Proteolysis and partial dephosphorylation of casein are affected by high somatic cell counts in sheep milk

- Milk SCC of ewes affected by subclinical mastitis impacts casein integrity.
- Partially dephosphorylated casein/peptides are formed by action of ALP.
- The main endogenous proteinase responsible for casein proteolysis is PL.
- Higher SCC, higher the ALP and PL activity.
- Partially dephosphorylated peptides can represent markers of the milk SC level.

Mastitis, an inflammatory affection of the mammary gland, causes an increase of milk somatic cells. The individual milk composition changes while that of bulk milk is remarkably constant, particularly in the lactose content. Milk samples were taken from 240 udder halves, with low (< 500 × 103/ml), medium (500-1000 × 103/ml) and high (1000-2000 × 103/ml) somatic cell count (SCC) of a flock of 120 primiparous Sarda ewes. A proteomic study has been conducted on milk from healthy udder halves (< 500 × 103/ml) as control group and infected udder halves (SCC > 1000 × 103/ml). In milk with high SCC, the majority of the peptides measured by ESI-Q-TOF MS arose from the four casein fractions by the decreasing order of affinity for plasmin (PL): i.e. β-casein (CN) > αs2-CN > αs1-CN ≫ κ-CN. The focus of the present work was on phosphopeptides (CPP) released by milk enzymes. In infected milk, αs2-CN and β-CN derived CPP were enriched on hydroxyapatite and seventeen CPP and twenty partially dephosphorylated CPP were identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. Although MALDI-TOF analysis faces quantitative limitations in the detection of highly phosphorylated peptides, it was, however, useful for identifying partially or fully dephosphorylated peptides. These results are well correlated with the alkaline phosphatase (ALP) activity in milk. Therefore, in addition to ALP, PL activity is indicated as valid predictor of endogenous proteolysis in infected sheep's milk.