.e. the curd state, gel images of SDS-PAGE, molecular weight distribution, free amino acid concentrations and in vitro protein digestion rate. HM underwent proteolysis before digestion and tended to have a higher protein digestion rate with finer curds during gastric digestion, than the IF. However, multifaceted analyses showed that the protein digestibility of HM and IF was not significantly different after gastrointestinal digestion. In conclusion, the infant INFOGEST method showed that the digestibility of HM and IF proteins differed to some extent before digestion and after gastric digestion, but not at the end of gastrointestinal digestion. The findings of this study will contribute to the refinement of IF with better protein digestibility in infant stomach.

Network analysis of the proteome and peptidome sheds light on human milk as a biological system

Proteins and peptides found in human milk have bioactive potential to benefit the newborn and support healthy development. Research has been carried out on the health benefits of proteins and peptides, but many questions still need to be answered about the nature of these components, how they are formed, and how they end up in the milk. This study explored and elucidated the complexity of the human milk proteome and peptidome. Proteins and peptides were analyzed with non-targeted nanoLC-Orbitrap-MS/MS in a selection of 297 milk samples from the CHILD Cohort Study. Protein and peptide abundances were determined, and a network was inferred using Gaussian graphical modeling (GGM), allowing an investigation of direct associations. This study showed that signatures of (1) specific mechanisms of transport of different groups of proteins, (2) proteolytic degradation by proteases and aminopeptidases, and (3) coagulation and complement activation are present in human milk. These results show the value of an integrated approach in evaluating large-scale omics data sets and provide valuable information for studies that aim to associate protein or peptide profiles from biofluids such as milk with specific physiological characteristics.

Human colostrum in vitro protein digestion: peptidomics by liquid chromatography-Orbitrap-high-resolution MS and prospection for bioactive peptides via bioinformatics

Breast milk is known to contain bioactive peptides that are released during digestion, being a major source of bioactive peptides to the new-born, some of which act against invading pathogens. However, the formation of bioactive peptides during digestion of human colostrum remains largely uninvestigated. This study aimed to investigate the formation of peptides during simulated digestion of human colostrum from adult women and to prospect antimicrobial peptides. For this purpose, we used high-resolution MS to monitor the release of peptides during in vitro digestion. Bioinformatics was used for the prospection of antimicrobial activity of peptides. During simulated digestion (oral, gastric and duodenal phases), 2318 peptide sequences derived from 112 precursor proteins were identified. At the end of simulated digestion, casein-derived peptide sequences were the most frequently observed. Among precursors, some proteins were seen for the first time in this study. The resulting peptides were rich in proline, glutamine, valine and leucine residues, providing characteristic traits of antimicrobial peptides. From bioinformatics analysis, seven peptides showed potentially high antimicrobial activity towards bacteria, viruses and fungi, from which the latter was the most prominent predicted activity. Antimicrobial peptides released during digestion may provide a defence platform with controlled release for the new-born.

A longitudinal study of fatty acid profiles, macronutrient levels, and plasmin activity in human milk

Introduction

Human milk provides nutrients essential for infant growth and health, levels of which are dynamic during lactation.

Methods

In this study, changes in macronutrients, fatty acids, and plasmin activities over the first six months of lactation in term milk were studied.

Results

There was a significant influence of lactation stage on levels of protein and plasmin activities, but not on levels of fat and carbohydrate in term milk. Concerning fatty acids in term milk, levels of caproic acid and α-linolenic acid increased significantly (p < 0.05), whereas those of arachidonic acid and docosahexaenoic acid decreased, in the six months after birth. Significant impacts of maternal pre-pregnancy BMI and infant gender on fatty acid profiles were also found. Multivariate statistical analysis showed that protein level, plasmin activity, and several fatty acids (α-linolenic acid, lignoceric acid, and docasadienoic acid) contributed strongly to discrimination of milk from different lactational stages.

Discussion

The study demonstrates that not all but some fatty acids were influenced by lactation, whereas protein and protease levels showed clear decreasing trends during lactation, which may help in understanding the nutritional requirements of infants.

The role of human milk fats in shaping neonatal development and the early life gut microbiota

Human breast milk (HBM) is the main source of nutrition for neonates across the critical early-life developmental period. The highest demand for energy is due to rapid neurophysiological expansion post-delivery, which is largely met by human milk lipids (HMLs). These HMLs also play a prebiotic role and potentially promote the growth of certain commensal bacteria, which, via HML digestion, supports the additional transfer of energy to the infant. In tandem, HMLs can also exert bactericidal effects against a variety of opportunistic pathogens, which contributes to overall colonisation resistance. Such interactions are pivotal for sustaining homeostatic relationships between microorganisms and their hosts. However, the underlying molecular mechanisms governing these interactions remain poorly understood. This review will explore the current research landscape with respect to HMLs, including compositional considerations and impact on the early life gut microbiota. Recent papers in this field will also be discussed, including a final perspective on current knowledge gaps and potential next research steps for these important but understudied breast milk components.

Goat milk-derived short chain peptides: Peptide LPYV as species-specific characteristic and their versatility bioactivities by MOF@Fe3O4@GO mesoporous magnetic-based peptidomics

Goat milk as an ideal substitute for human milk has not been sufficiently explored. An in-situ synthesized MOF@Fe₃O₄@GO was demonstrated as a magnetic mesoporous adsorbent for efficiently enriching short chain peptides (SCPs) in milk compared with the routine solid phase extraction approach with graphite carbon black or C₁₈ as the packing material in terms of the number of enriched SCPs and data stability. A total of 61 and 126 SCPs were identified and quantified in bovine milk (0.09-89.34 μg L^-1) and goat milk (10.5-1267.06 μg L^-1), respectively, and peptide LPYV can be used as a potential marker for adulteration of goat milk. Relative high expression of chymotrypsin and pepsin by EnzymePredictor analysis could partially elaborate the reason of the abundance of SCPs in goat milk. Compared with bovine milk, further bioinformatics analysis indicated that goat milk could own higher nutritional value because of relative higher concentrations (>1 mg/L) of SCPs (LLV, FL, LVYP) with confirmed bioactivities including angiotensin-converting enzyme (ACE) inhibitor, antioxidant, dipeptidylpeptidase (DPP) III and DPP IV inhibitor, etc. Overall, this study opened a novel avenue for understanding versatility benefit of dairy products from a perspective of SCPs by using a developed MOF@Fe₃O₄@GO mesoporous magnetic-based peptidomics.

