Quantification of αS1-casein in breast milk using a targeted mass spectrometry-based approach

The caseins comprise a milk protein fraction of high nutritional value and, as more recently discovered, of immunologic relevance. In particular, αS1-casein (CSN1S1) is of interest being a potential autoantigen. So far, the concentration of caseins in human milk was primarily determined by indirect methods. The aim of this study was to directly measure the CSN1S1 content in breast milk using mass spectrometry (MS). The quantification was based on tryptic CSN1S1 peptides with the best response in liquid chromatography (LC)-MS/MS analysis. Targeted experiments allowed both specific and sensitive detection at the low fmol level. For this pilot study, twenty breast milk samples of the first week post-partum were analyzed and contained between 3 and 540μg/ml CSN1S1. Limitations of CSN1S1 quantification are discussed.