Natural Association of Lysozyme and Casein Micelles in Human Milk

Abstract

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.

Comparative proteomics of human milk casein fraction collected from women of Korean and Han ethnic groups in China

“…As shown in Supplementary Table 1 , lysozyme (P61626), the main whey protein, was also detected in the casein fraction. As indicated in a recent study, approximately 75% of lysozyme in human milk is naturally bound to casein, and this association does not affect the antibacterial activity of lysozyme ( 7 ).…”

“…Casein content in milk depends on the lactation period—it is approximately 20% in the early stage and 45% in the late phase ( 6 ). Caseins in milk mainly exist in the form of micelles with an average size of 40–100 nm ( 7 ). Caseins have been shown to have multiple biological functions in newborn babies, especially in transporting calcium phosphate from the mother to the infants.…”

Lactation in the human

“…The most abundant soluble or whey proteins in human milk are alpha-lactalbumin (~3.3 g/L), lactoferrin (~2.0 g/L), sIgA (~1.0 g/L), lysozyme (29 to 96 µg/ml; Jaeser et al, 2022 ), bile salt stimulated lipase, and serum albumin (0.4 g/L). There are many others most of whose functions in milk are not currently clear ( Smilowitz et al, 2023 ).…”

“…β-casein is highly phosphorylated and about half the molecular weight of human casein is due to its heavy glycosylation ~50% vs. 10% in the bovine ( Lonnerdal and Atkinson, 1995 ). The casein micelle binds about 75% of the lysozyme in human milk ( Jaeser et al, 2022 ); however, the activity of the bound lysozyme was not different from that of the free lysozyme.…”