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...

Quantitative N-glycoproteome analysis of bovine milk and yogurt

Post-translational modification structure of food's proteins might be changed during processing, thereby affecting the nutritional characteristics of the food product. In this study, differences in protein N-glycosylation patterns between milk and yogurt were quantitatively compared based on glycopeptide enrichment, liquid chromatography separation, and tandem mass spectrometry analysis. A total of 181 N-glycosites were identified, among which 142 were quantified in milk and yogurt. Significant alterations in the abundance of 13 of these N-glycosites were evident after the fermentation of milk into yogurt. Overall, the N-glycosylation status of the majority of milk proteins remained relatively unchanged in yogurt, suggesting that their conformations, activities, and functions were maintained despite the fermentation process. Among the main milk proteins, N²⁴¹ of cathepsin D and N³⁵⁸ of lactoperoxidase were markedly reduced after undergoing lactic acid fermentation to produce yogurt. Furthermore, a comparative analysis of current and previously reported N-glycoproteomic data revealed heterogeneity in the N-glycosylation of milk proteins. To sum up, a quantitative comparison of the N-glycoproteomes of milk and yogurt was presented here for the first time, providing evidence that the fermentation process of yogurt could cause changes in the N-glycosylation of certain milk proteins.

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181 N-glycosites from 118 N-glycoproteins were identified in milk and yogurt.

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13 N-glycosites changed significantly after fermentation of milk into yogurt.

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N²⁴¹ of cathepsin D and N³⁵⁸ of lactoperoxidase was markedly reduced in yogurt.

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Heterogeneity of N-glycosylation of milk protein has been documented.

Identification of three novel antioxidative peptides from Auxenochlorella pyrenoidosa protein hydrolysates based on a peptidomics strategy

Auxenochlorella pyrenoidosa is recognized as a potential sustainable protein material in food industry, however, its application remains still very limited. Herein, this study aimed to investigate the antioxidative properties of Auxenochlorella pyrenoidosa protein hydrolysates and identify novel antioxidative peptides from protein hydrolysates through a workflow mainly including enzymatic hydrolysis, peptidome quantification, quantitative structure-activity relationship (QSAR) modeling, in silico screening, and validation. Three novel antioxidative peptides including AGWACLVG, IDLAY and YPLDL were identified from protein hydrolysates by papain with the hydrolysis time of 4 h, in which, AGWACLVG showed strong 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity with the IC₅₀ value of 68.88 µM and Trolox equivalent antioxidative capacity of 6.20 ± 0.23 mmol TE/g. This study suggested that Auxenochlorella pyrenoidosa protein hydrolysates could be used as potential antioxidative ingredients in food industry, and the identification of novel antioxidative peptides would contribute to the construction of more robust QSAR models in the future.

Monitoring Human Milk β-Casein Phosphorylation and O-Glycosylation Over Lactation Reveals Distinct Differences between the Proteome and Endogenous Peptidome

Human milk is a vital biofluid containing a myriad of molecular components to ensure an infant's best start at a healthy life. One key component of human milk is β-casein, a protein which is not only a structural constituent of casein micelles but also a source of bioactive, often antimicrobial, peptides contributing to milk's endogenous peptidome. Importantly, post-translational modifications (PTMs) like phosphorylation and glycosylation typically affect the function of proteins and peptides; however, here our understanding of β-casein is critically limited. To uncover the scope of proteoforms and endogenous peptidoforms we utilized mass spectrometry (LC-MS/MS) to achieve in-depth longitudinal profiling of β-casein from human milk, studying two donors across 16 weeks of lactation. We not only observed changes in β-casein's known protein and endogenous peptide phosphorylation, but also in previously unexplored O-glycosylation. This newly discovered PTM of β-casein may be important as it resides on known β-casein-derived antimicrobial peptide sequences.

Composition of uterine milk and its changes with gestational period in red stingrays (Hemitrygon akajei)

Uterine milk is secreted in the uterus for embryo nutrition in several elasmobranch species and may contribute to rapid embryonic growth, but the details of its composition and its functions are poorly understood. In this study, to explore the roles of uterine milk for embryos, its components throughout the gestational period were analysed in detail. Uterine milk was collected from pregnant red stingrays (Hemitrygon akajei) in the early, middle and late gestational periods, respectively (n= 3 for each period). The crude composition, constituent proteins and fatty acids in the milk were analysed. The uterine milk was rich in proteins throughout the gestational period, whereas lipids dramatically increased in the middle period and reduced slightly towards the late period. Some proteins potentially associated with nutrition, cartilage growth and embryonic immunity were found. Several enzymes related to central metabolism were also detected. The constituent fatty acids in the middle and late periods were similar to those in the egg yolks of elasmobranchs, except for C18:2, which was rich only in the uterine milk. The most abundant fatty acid in the milk was C16:1, which could function as a lipokine to promote lipid metabolism in the embryo. This study's data suggest that uterine milk may be secreted in addition to the egg yolk in elasmobranchs to support rapid and healthy embryonic growth.

