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Phosphorylation is a widespread posttranslational protein modification and is important in various biological processes. However, milk fat globule membrane (MFGM) phosphoproteins have not been explored systematically in human milk. Here, we used quantitative phosphoproteomics to analyze phosphorylation sites in human MFGM proteins and their differences at different stages of lactation; 305 phosphorylation sites on 170 proteins and 269 phosphorylation sites on 170 proteins were identified in colostrum and mature MFGM, respectively. Among these, 71 phosphorylation sites on 48 proteins were differentially expressed between the different stages of lactation. Osteopontin in human MFGM was the most heavily phosphorylated protein, with a total of 39 identified phosphorylation sites. Our results shed light on phosphorylation sites, composition, and biological functions of MFGM phosphoproteins in human colostrum and mature milk, and provide novel insights into the crucial roles of protein phosphorylation during infant development.

Cow milk (CM) allergy is one of the most common food allergies worldwide; the most abundant CM proteins, such as casein (CN), β‐lactoglobulin (β‐LG), and ɑ‐lactalbumin (ɑ‐LA), are all potentially allergenic. Reducing the antigenicity of CM continues to be a major challenge. However, previous studies have focused on the antigenicity of individual allergic CM proteins. Thus, in the present study, we aimed to evaluate the effects of different food‐grade enzymes on the antigenicity of CN, β‐LG, ɑ‐LA in natural CM. The degree of hydrolysis (DH) and molecular mass (MW) distribution of CM hydrolysates were assessed. Additionally, the residual antigenicity of CM hydrolysates was evaluated through enzyme‐linked immunosorbent assay and Western blotting with anti‐CN, anti‐β‐LG, and anti‐ɑ‐LA rabbit polyclonal antibodies. The results showed that Alcalase‐ and Protamex‐mediated hydrolysis could efficiently reduce the antigenicity of CN, β‐LG, and ɑ‐LA, inducing a higher DH, the loss of density of CM proteins, and the increasing levels of low MW (<3 kDa) peptides in CM hydrolysates. Further, Protamex and Alcalase could more efficiently hydrolyze the major allergenic components of CM than the other enzymes, which could represent an advantage for the development of hypoallergenic CM. These findings add further knowledge about the study and development of hypoallergenic CM.

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