Glycoproteins in the heat-and acid-stable fraction of ovine milk

Protein signatures of spontaneous lipolysis and lipoprotein lipase activity in cow's milk

Spontaneous milk lipolysis refers to the breakdown of triacylglycerols in milk. Lipolysis impacts the organoleptic value of milk by causing off-flavours and reduces the technological properties of milk. Lipolysis is caused by lipoprotein lipase (LPL), a tightly regulated enzyme in milk. Our objective was to identify robust biomarkers of lipolysis and putative regulators of LPL enzyme in bovine milk. To achieve this goal, we used feed restriction as a lever to generate highly contrasted samples with regard to milk lipolysis. We combined statistical methods on proteomics data, milk lipolysis and LPL activity values. Following this strategy, we identified CD5L and GP2 as robust biomarkers of high lipolysis in cow milk. We also identified HID1, SURF4 and CUL9 as putative inhibitors of the lipolytic process in the milk. We thus proposed 5 putative biomarkers to be considered in future tools to manage milk lipolysis. SIGNIFICANCE: This manuscript is notable in three aspects. First, this is the first evaluation of the milk proteome relative to milk lipolysis or LPL activity. Second, the relationship between the abundance of proteins and milk traits was evaluated by a combination of univariate and multivariate analyses. Third, we provide a short list of five proteins to be tested in a larger population to feed the pipeline of biomarker discovery.

The primary structure of caprine PP3: amino acid sequence, phosphorylation, and glycosylation of component PP3 from the proteose-peptone fraction of caprine milk

Proteose-peptone component 3 is a phosphorylated glycoprotein that was isolated from the proteose-peptone fraction of caprine milk. By mass spectrometric analysis, amino acid sequencing, and polymerase chain reaction analysis, the primary structure has been determined and has been shown to contain 136 amino acids. Phosphorylations were identified at Ser30 and Ser41. A partial glycosylation was present at Thr16, and a N-linked glycosylation was present at Asn78. Galactosamine was the amino sugar detected at Thr16. Glucosamine and galactosamine were the amino sugars found in the carbohydrate group linked to Asn78. The caprine amino acid sequence exhibits 88% identity with the bovine proteose-peptone component 3 sequence. However, when compared with the bovine sequence, the caprine sequence contains an insertion of a serine residue at position 25.

The localization and multimeric nature of component PP3 in bovine milk: purification and characterization of PP3 from caprine and ovine milks

The distribution of proteose-peptone component PP3 in bovine whey, milk fat globule membrane, and casein has been investigated with antibodies raised against highly purified PP3. Using Western blot analysis, we show that PP3 is present in the milk fat globule membrane and in whey but is absent in the casein fraction. The proposed multimeric structure of bovine PP3 was analyzed by mass spectrometry and gel filtration. Calibrated gel filtration of acidic whey showed that PP3 eluted at a volume corresponding to 190 kDa, indicating that PP3 exists as a multimeric aggregate in bovine milk. Western blot analysis with anti-bovine PP3 immunoglobulins was used to analyze caprine, ovine, and human milks, and immunoreactive proteins were detected in caprine and ovine milks. Finally, the immunoreactive proteins from caprine and ovine milks were purified and characterized as PP3 analogues by amino acid analysis and N-terminal sequence analysis.