Bioactive Compounds in Infant Formula and Their Effects on Infant Nutrition and Health: A Systematic Literature Review

Infant formulas are an alternative to replace or supplement human milk when breastfeeding is not possible. The knowledge of human milk's bioactive compounds and their beneficial effects has attracted the interest of researchers in the field of infant nutrition, as well as researchers of technology and food sciences that seek to improve the nutritional characteristics of infant formulas. Several scientific studies evaluate the optimization of infant formula composition. The bioactive compound inclusion has been used to upgrade the quality and nutrition of infant formulas. In this context, the purpose of this systematic literature review is to assess the scientific evidence of bioactive compounds present in infant formulas (α-lactalbumin, lactoferrin, taurine, milk fat globule membrane, folates, polyamines, long-chain polyunsaturated fatty acids, prebiotics, and probiotics) and their effects on infant nutrition and health. Through previously determined criteria, studies published in the last fifteen years from five different databases were included to identify the advances in the optimization of infant formula composition. Over the last few years, there has been optimization of the infant formula composition, not only to increase the similarities in their content of macro and micronutrients but also to include novel bioactive ingredients with potential health benefits for infants. Although the infant food industry has advanced in the last years, there is no consensus on whether novel bioactive ingredients added to infant formulas have the same functional effects as the compounds found in human milk. Thus, further studies about the impact of bioactive compounds in infant nutrition are fundamental to infant health.

Protein levels and protease activity in milk from mothers of pre-term infants: A prospective longitudinal study of human milk macronutrient composition

BACKGROUND AND AIMS

The composition and enzymology of human milk changes throughout the lactation period, and differ for mothers who give birth prematurely compared to those who deliver at full-term. Understanding the composition of milk from mothers of very low birth weight premature infants is of great significance, and the objective of this study was to evaluate the composition, protein profile and plasmin activity of milk from mothers who delivered infants at different gestational ages.

METHODS

Samples of human milk were donated by women (n = 74) in the Cork, Ireland, area who gave birth to full-term (>37 weeks gestation, FT), pre-term (32-37 weeks, PT) and very pre-term (≤32 weeks, VPT) infants. FT milk was collected at 1, 3, 6 and 10 weeks post-partum (PP), while PT and VPT milk was collected weekly until the FT due date of the infant and subsequently followed the FT protocol.

RESULTS

Gestational age did not significantly affect lactose or fat content or total energy content of milk. However, protein content, and levels of some individual proteins, were significantly affected by both gestational age at birth and duration of lactation, with significantly higher protein levels in PT or VPT milk samples at 0-7 days and 1-2 months, respectively. Plasmin activity was significantly higher in VPT milk, indicating differences in proteolytic processing in milk.

CONCLUSION

Compositional differences between the milk of mothers of term and pre-term infants were greatest in terms of the protein profile, which showed both qualitative and quantitative differences, as well as difference in proteolytic activity.

Metabolomic Profile of Personalized Donor Human Milk

Human milk could be considered an active and complex mixture of beneficial bacteria and bioactive compounds. Since pasteurization drastically reduces the microbial content, we recently demonstrated that pasteurized donor human milk (DHM) could be inoculated with different percentages (10% and 30%) of mother's own milk (MOM) to restore the unique live microbiota, resulting in personalized milk (RM10 and RM30, respectively). Pasteurization affects not only the survival of the microbiota but also the concentration of proteins and metabolites, in this study, we performed a comparative metabolomic analysis of the RM10, RM30, MOM and DHM samples to evaluate the impact of microbial restoration on metabolite profiles, where metabolite profiles clustered into four well-defined groups. Comparative analyses of DHM and MOM metabolomes determined that over one thousand features were significantly different. In addition, significant changes in the metabolite concentrations were observed in MOM and RM30 samples after four hours of incubation, while the concentration of metabolites in DHM remained constant, indicating that these changes are related to the microbial expansion. In summary, our analyses indicate that the metabolite profiles of DHM are significantly different from that of MOM, and the profile of MOM may be partially restored in DHM through microbial expansion.

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.

Lysosomal overloading and necrotizing enterocolitis

PURPOSE

Intestinal absorption in premature infants occurs via direct epithelial cellular endocytosis and degradation by intracellular lysosomes. Autophagy is a mechanism by which cytoplasmic organelles contribute to lysosomal degradation. However, excessive autophagy can lead to cell death. The purpose of this study was to investigate whether autophagy and endocytosis are present in the small intestinal mucosa during experimental necrotizing enterocolitis (NEC).

METHODS

NEC was induced by gavage feeding of hyperosmolar formula, lipopolysaccharide and hypoxia between P5 and P9 (ethical approval 44032). Breastfed mice were used as control. Distal ileum was harvested on P5, P7 and P9 and analyzed for intestinal epithelial cellular morphology as well as autophagy/lysosomal activity, and cell death. Groups were compared using Student's t test.

RESULTS

During NEC, giant lysosomes were present in the intestinal villi, with some exceeding their degradation ability leading to their rupture. The NEC group had significantly increased inflammation and autophagy activity, decreased lysosome activity, and increased apoptosis compared to control.

CONCLUSIONS

NEC induction causes excessive autophagy and endocytosis leading to lysosomal overloading, lysosomal membrane permeabilization and rupture which results in cell death. These novel findings may help to clarify the pathogenesis of NEC. Reduction of lysosome overload and assisting in their degradation capability may reduce the burden of NEC.

Macronutrient Digestion and Absorption in the Preterm Infant

The human fetus receives oral nutrition through swallowed amniotic fluid and this makes a significant nutritional contribution to the fetus. Postnatally, macronutrient absorption and digestion appear to function well in the preterm infant. Although pancreatic function is relatively poor, the newborn infant has several mechanisms to overcome this. These include a range of digestive enzymes in human milk, novel digestive enzymes involved in fat and protein digestion that do not appear to be present in the older child or adult, and the presence of a Bifidobacterium-rich colonic microbiome that may "scavenge" unabsorbed macronutrients and make them available to the infant.

