Isolation and characterization of a 21 kDa whey protein in rhesus monkey (Macaca mulatta) milk

A soluble protein in Rhesus monkey milk was isolated to apparent homogeneity by FPLC gel filtration, anion-exchange and reverse-phase chromatography. It is a major milk protein and is present at 2.5-3.0 mg/ml milk throughout lactation. It is only found in the whey fraction of milk; acid precipitation of casein does not result in any significant change in its concentration. A molecular weight (MW) of about 21.6 kDa was estimated from gel filtration and SDS gel electrophoresis and also calculated from its amino acid composition. The amino acid composition of this protein is similar to that of bovine beta-lactoglobulin (beta-Lg), but it is larger in size, possibly representing a family of primate beta-Lgs.

Chromatographic and electrophoretic methods used for analysis of milk proteins

Current knowledge of milk proteins and their behavior in dairy foods is based on early applications of chromatography and electrophoresis. Electrophoretic identification of the number and genetic variety of milk proteins inaugurated a research effort in which chromatographic techniques were successfully applied to the isolation of each milk protein, thus facilitating the characterization and further study of milk and dairy products. This review focuses on recent applications of chromatography for separations and analysis and on analytical applications of electrophoresis.

Immunochemical characterization with monoclonal antibodies of three major caseins and alpha-lactalbumin from rat milk

Rat milk contains at least three major caseins with apparent molecular weights of 41,000 (alpha-casein), 25,000 (beta-casein), and 22,000 (gamma-casein) (estimated in 10% sodium dodecyl sulfate-polyacrylamide gels). These three caseins and alpha-lactalbumin, a major whey protein, were purified from rat milk. The purified caseins and alpha-lactalbumin were used to immunize BALB/c mice, and spleen cells from these mice were hybridized with cells of the mouse myeloma SP-2/0 cell-line. We have isolated a small library of hybridoma cell-lines secreting monoclonal antibodies specific for each of the major caseins and alpha-lactalbumin from rat milk. Antibodies were tested for immunoreactivity with each of the purified milk proteins and with total rat milk proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Some heterogeneity in apparent molecular weight was observed for purified alpha-casein, gamma-casein, and alpha-lactalbumin. Monoclonal antibodies against alpha-casein, gamma-casein, and alpha-lactalbumin recognized all of the molecular weight forms of the antigen for which they were specific. Each monoclonal antibody was specific for one of the caseins or alpha-lactalbumin and did not react with the other caseins or alpha-lactalbumin, suggesting that there is limited structural homology among these proteins. All of the monoclonal antibodies against the rat caseins reacted with components of mouse milk, and the monoclonal antibodies against rat gamma-casein reacted with a component of human milk of apparent molecular weight 27,000. No interspecies reactivity was observed with the antibodies against rat alpha-lactalbumin. These monoclonal antibodies are being used to develop sensitive assays for each of these major rat milk proteins.

Purification and characterization of four components of rat caseins

Rat casein components (C1-, C3A-, C3B- and C4-casein) were extensively purified from rat milk, and the properties of these proteins were compared with those of other caseins including rat C2-casein. C1-casein was precipitated by a low concentration of CaCl2 (1.5 mM). Both C3A- and C3B-casein were less sensitive to Ca2+ than were C1- and C2-casein, and the presence of 20 mM CaCl2 was required at 37 degrees C for their precipitation. C4-casein was absolutely insensitive to Ca2+. This protein exhibited the ability to stabilize all of the other rat casein components against Ca2+-dependent precipitation. In addition, C4-casein contained sialic acid, galactose and N-acetylgalactosamine. Therefore, C4-casein appears to be a bovine kappa-casein-like protein.