Differences in post-translational modifications of proteins in milk from early and mid-lactation dairy cows as studied using total ion chromatograms from LC-ESI/MS

Abstract

Liquid chromatography top-down mass-spectrometry combined with an in-house library with 5180 entries of known milk protein masses was used to identify common and rare isoforms of major milk proteins from individual cow's milk samples from early- and mid-lactating (EL and ML, respectively) cows. The milk samples were from 24 Danish Holstein-Friesians, with 12 cows each in EL and ML. A Latin square design with four periods was used to obtain one composite milk sample from each cow per period. The deconvoluted masses were used for identifying common and rare isoforms of the major milk proteins (αS1-CN, αS2-CN, β-CN, κ-CN, α-LA, and β-LG). αS1-CN splice variants were found in all samples, αS- and κ-CN isoforms differed between EL and ML. κ-CN post-translational modifications were found to relate to κ-CN genotype. This approach could identify 66 isoforms of major milk proteins, where 15 were of the αS-CNs and 39 from κ-CN.