Quantitative Analysis of Antibody Survival across the Infant Digestive Tract Using Mass Spectrometry with Parallel Reaction Monitoring

Orally delivered antibodies may be useful for the prevention of enteric pathogen infection, but to be effective they need to survive intact across digestion through the gastrointestinal tract. As a test case, we fed a recombinant human antibody, palivizumab, spiked into human milk to four infants and collected gastric, intestinal and stool samples. We identified a tryptic peptide from palivizumab (LLIYDTSK) that differs from all endogenous human antibodies and used this for quantitation of the intact palivizumab. To account for dilution by digestive fluids, we co-fed a non-digestible, non-absorbable molecule-polyethylene glycol 28-quantified it in each sample and used this value to normalize the observed palivizumab concentration. The palivizumab peptide, a stable isotope-labeled synthetic peptide and polyethylene glycol 28 were quantified via a highly sensitive and selective parallel-reaction monitoring approach using nano-liquid chromatography/Orbitrap mass spectrometry. On average, the survival of intact palivizumab from the feed to the stomach, upper small intestine and stool were 88.4%, 30.0% and 5.2%, respectively. This approach allowed clear determination of the extent to which palivizumab was degraded within the infant digestive tract. This method can be applied with some modifications to study the digestion of any protein.

Proteolytic activity of Lactobacillus strains isolated from Mongolian traditional dairy products: A multiparametric analysis

The aim of our study was to characterize the proteolytic activity of 170 Lactobacillus strains isolated from traditional Mongolian dairy products (yogurt and fermented milk), and to investigate their capacity to generate bioactive peptides during milk fermentation. All isolates were screened for proteolytic activity using skim milk agar-well diffusion test. Fifteen strains (9 Lactobacillus helveticus and 6 Lactobacillus delbrueckii subsp. bulgaricus) were then selected and further evaluated using an original strategy based on multiparametric analysis, taking into account growth rate, acidification capacity, proteolytic activity, cell envelope associated peptidase (CEP) profile and LC-MS/MS analysis of peptides. All parameters were analyzed using principal component analysis (PCA). Results showed that strain growth and acidification correlate with peptide production and that Mongolian L. helveticus strains differ from Western strains in terms of CEP distribution. The PCA revealed that CEP profiles are major determinants of β-casein hydrolysis patterns. Strains with distinctive proteolytic activities were identified.

Proteomic pattern of breast milk discriminates obese mothers with infants of delayed weight gain from normal-weight mothers with infants of normal weight gain

We previously reported that exclusively breastfed infants born to mothers with pregestational obesity gain less weight during the first month after birth than those born to mothers of normal pregestational weight. This issue is potentially important since lower weight gain in breastfed infants of obese mothers might increase the risk of developing later obesity. Breast milk quality and quantity, together with breastfeeding practice, possibly influence infants' feeding behavior, appetite control, and regulation of growth later in life. The issue of whether breast milk protein patterns from obese mothers differ in composition from those of non-obese mothers remains largely unexplored. Here, we established a breast milk proteomic pattern that discriminates obese mothers and infants with delayed weight gain at 1 month after birth from normal-weight mothers with infants of the same age and with normal weight gain. Obese mothers were matched to normal-weight mothers (n = 26; body mass index 33.5 ± 3.2 vs 21.5 ± 1.5 kg·m^-2). The mean weight gain of infants in the obese group at 1 month after birth was 430.8 g lower than that of the infants in the control group. Analysis of the breast milk delipidized fraction by surface-enhanced laser desorption/ionization on CM10 and Q10 arrays was followed by MS-assisted purification and LC-MS/MS microsequencing of a selected biomarker. We identified 15 candidate protein biomarkers, seven of which were overexpressed in the obese group and eight in the normal-weight group. One of the most significant candidate biomarkers, overexpressed in the obese group, was identified as a fragment of the sixth extracellular domain of the polymeric immunoglobulin receptor. Further structural identification of these candidate biomarkers and their validation in clinical assays may facilitate the development of a predictive immunoassay.

Peptidomics Analysis of Milk Protein-Derived Peptides Released over Time in the Preterm Infant Stomach

Over the course of milk digestion, native milk proteases and infant digestive proteases fragment intact proteins into peptides with potential bioactivity. This study investigated the release of peptides over 3 h of gastric digestion in 14 preterm infant sample sets. The peptide content was extracted and analyzed from milk and gastric samples via Orbitrap tandem mass spectrometry. The relative ion intensity (abundance) and count of peptides in each sample were compared over time and between infants fed milk fortified with bovine milk fortifier and infants fed unfortified milk. Bioactivity of the identified peptides was predicted by sequence homology to known bioactive milk peptides. Both total and bioactive peptide abundance and count continuously increased over 3 h of gastric digestion. After accounting for infant weight, length, and postconceptual age, fortification of milk limited the release of peptides from human milk proteins. Peptides that survived further gastric digestion after their initial release were structurally more similar to bioactive peptides than nonsurviving peptides. This work is the first to provide a comprehensive profile of milk peptides released during gastric digestion over time, which is an essential step in determining which peptides are most likely to be biologically relevant in the infant. Data are available via ProteomeXchange with identifier PXD012192.

Impact of human milk pasteurization on the kinetics of peptide release during in vitro dynamic digestion at the preterm newborn stage

Holder pasteurization (62.5 °C, 30 min) of human milk denatures beneficial proteins. The present paper aimed to assess whether this can affect the kinetics of peptide release during digestion at the preterm stage. Raw (RHM) or pasteurized (PHM) human milk were digested in triplicates using an in vitro dynamic system. Mass spectrometry and multivariate statistics were conducted. Pre-proteolysis occurred mostly on β-casein, for which cumulative peptide abundance was significantly greater in PHM over 28% of the hydrolysed sequence. Eight clusters resumed the kinetics of peptide release during digestion, which differed on seven clusters (69% of the 1134 peptides). Clusters associated to the heat-denaturated proteins, lactoferrin and bile salt-stimulated lipase, presented different kinetics of release during digestion, unlike that for β-casein. Some bioactive peptides from β-casein presented significant different abundances between PHM and RHM before digestion (1-18, 185-211) or in during intestinal digestion (154-160, 161-166). Further physiological consequences should be investigated.

