Sialic Acid as an Index of the κ-Casein Content of Bovine Skimmilk

Abstract

Summary The sialic acid content of skimmilk proteins has been examined, using a modification of Warren's thiobarbituric acid method. Lactose interferes with this method and was removed by thorough washing of the proteins. κ -casein and proteose-peptone appear to be the only two skimmilk proteins containing significant amounts of sialic acid. Treatments with DEAE cellulose almost completely removed the low concentrations of sialic acid present in α s - and β -caseins, and examination of these caseins by starch gel electrophoresis indicates that such removal involved traces of κ -casein. Since the stability of α s - κ -casein mixtures in the presence of calcium appeared to be directly related to the sialic acid content of the mixture at all levels below the 0.37% (uncorrected for moisture) needed to give complete stability, it is concluded that sialic acid measurements can be used as an index of the intact κ -casein content of such mixtures. Proteose-peptone is precipitated by 12% TCA, but remains in solution at pH 4.5, so analysis of sialic acid can also be used as an index of the κ -casein concentration of whole acid casein. Values of 0.26–0.59% sialic acid were obtained in washed whole acid caseins prepared from individual lots of milk, indicating that the proportion of κ -casein is variable. The sialic acid content of κ -casein appears to be close to 2.3%; on this basis, the proportion of κ -casein in whole acid casein varies between 11–26%.