Analysis of beta-casein and its phosphoforms in human milk

Quantitation of beta-casein was achieved through the use of urea-polyacrylamide gel electrophoresis of whole milk and scanning densitometry. This method also provided electrophorectic separation of the different phosphoforms of S-casein which were also quantitated. Fifty-eight human milk samples collected in 4 different countries were analyzed for beta-casein and beta-casein phosphoform concentrations. The average B-casein concentration obtained using the whole milk methodology was 4.72 +/- 1.44 mg/ml. We found that a-casein is found in all the fractions of milk that has been centrifuged to remove the lipid or acid precipitated to collect the caseins. This study used whole human milk and therefore all the beta-casein present was included in the analysis. The whole milk analysis of B-casein indicated that on average the phosphoforms are present in the following order ranked by concentration: tetra- > di- > non- > mono- > tri- > penta-phosphorylated beta-casein. However, the phosphoform distribution of individual donors varied widely. Four different methods were used to determine the concentration of total protein in human milk samples. UV absorbance-based and colorimetric methods produced higher values of protein concentration than the Kjeldahl method. (C) 1998 Elsevier Science Inc.