Release of functional peptides from mother's milk and fortifier proteins in the premature infant stomach

Digestion of milk proteins in the premature infant stomach releases functional peptides; however, which peptides are present has not been reported. Premature infants are often fed a combination of human milk and bovine milk fortifiers, but the variety of functional peptides released from both human and bovine milk proteins remains uncharacterized. This study applied peptidomics to investigate the peptides released in gastric digestion of mother’s milk proteins and supplemental bovine milk proteins in premature infants. Peptides were assessed for homology against a database of known functional peptides—Milk Bioactive Peptide Database. The peptidomic data were analyzed to interpret which proteases most likely released them from the parent protein. We identified 5,264 unique peptides from bovine and human milk proteins within human milk, fortifier or infant gastric samples. Plasmin was predicted to be the most active protease in milk, while pepsin or cathepsin D were predicted to be most active in the stomach. Alignment of the peptide distribution showed a different digestion pattern between human and bovine proteins. The number of peptides with high homology to known functional peptides (antimicrobial, angiotensin-converting enzyme-inhibitory, antioxidant, immunomodulatory, etc.) increased from milk to the premature infant stomach and was greater from bovine milk proteins than human milk proteins. The differential release of bioactive peptides from human and bovine milk proteins may impact overall health outcomes in premature infants.

Short Communication: Differences in Levels of Free Amino Acids and Total Protein in Human Foremilk and Hindmilk

Free amino acids (FAAs) in human milk are indicated to have specific functional roles in infant development. Studies have shown differences between human milk that is expressed at the beginning of a feed (i.e., foremilk) and the remainder of the milk expressed (i.e., hindmilk). For example, it is well established that human hindmilk is richer in fat and energy than foremilk. Hence, exclusively feeding hindmilk is used to enhance weight gain of preterm, low birthweight infants. Whether FAAs occur differently between foremilk and hindmilk has never been reported, but given their bioactive capacities, this is relevant to consider especially in situations where hindmilk is fed exclusively. Therefore, this study analyzed and compared the FAA and total protein content in human foremilk and hindmilk samples donated by 30 healthy lactating women. The total protein content was found to be significantly higher in hindmilk (p < 0.001), whereas foremilk contained a significantly higher total content of FAAs (p = 0.015). With regards to individual FAAs, foremilk contained significantly higher levels of phenylalanine (p = 0.009), threonine (p = 0.003), valine (p = 0.018), alanine (p = 0.004), glutamine (p < 0.001), and serine (p = 0.012) than hindmilk. Although statistical significance was reached, effect size analysis of the milk fraction on FAA levels in milk revealed that the observed differences were only small. To what extent these differences are of physiological importance for infant development remains to be examined in future research.

Human milk peptides differentiate between the preterm and term infant and across varying lactational stages

Variations in endogenous peptide profiles, functionality, and the enzymes responsible for the formation of these peptides in human milk are understudied. Additionally, there is a lack of knowledge regarding peptides in donor human milk, which is used to feed preterm infants when mother's own milk is not (sufficiently) available. To assess this, 29 human milk samples from the Dutch Human Milk Bank were analyzed as three groups, preterm late lactation stage (LS) (n = 12), term early (n = 8) and term late LS (n = 9). Gestational age (GA) groups were defined as preterm (24-36 weeks) and term (≥37 weeks). LS was determined as days postpartum as early (16-36 days) or late (55-88 days). Peptides, analyzed by LC-MS/MS, and parent proteins (proteins from matched peptide sequences) were identified and quantified, after which peptide functionality and the enzymes responsible for protein cleavage were determined. A total of 16 different parent proteins were identified from human milk, with no differences by GA or LS. We identified 1104 endogenous peptides, of which, the majority were from the parent proteins β-casein, polymeric immunoglobulin receptor, α_s1-casein, osteopontin, and κ-casein. The absolute number of peptides differed by GA and LS with 30 and 41 differing sequences respectively (p < 0.05) Odds likelihood tests determined that 32 peptides had a predicted bioactive functionality, with no significant differences between groups. Enzyme prediction analysis showed that plasmin/trypsin enzymes most likely cleaved the identified human milk peptides. These results explain some of the variation in endogenous peptides in human milk, leading to future targets that may be studied for functionality.

Changes in Proteases, Antiproteases, and Bioactive Proteins From Mother's Breast Milk to the Premature Infant Stomach

OBJECTIVE

Our previous studies suggested that human milk proteases begin to hydrolyze proteins in the mammary gland and continue within the term infant' stomach. No research has measured milk protease and pepsin activity in the gastric aspirates of preterm infants after human milk feeding. This study investigated how the concentrations of human milk proteases and protease inhibitors changed in the premature infant stomach.

METHODS

Human milk and infant gastric samples were collected from 18 preterm-delivering mother-infant pairs (24-32 week gestational age). Paired human milk and gastric samples were collected across postnatal age (2-47 days). Protease concentrations were determined by spectrophotometric or fluorometric assays, and the concentrations of protease inhibitors and bioactive proteins were determined by enzyme-linked immunosorbent assay. Paired t tests were applied to compare enzymes, antiproteases, and bioactive proteins between human milk and gastric samples.

RESULTS

Our study reveals that although human milk proteases, including carboxypeptidase B2, kallikrein, plasmin, cathepsin D, elastase, thrombin, and cytosol aminopeptidase, are present in the preterm infant stomach, only plasmin and cathepsin D can actively hydrolyze proteins at gastric pH. Enzyme-linked immunosorbent assay and peptidomic evidence suggest that all milk antiproteases as well as lactoferrin and immunoglobulin A are partially digested in the preterm stomach.

CONCLUSIONS

Most human milk proteases are active in milk but not at preterm infant gastric pH. Only cathepsin D and plasmin have potential to continue degrading milk proteins within the preterm infant stomach.

