Alterations in Whey Protein Abundance Correlated with the Somatic Cell Count Identified via Label-Free and Selected Reaction Monitoring Proteomic Approaches

Abstract

The somatic cell count (SCC) is widely used to assess milk quality and diagnose intramammary infections. Several whey proteins have been shown to correlate significantly with SCC and are considered potential indicators of udder health. However, the relationship between milk whey proteins and SCC has not been fully elucidated. In this study, milk samples were grouped into five categories based on SCC levels. Subsequently, whey proteins were identified using a label-free proteomics approach, and the differential abundance of proteins was validated through a selected reaction monitoring (SRM) method. The levels of various proteins, including azurocidin 1 and kininogen-2, exhibited an increase, whereas topoisomerase I, tropomyosin-1, and desmin showed a significant decrease depending on the SCCs. Principal component analysis unveiled that these proteins contributed to the developmental alterations in milk proteins. A majority of these differentially abundant proteins were associated with response to stimulus, localization, and defense response. Our results provide fundamental information on the SCC that can be utilized for evaluating milk quality and serve as potential indicators for detecting intramammary infections.

Keywords: milk; proteomics; somatic cell count; whey protein.

Figure 1

Biological processes (a) and cellular components (b) of differentially abundant proteins of milk whey among the different somatic cell count milk groups predicted using the DAVID functional annotation tool.

Figure 2

The constructed protein–protein interaction network of differentially abundant proteins in milk whey among the different somatic cell count milk groups. A1BG—Alpha-1-B Glycoprotein; ALDOA—Fructose-Bisphosphate Aldolase; ALB—Albumin; APOH—Apolipoprotein H; A2M—Alpha-2-Macroglobulin; B2M—Beta-2-Microglobulin; BTN1A1—Butyrophilin Subfamily 1 Member A1; C3—Complement Component 3; C4A—Complement Component 4A; CD14—Monocyte Differentiation Antigen CD14; CD36—Platelet Glycoprotein 4; CDH1—Cadherin 1; CFB—Complement Factor B; CFH—Complement Factor H; CLEC3B—Tetranectin; CLU—Clusterin; CP—Ceruloplasmin; CRISP3—Cysteine Rich Secretory Protein 3; CSN1S1—Alpha-S1-Casein; CSN3—Kappa Casein; DES—Desmin; FGA—Fibrinogen Alpha Chain; FGB—Fibrinogen Beta Chain; FGG—Fibrinogen Gamma Chain; GC—Vitamin D Binding Protein; GLYCAM1—Glycosylation Dependent Cell Adhesion Molecule 1; HNRNPA1—Heterogeneous Nuclear Ribonucleoprotein A1; HNRNPF—Heterogeneous Nuclear Ribonucleoprotein F; HNRNPK—Heterogeneous Nuclear Ribonucleoprotein K; HPX—Hemopexin; HSPA5—Endoplasmic Reticulum Chaperone BiP; HSPA8—Heat Shock Cognate 71 kDa Protein; KNG1—Kininogen 1; LALBA—Alpha Lactalbumin; LBP—Lipopolysaccharide Binding Protein; LCN2—Lipocalin 2; LPL—Lipoprotein Lipase; LPO—Lactoperoxidase; OS9—Protein OS-9; PAEP—Beta-Lactoglobulin; PCBP2—Poly(rC) Binding Protein 2; PDIA3—Protein Disulfide-Isomerase A3; PDIA6—Protein Disulfide-Isomerase A6; PGLYRP1—Peptidoglycan Recognition Protein 1; PLG—Plasminogen; PLIN2—Perilipin 2; PPIB—Peptidylprolyl Isomerase B; RBP4—Retinol Binding Protein 4; SERPINA1—Alpha-1-Antiproteinase; SERPINC1—Antithrombin-III; TF—Transferrin; TKT—Transketolase; TPM1—Tropomyosin 1; VIM—Vimentin; XDH—Xanthine Dehydrogenase/Oxidase.

Figure 3

Hierarchical clustering (a), and PCA score plot (b) of differential proteins among the somatic cell count groups. S1 to S5 groups represent somatic cell counts of 6–9, 17–20, 38–42, 68–80, and 176–243 × 10⁴ cells/mL, respectively.