ELSEVIER Journal of Chromatography A, 768 (1997) 47-56 JOURNALOF CHROMATOGRAPHY A Capillary electrophoretic analysis of genetic variants of milk proteins from different species Isidra Recio, Maria-Luisa Perez-Rodrlguez , Mercedes Ramos, Lourdes Amigo* lnstituto de Fermentaciones lndustriales (CS1C), Juan de la Cierva 3, 28006 Madrid, Spain Abstract Polymorphism of bovine, ovine and caprine milk proteins was studied by CE. Identification of some rare bovine variants was carried out by isoelectric focussing (IEF) using PhastSystem. Genetic variants A and D of bovine a 2-casein, [3-casein variants A', A2, A 3, B and C and %~-casein variants B and C were determined by CE. In addition, the different casein fractions including some genetic variants of ovine and caprine milk were identified by CE. In order to carry out this identification, collected fractions from a cation-exchange FPLC separation were injected by CE. Keywords: Milk; Proteins; Caseins 1. Introduction Milk composition and the main characteristics of its various constituents are now well known. In particular the amino acid sequences of its seven main protein components have been elucidated. At the moment, there is no other food product whose proteins are so well characterized. Moreover, for six of these seven protein types a qualitative polymor- phism has been described [1]. Genetic polymorphism in the milk proteins is due to either substitutions of amino acids, or deletion of a certain amino acid sequence along the peptide chain, consequently these mutations are caused by changes in the sequence of base pairs of the DNA molecule which constitute the protein gene. It has been described that polymorphism is also due to post- transcriptional modifications, such as different de- *Corresponding author. 'Present address: Facultad de Farmacia, Universidad Com- plutense, 28040 Madrid, Spain. grees of phosphorylation and glycosylation of the protein. Besides the structural polymorphism, a quantitative allelic variability has been found which renders differences in protein expression. This phe- nomenon has been described in cow [2] and goat milk [3]. Polymorphism of milk proteins is related to pro- duction traits, milk composition and milk quality. Rapid screening of milk protein variants is important for the genetic improvement of milk with regard to the potential technological application and breeding selection programs [4]. Until now, the detection of genetic polymorphism of milk proteins has been achieved through various electrophoretic and chromatographic techniques [1]. However, the conventional electrophoretic methods are restricted to the resolution of proteins with differing net charges caused by the substitution of one amino acid for another and it is only possible to achieve semiquantitative results. Peptide mapping by HPLC and DNA mapping have also been used for the detection of silent variants (mutations in proteins 0021-9673/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0021-9673(96)00889-8