Investigation into the antimicrobial action and mechanism of a novel endogenous peptide β-casein 197 from human milk

A novel endogenous peptide cleaved from 197–213 AA of β-casein, named β-casein 197, was identified by tandem mass spectrometry. β-casein 197 constituted a significant proportion of the peptide content in preterm milk. This study investigated the antibacterial effects and mechanisms against common pathogenic bacteria. Six bacterial strains were selected for this study: Escherichia coli, Staphylococcus aureus, Yersinia enterocolitica, Listeria monocytogenes, Klebsiella pneumonia and Bacillus subtilis. After synthesis, serial twofold dilutions of β-casein 197 were added to select for sensitive bacteria. The disk diffusion method and analysis of bacterial staining were used to identify antibacterial effect, while DNA-binding, scanning electron microscopy and transmission electron microscopy were used to explore antimicrobial mechanisms. Disk diffusion showed that E. coli, S. aureus and Y. enterocolitica were sensitive to the β-casein 197. In addition, live/dead fluorescent staining also confirmed antibacterial effects. Scanning electron and transmission electron microscopy revealed affected extracellular and intracellular structure for three species of bacteria, while a DNA-binding assay showed that the antimicrobial activity did not occur through DNA binding. This study suggests that β-casein 197 has antimicrobial activity against common pathogenic bacteria in newborns with infection. The peptide induced membrane permeabilization but did not bind to genomic DNA. Based on our findings, β-casein 197 has potential clinical value for preventing infections of premature infants.

Milk Proteins Are Predigested Within the Human Mammary Gland

Previous work demonstrates that proteases present in human milk release hundreds of peptides derived from milk proteins. However, the question of whether human milk protein digestion begins within the mammary gland remains incompletely answered. The primary objective of this study was to determine whether proteolytic degradation of human milk proteins into peptides begins within the mammary gland. The secondary objectives were to determine which milk proteases participate in the proteolysis and to predict which released peptides have bioactivity. Lactating mothers (n = 4) expressed their milk directly into a mixture of antiproteases on ice followed by immediate freezing of the milk to limit post-expression protease activity. Samples were analyzed for their peptide profiles via mass spectrometry and database searching. Peptidomics-based protease prediction and bioactivity prediction were each performed with several different approaches. The findings demonstrate that human milk contains more than 1,100 unique peptides derived from milk protein hydrolysis within the mammary gland. These peptides derived from 42 milk proteins and included 306 potential bioactive peptides. Based on the peptidomics data, plasmin was predicted to be the milk protease most active in the hydrolysis of human milk proteins within the mammary gland. Milk proteases actively cleave milk proteins within the mammary gland, initiating the release of functional peptides. Thus, the directly breastfed infant receives partially pre-digested proteins and numerous bioactive peptides.

Analysis of Milk from Mothers Who Delivered Prematurely Reveals Few Changes in Proteases and Protease Inhibitors across Gestational Age at Birth and Infant Postnatal Age

Background: Peptidomics research has demonstrated that protease activity is higher in breast milk from preterm-delivering mothers than from term-delivering mothers. However, to our knowledge, the effect of the degree of prematurity and postnatal age on proteases and protease inhibitors in human milk remains unknown.Objective: We aimed to determine the change of proteases and protease inhibitors in milk from mothers who delivered prematurely across gestational age (GA) and postnatal age.Methods: Milk samples were collected from 18 mothers aged 26-40 y who delivered preterm infants and who lacked mastitis. For analysis, samples were separated into 2 groups: 9 from early GA (EGA) (24-26 wk GA)-delivering mothers and 9 from late GA (LGA) (27-32 wk GA)-delivering mothers. Within the 9 samples in each group, the collection time ranged from postnatal days 2 to 47. The activity and predicted activity of proteases in preterm milk were determined with the use of fluorometric and spectrophotometric assays and peptidomics, respectively. Protease and protease inhibitor concentrations were determined with the use of ELISA. Linear mixed models were applied to compare enzymes across GA and postnatal age.Results: Carboxypeptidase B2, kallikrein, plasmin, elastase, thrombin, and cytosol aminopeptidase were present and active in the milk of preterm-delivering mothers. Most milk protease and antiprotease concentrations did not change with GA or postnatal age. However, the concentration and activity of kallikrein, the most abundant and active protease in preterm milk, increased by 25.4 ng · mL-1 · d-1 and 0.454 μg · mL-1 · d-1 postnatally, respectively, in EGA milk samples while remaining stable in LGA milk samples.Conclusions: This research demonstrates that proteases are active in human milk and begin to degrade milk protein within the mammary gland before consumption by infants. Proteases and protease inhibitors in milk from mothers of premature infants mostly did not vary substantially across GA and postnatal age.

High-temperature short-time pasteurisation of human breastmilk is efficient in retaining protein and reducing the bacterial count

AIM

Milk banks are advised to use Holder pasteurisation to inactivate the cytomegalovirus, but the process adversely affects the bioactive properties of human breastmilk. This study explored the antibacterial efficacy of an alternative high-temperature short-time (HTST) treatment of human breastmilk and its effect on marker proteins, compared with the Holder method.

METHODS

Breastmilk samples were obtained from 27 mothers with infants in a German neonatal intensive care unit. The samples were either heated to 62°C for five seconds using HTST or processed using Holder pasteurisation, at 63 ± 0.5°C for 30 minutes. Immunoglobulin A, lactoferrin, lysozyme, alkaline phosphatase and bile salt-stimulated lipase concentrations and bacterial colony-forming units/mL were measured before and after heating.

RESULTS

HTST-treated samples retained higher rates of immunoglobulin A (95% versus 83%), alkaline phosphatase (6% versus 0%) and bile salt-stimulated lipase (0.8% versus 0.4%) than Holder pasteurisation samples (all p < 0.01), but not lactoferrin (32% versus 20%, p = 0.18) and lysozyme (72% versus 65%, p = 1). No difference in antibacterial efficacy was noted between the two groups (p = 0.29).

CONCLUSION

Using the HTST treatment protocol retained some of the bioactive properties of human breastmilk and appeared to have similar antibacterial efficacy to Holder pasteurisation.

