Concentrations and Anti-Haemophilus influenzae Activities of β-Casein Phosphoforms in Human Milk

Abstract

The beta-casein level in untreated human milk is significantly higher than previously reported. The phosphoform distribution of beta-casein in individual donors varies widely. Anti-H. influenzae activity was detected in vitro among human beta-casein molecules with three or more phosphate groups.

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

“…Overall, site Ser24 showed the highest number of PSMs (e.g., the mean phosphosite PSMs across all time points being 197 and 663 for respectively the proteome and peptidome for donor one), followed by Ser25, Ser23, Ser21, and Thr18 regardless of the donor, lactational stage, proteome or peptidome data ( Figure 4 , Supplemental Figure S4 and Supplemental Table S3 ). This overall observed trend in phosphosite occupancy is in line with literature data [ 48 , 51 ]. The identified phosphosites follow the Ser/Thr-Xxx-Glu/pSer sequence motif recognized by the FAM20C kinase, which has been identified as the Golgi casein kinase responsible for this phosphorylation [ 52 , 53 ].…”

“…The degree of phosphorylation on β-casein not only regulates the size of the casein micelle, it also influences the function of the protein [ 31 , 51 , 59 ]. This, in turn, affects the overall digestibility, release, and absorption of functional proteins and peptides in the infant intestinal tract.…”

“…The phosphosites of human milk caseins have been extensively studied [ 31 , 48 , 49 , 50 ] and even the site occupancy over the first month of lactation has been well-characterized in terms of the intact protein [ 48 ], but work on the peptidome has lagged behind. Previous studies have shown that the preferred phosphorylation order is Ser24 > Ser25 > Ser23 > Ser21 > Thr18, with a preference for four sites being occupied at any given time throughout lactation [ 48 , 51 ]. Our data are in line with these reports regarding the preferential order of phosphosites, but our data suggests that occupancy of more than three phosphosites predominantly occurs on the endogenous peptides and not the intact protein.…”

“…The degree of phosphorylation of β-casein is known to directly affect its function. For instance, different phosphoproteoforms are better at inhibiting H. influenzae than others [ 51 ]. This study found that the most common phosphoproteoforms of β-casein, by concentration, were tetra- > di- > non- > mono- > tri- > penta-phosphorylated.…”

Casein proteolysis in human milk: tracing the pattern of casein breakdown and the formation of potential bioactive peptides

“…ii) The existence of 17 . 5 % of the protein fraction still unidentified in human milk (Kunz & Lonnerdal, 1989a ;Kroening et al 1999). This is composed of casein material but contained in the whey fraction, and this behaviour could be actually due to the low size of the peptides, corresponding to the casein fragments we have identified.…”