Milk lysozyme

Exclusive human milk feeding of the newborn is recommended during the first 6 months of life to promote optimal health outcomes during early life and beyond. Human milk contains a variety of bioactive factors such as hormones, cytokines, leukocytes, immunoglobulins, lactoferrin, lysozyme, stem cells, human milk oligosaccharides (HMOs), microbiota, and microRNAs. Recent findings highlighted the potential importance of adding HMOs into infant formula for their roles in enhancing host defense mechanisms in neonates. Therefore, understanding the roles of human milk bioactive factors on immune function is critical to build the scientific evidence base around breastfeeding recommendations, and to enhance positive health outcomes in formula fed infants through modifications to formulas. However, there are still knowledge gaps concerning the roles of different milk components, the interactions between the different components, and the mechanisms behind health outcomes are poorly understood. This review aims to show the current knowledge about HMOs, milk microbiota, immunoglobulins, lactoferrin, and milk microRNAs (miRNAs) and how these could have similar mechanisms of regulating gut and microbiota function. It will also highlight the knowledge gaps for future research.

Human milk (HM) contains hundreds of proteins with very diverse functions that likely contribute to the short- and long-term beneficial effects of breastfeeding. These functions include serving as a source of amino acids, improving the bioavailability of micronutrients, including vitamins, minerals, and trace elements, providing immunologic defense, stimulating intestinal growth and maturation, shaping the microbiome, and enhancing learning and memory. Human milk proteins can be broadly classified into 3 categories: caseins, whey proteins, and mucins, which are present in the milk fat globule membrane. HM is whey predominant; however, the whey/casein ratio of HM changes from 90/10 in colostrum to 60/40 in mature HM. The whey proteins present in significant quantities in the whey fraction are α-lactalbumin, lactoferrin, IgA, osteopontin, and lysozyme. Additionally, bioactive peptides are formed during digestion of casein and whey, and glycans from glycoproteins are bifidogenic, adding further complexity to the functional properties of HM proteins. Recent advances in dairy technology have enabled isolation of bioactive milk proteins from bovine milk in sufficient quantities for clinical studies and, in some cases, addition to commercially available infant formula. Herein, the current evidence on HM protein composition and bioactivity of HM proteins is reviewed.

Using reversed phase high-performance liquid chromatography with ultraviolet (UV) detection and electrospray ionization (ESI)-quadrupole time-of-flight mass spectrometry (RP-HPLC-UV-ESI-Q-TOF), the lysozyme content in the milk of 10 volunteering mothers was quantified, ranging from 29 to 96 μg/mL. Following ultracentifugation, it was found that the lysozyme in human milk, unlike other whey proteins, is mainly bound to casein micelles (ca. 75%). The enzymatic activity of human lysozyme, measured as lytic activity against cell walls of Micrococcus lysodeikticus, was similar for the micelle-bound and free protein, indicating that the micellar structure should not affect the antibacterial activity of lysozyme. The results indicate that lysozyme is an integral component of casein micelles in human milk.