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

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Abstract
The sialic acid content of skimmilk proteins has been examined, using a modification of Warren's thiobarbiturie acid method. Lactose interferes with this method and was removed by thorough washing of the proteins. K-casein and proteose-peptone appear to be the only two skimmilk proteins containing significant amounts of sialie acid. Treatments with DEAE cellulose ahnost completely renmved the low concentrations of sialie acid present in a~: and fl-caseins, and examination of these caseins by starch gel eleetrophoresis indicates that such removal involved traces of K-casein. Since the stability of a~-K-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 mcasurenmnts can be used as an index of the intact K-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 K-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 K-casein is variable. The sialic acid content of K-casein appears to be close to 2.3%; on this basis, the proportion of K-casein in whole acid casein varies between 11-26%.

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