Enzymes in Human Milk

Milk proteins are a complex and diverse source of biological activities. Beyond their function, intact milk proteins also act as carriers of encrypted functional sequences that, when released as peptides, exert biological functions, including antimicrobial and immunomodulatory activity, which could contribute to the infant's competitive success. Research has now revealed that the release of these functional peptides begins within the mammary gland itself. A complex array of proteases produced in mother's milk has been shown to be active in the milk, releasing these peptides. Moreover, our recent research demonstrates that these milk proteases continue to digest milk proteins within the infant's stomach, possibly even to a larger extent than the infant's own proteases. As the neonate has relatively low digestive capacity, the activity of milk proteases in the infant may provide important assistance to digesting milk proteins. The coordinated release of these encrypted sequences is accomplished by selective proteolytic action provided by an array of native milk proteases and infant-produced enzymes. The task for scientists is now to discover the selective advantages of this protein-protease-based peptide release system.

An interactomics overview of the human and bovine milk proteome over lactation

Background

Milk is the most important food for growth and development of the neonate, because of its nutrient composition and presence of many bioactive proteins. Differences between human and bovine milk in low abundant proteins have not been extensively studied. To better understand the differences between human and bovine milk, the qualitative and quantitative differences in the milk proteome as well as their changes over lactation were compared using both label-free and labelled proteomics techniques. These datasets were analysed and compared, to better understand the role of milk proteins in development of the newborn.

Methods

Human and bovine milk samples were prepared by using filter-aided sample preparation (FASP) combined with dimethyl labelling and analysed by nano LC LTQ-Orbitrap XL mass spectrometry.

Results

The human and bovine milk proteome show similarities with regard to the distribution over biological functions, especially the dominant presence of enzymes, transport and immune-related proteins. At a quantitative level, the human and bovine milk proteome differed not only between species but also over lactation within species. Dominant enzymes that differed between species were those assisting in nutrient digestion, with bile salt-activated lipase being abundant in human milk and pancreatic ribonuclease being abundant in bovine milk. As lactation advances, immune-related proteins decreased slower in human milk compared to bovine milk. Notwithstanding these quantitative differences, analysis of human and bovine co-expression networks and protein-protein interaction networks indicated that a subset of milk proteins displayed highly similar interactions in each of the different networks, which may be related to the general importance of milk in nutrition and healthy development of the newborn.

Conclusions

Our findings promote a better understanding of the differences and similarities in dynamics of human and bovine milk proteins, thereby also providing guidance for further improvement of infant formula.

Electronic supplementary material

The online version of this article (doi:10.1186/s12953-016-0110-0) contains supplementary material, which is available to authorized users.

Antibody-independent identification of bovine milk-derived peptides in breast-milk

Exclusively breast-fed infants can exhibit clear signs of IgE or non IgE-mediated cow's milk allergy. However, the definite characterization of dietary cow's milk proteins (CMP) that survive the maternal digestive tract to be absorbed into the bloodstream and secreted into breast milk remains missing. Herein, we aimed at assessing possible CMP-derived peptides in breast milk. Using high performance liquid chromatography (HPLC)-high resolution mass spectrometry (MS), we compared the peptide fraction of breast milk from 12 donors, among which 6 drank a cup of milk daily and 6 were on a strict dairy-free diet. We identified two bovine β-lactoglobulin (β-Lg, 2 out 6 samples) and one αs1-casein (1 out 6 samples) fragments in breast milk from mothers receiving a cup of bovine milk daily. These CMP-derived fragments, namely β-Lg (f42-54), (f42-57) and αs1-casein (f180-197), were absent in milk from mothers on dairy-free diet. In contrast, neither intact nor hydrolyzed β-Lg was detected by western blot and competitive ELISA in any breast milk sample. Eight additional bovine milk-derived peptides identified by software-assisted MS were most likely false positive. The results of this study demonstrate that CMP-derived peptides rather than intact CMP may sensitize or elicit allergic responses in the neonate through mother's milk. Immunologically active peptides from the maternal diet could be involved in priming the newborn's immune system, driving a tolerogenic response.

Peptidomic analysis reveals proteolytic activity of kefir microorganisms on bovine milk proteins

SCOPE

The microorganisms that make up kefir grains are well known for lactose fermentation, but the extent to which they hydrolyze and consume milk proteins remains poorly understood. Peptidomics technologies were used to examine the proteolytic activity of kefir grains on bovine milk proteins.

METHODS AND RESULTS

Gel electrophoresis revealed substantial digestion of milk proteins by kefir grains, with mass spectrometric analysis showing the release of 609 protein fragments and alteration of the abundance of >1500 peptides that derived from 27 milk proteins. Kefir contained 25 peptides identified from the literature as having biological activity, including those with antihypertensive, antimicrobial, immunomodulatory, opioid and anti-oxidative functions. 16S rRNA and shotgun metagenomic sequencing identified the principle taxa in the culture as Lactobacillus species.

CONCLUSION

The model kefir sample contained thousands of protein fragments released in part by kefir microorganisms and in part by native milk proteases.

