Isoelectric Focusing of Bovine Milk Caseins

Abstract

Isoelectric focusing performed in polyacrylamide gels and sucrose gradient columns containing 7 M urea and under controlled conditions effectively separated the major casein species of bovine milk. A number of individual αs- and β-caseins not detected by alkaline starch gel electrophoresis were resolved by isoelectric focusing in narrow and wide pH gradients. The evidence indicated that genetic variants of casein species and, to some extent, ampholyte-casein artifacts formed during focusing were responsible for the apparent heterogeneity. κ-Caseins A and B were focused when layered casein samples (for gels) or sucrose gradients (for columns) contained 2-mercaptoethanol. γ-Components were separated by gels but a problem with recovery from column fractions was not completely solved. Apparent isoelectric points (pI's) ranged in increasing order from pH 4.9 to 6.5 for αs, β-, and κ-caseins and from 6.7 to 8.0 for γ-components. Correction of pI values for the apparent effect of 7 M urea allowed for an evaluation of possible differences in casein conformation. © 1972, American Dairy Science Association. All rights reserved.