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Published on: February 7, 2022
Milk Bottom-Up Proteomics: Method Optimization
Delphine Vincent1, Vilnis Ezernieks1, Aaron Elkins1
1Department of Economic Development, Jobs, Transport and Resources, AgriBio Centre, La Trobe University Bundoora, VIC, Australia.
Insights
Comparing three milk protein extraction methods, methanol/chloroform offered the best protein recovery and SDS-PAGE resolution. Urea extraction identified the most unique milk proteins, while TCA/acetone was least compatible for downstream analysis.
Area of Science:
- Proteomics
- Dairy Science
- Analytical Chemistry
Background:
- Milk proteome analysis requires reproducible and high-throughput sample preparation.
- Understanding milk protein composition is crucial for assessing factors like diet, health, and genetics.
- Previous extraction methods may not be optimal for diverse milk protein identification.
Purpose of the Study:
- To compare the efficiency and reproducibility of three novel milk protein extraction methods: urea, trichloroacetic acid (TCA)/acetone, and methanol/chloroform.
- To evaluate protein recovery, SDS-PAGE profile resolution, and compatibility with downstream proteomic analyses.
- To identify breed-specific protein differences in Jersey and Holstein-Friesian cows' milk.
Main Methods:
- Skim milk samples from Jersey and Holstein-Friesian cows were used.
- Three extraction procedures were evaluated: Method A (urea), Method B (TCA/acetone), and Method C (methanol/chloroform).
- Protein assays, SDS-PAGE, and nano-HPLC-electrospray ionization-tandem mass spectrometry (nLC-ESI-MS/MS) were employed, with replicates at each step to ensure reproducibility.
Main Results:
- A total of 186 unique protein accessions were identified using a combination of methods.
- Method C (methanol/chloroform) provided the most resolved SDS-PAGE patterns and highest protein recovery.
- Method A (urea) identified the largest number of unique protein accessions.
- Method B (TCA/acetone) showed limited compatibility with subsequent analytical procedures.
- Breed-specific protein differences between Jersey and Holstein-Friesian milk were observed.
Conclusions:
- The methanol/chloroform extraction method is superior for achieving high protein recovery and clear SDS-PAGE profiles in cow's milk.
- Urea-based extraction is effective for maximizing the number of identified milk proteins.
- The choice of extraction method significantly impacts downstream proteomic analysis outcomes and protein identification.
- This study provides valuable insights into cow's milk proteome and highlights breed-specific variations.
Abstract:
Milk is a complex fluid whose proteome displays a diverse set of proteins of high abundance such as caseins and medium to low abundance whey proteins such as ß-lactoglobulin, lactoferrin, immunoglobulins, glycoproteins, peptide hormones, and enzymes. A sample preparation method that enables high reproducibility and throughput is key in reliably identifying proteins present or proteins responding to conditions such as a diet, health or genetics. Using skim milk samples from Jersey and Holstein-Friesian cows, we compared three extraction procedures which have not previously been applied to samples of cows' milk. Method A (urea) involved a simple dilution of the milk in a urea-based buffer, method B (TCA/acetone) involved a trichloroacetic acid (TCA)/acetone precipitation, and method C (methanol/chloroform) involved a tri-phasic partition method in chloroform/methanol solution. Protein assays, SDS-PAGE profiling, and trypsin digestion followed by nanoHPLC-electrospray ionization-tandem mass spectrometry (nLC-ESI-MS/MS) analyses were performed to assess their efficiency. Replicates were used at each analytical step (extraction, digestion, injection) to assess reproducibility. Mass spectrometry (MS) data are available via ProteomeXchange with identifier PXD002529. Overall 186 unique accessions, major and minor proteins, were identified with a combination of methods. Method C (methanol/chloroform) yielded the best resolved SDS-patterns and highest protein recovery rates, method A (urea) yielded the greatest number of accessions, and, of the three procedures, method B (TCA/acetone) was the least compatible of all with a wide range of downstream analytical procedures. Our results also highlighted breed differences between the proteins in milk of Jersey and Holstein-Friesian cows.