A colostrum trypsin inhibitor gene expressed in the Cape fur seal mammary gland during lactation

The colostrum trypsin inhibitor (CTI) gene and transcript were cloned from the Cape fur seal mammary gland and CTI identified by in silico analysis of the Pacific walrus and polar bear genomes (Order Carnivora), and in marine and terrestrial mammals of the Orders Cetartiodactyla (yak, whales, camel) and Perissodactyla (white rhinoceros). Unexpectedly, Weddell seal CTI was predicted to be a pseudogene. Cape fur seal CTI was expressed in the mammary gland of a pregnant multiparous seal, but not in a seal in its first pregnancy. While bovine CTI is expressed for 24-48 h postpartum (pp) and secreted in colostrum only, Cape fur seal CTI was detected for at least 2-3 months pp while the mother was suckling its young on-shore. Furthermore, CTI was expressed in the mammary gland of only one of the lactating seals that was foraging at-sea. The expression of β-casein (CSN2) and β-lactoglobulin II (LGB2), but not CTI in the second lactating seal foraging at-sea suggested that CTI may be intermittently expressed during lactation. Cape fur seal and walrus CTI encode putative small, secreted, N-glycosylated proteins with a single Kunitz/bovine pancreatic trypsin inhibitor (BPTI) domain indicative of serine protease inhibition. Mature Cape fur seal CTI shares 92% sequence identity with Pacific walrus CTI, but only 35% identity with BPTI. Structural homology modelling of Cape fur seal CTI and Pacific walrus trypsin based on the model of the second Kunitz domain of human tissue factor pathway inhibitor (TFPI) and porcine trypsin (Protein Data Bank: 1TFX) confirmed that CTI inhibits trypsin in a canonical fashion. Therefore, pinniped CTI may be critical for preventing the proteolytic degradation of immunoglobulins that are passively transferred from mother to young via colostrum and milk.

Proteolytic Systems in Milk: Perspectives on the Evolutionary Function within the Mammary Gland and the Infant

Milk contains elements of numerous proteolytic systems (zymogens, active proteases, protease inhibitors and protease activators) produced in part from blood, in part by mammary epithelial cells and in part by immune cell secretion. Researchers have examined milk proteases for decades, as they can cause major defects in milk quality and cheese production. Most previous research has examined these proteases with the aim to eliminate or control their actions. However, our recent peptidomics research demonstrates that these milk proteases produce specific peptides in healthy milk and continue to function within the infant's gastrointestinal tract. These findings suggest that milk proteases have an evolutionary function in aiding the infant's digestion or releasing functional peptides. In other words, the mother provides the infant with not only dietary proteins but also the means to digest them. However, proteolysis in the milk is controlled by a balance of protease inhibitors and protease activators so that only a small portion of milk proteins are digested within the mammary gland. This regulation presents a question: If proteolysis is beneficial to the infant, what benefits are gained by preventing complete proteolysis through the presence of protease inhibitors? In addition to summarizing what is known about milk proteolytic systems, we explore possible evolutionary explanations for this proteolytic balance.

Endogenous human milk peptide release is greater after preterm birth than term birth

BACKGROUND

Hundreds of naturally occurring milk peptides are present in term human milk. Preterm milk is produced before complete maturation of the mammary gland, which could change milk synthesis and secretion processes within the mammary gland, leading to differences in protein expression and enzymatic activity, thereby resulting in an altered peptide profile.

OBJECTIVE

This study examined differences in peptides present between milk from women delivering at term and women delivering prematurely.

METHODS

Nano-LC tandem mass spectrometry was employed to identify naturally occurring peptides and compare their abundances between term and preterm human milk samples at multiple time points over lactation. Term milk samples were collected from 8 mothers and preterm milk was collected from 14 mothers. The 28 preterm and 32 term human milk samples were divided into 4 groups based on day of collection (<14, 14-28, 29-41, and 42-58 d).

RESULTS

Preterm milk peptide counts, ion abundance, and concentration were significantly higher in preterm milk than term milk. Bioinformatic analysis of the cleavage sites for peptides identified suggested that plasmin was more active in preterm milk than term milk and that cytosol aminopeptidase and carboxypeptidase B2 likely contribute to extensive milk protein breakdown. Many identified milk peptides in both term and preterm milk overlapped with known functional peptides, including antihypertensive, antimicrobial, and immunomodulatory peptides.

CONCLUSION

The high protein degradation by endogenous proteases in preterm milk might attenuate problems because of the preterm infant's immature digestive system. This trial was registered at clinicaltrials.gov as NCT01817127.

Effects of bisphenol S on the structures and activities of trypsin and pepsin

The effects of bisphenol S on the structures and activities of trypsin and pepsin were investigated by various methods like UV-visible absorbance, fluorescence, circular dichroism, and molecular docking. The secondary and tertiary structures of trypsin and pepsin were altered by bisphenol S binding, which resulted in the loosening of the skeletons of trypsin and pepsin. In addition, bisphenol S induced microenvironmental changes around tyrosine and tryptophan residues of trypsin and pepsin. The activity experimental results showed that the activity of pepsin decreases obviously with the increasing concentration of BPS, while the activity of trypsin does not change remarkably. The binding and thermodynamic parameters obtained by molecular docking and fluorescence spectroscopy showed that the bindings of bisphenol S to trypsin and pepsin were spontaneous processes and hydrogen bonding and hydrophobic interactions played a vital role in stabilizing the bisphenol S-trypsin and bisphenol S-pepsin complexes. The binding constants (K(A)) of bisphenol S with trypsin were 7.42 × 10(4) (298 K) and 5.91 × 10(4) L/mol (310 K), and those of pepsin were 5.78 × 10(4) (298 K) and 4.44 × 10(4) L/mol (310 K). Moreover, there was one main kind of binding site for bisphenol S on trypsin or pepsin.

Following the digestion of milk proteins from mother to baby

Little is known about the digestive process in infants. In particular, the chronological activity of enzymes across the course of digestion in the infant remains largely unknown. To create a temporal picture of how milk proteins are digested, enzyme activity was compared between intact human milk samples from three mothers and the gastric samples from each of their 4-12 day postpartum infants, 2 h after breast milk ingestion. The activities of 7 distinct enzymes are predicted in the infant stomach based on their observed cleavage pattern in peptidomics data. We found that the same patterns of cleavage were evident in both intact human milk and gastric milk samples, demonstrating that the enzyme activities that begin in milk persist in the infant stomach. However, the extent of enzyme activity is found to vary greatly between the intact milk and gastric samples. Overall, we observe that milk-specific proteins are cleaved at higher levels in the stomach compared to human milk. Notably, the enzymes we predict here only explain 78% of the cleavages uniquely observed in the gastric samples, highlighting that further investigation of the specific enzyme activities associated with digestion in infants is warranted.