Quantification of beta-casein in human milk

Abstract

A method is described for preparing immunologically homogeneous human milk beta-casein, against which monospecific rabbit antiserum was prepared. The antiserum was used to quantify beta-casein, the major human casein, by rocket immunoelectrophoresis in individual milk samples. However, it was found that in most samples beta-casein occurred together with degradation products originating from its proteolysis by plasmin. Immunological quantification of human beta-casein, treated with plasmin for various time periods, showed that rocket height was not affected by proteolysis up to degradation states clearly more advanced than those observed in all samples of fresh human milk tested. Assays of 150 individual milk samples from 80 women, covering a lactation period of up to 730 d, gave an average concentration of beta-casein (native + degraded) of 4.67 +/- 0.89 standard deviation (g/l); extremes at 2.1 and 7.3 g/l did not vary significantly during the period under study. Comparison of this average value with an accepted casein content of 4.4 g/l (Macy & Kelly, 1961) showed that the casein content of human milk is underestimated when obtained by N determinations on milk and on its supernatants at pH 4.6 (whey). Caseins other than beta-casein occurred only in minute amounts, if at all.