Isolation of some minor milk proteins, distributed in acid whey from approximately 100,000 to 250,000 daltons of particle size

Milk proteins in acid whey were separated into five fractions according to molecular size by gel filtration chromatography. The second peak, P2, contained proteins between approximately 250,000 and 100,000 daltons. Proteins in P2 were concentrated. After separation into albumins and globulins, each protein group was isolated by DEAE chromatography and hydrophobic interaction chromatography, Isolated albumin fractions were a yellow-colored protein of 89,000 daltons, an unidentified protein of 73,000 daltons, a beta-lactoglobulin of 18,300 daltons, and a red-colored protein of 87,000 daltons. Two types of globulin fractions were isolated: 1) a globulin fraction that coagulated in saturated sodium sulfate but did not coagulate when dialyzed against deionized water included a brown-colored protein of 150,000 daltons, and 2) a bovine serum albumin of 67,000 daltons with unidentified 170,000 and 30,000 daltons bands. A true globulin fraction contained a 77,000 dalton unidentified protein with several faint bands. The red-colored protein was identified as lactoferrin and the brown-colored protein as xanthine oxidase (EC 1.2.3.2.). A yellow-colored protein was concluded to be the denatured protein of contaminated lactoperoxidase (EC 1.11.1.7).

Substrate specificities of insulin and epidermal growth factor receptor kinases

The abilities of insulin and EGF stimulated protein kinases to phosphorylate a series of exogenous substrates were compared using wheat germ lectin purified preparations of solubilized rat liver membranes. Three different kinds of substrates were found: substrates phosphorylated primarily by insulin stimulated kinase, substrates phosphorylated primarily by EGF stimulated kinase and substrates phosphorylated by both kinases to a similar extent. These results indicate that the insulin and the EGF receptor kinase have different, but overlapping, substrate specificities. In vivo, phosphorylation of cellular proteins by various hormone receptor kinases may be part of the signal transmission process for actions of the hormones. Different substrate specificities of kinases of different hormone receptors may therefore represent an important mechanism to preserve the specificity of the hormonal signal at the post receptor level.

Determination of native and denatured milk proteins by high-performance size exclusion chromatography

High-performance size exclusion chromatography on a TSK 3000 SW column was used to separate and quantify the individual milk proteins in their native and/or denatured state. Quantitation of the individual native whey proteins within +/- 10% was achieved. The denatured proteins were dissociated into their monomer units by using sodium dodecyl sulphate and beta-mercaptoethanol. A good separation of the whey protein monomers was achieved and the quantitation of the individual whey proteins within +/- 10% was possible. Under these conditions, alpha s1- and beta-casein were eluted as one peak, whereas kappa-casein and gamma-casein were eluted in the same position as beta-lactoglobulin and alpha-lactalbumin respectively. A method for calculating the areas and hence the concentration of the individual proteins in a mixture of casein and whey proteins is described.