The Purification and Properties of the A Protein of Lactose Synthetase

Abstract

Abstract The A protein of lactose synthetase has been purified from bovine skim milk. After chromatography on DEAE-Sephadex and cellulose phosphate, the active fractions were purified to constant specific activity by chromatography on columns of α-lactalbumin attached covalently to Sepharose. The A protein was bound specifically to this column in buffers containing glucose and was eluted by omitting glucose from the developing buffer. These highly purified preparations are inactivated easily but can be stabilized by N-acetylglucosamine or by bovine serum albumin. The final preparation was shown to be homogeneous by gel electrophoresis in sodium dodecyl sulfate, by ultracentrifugal analysis, and by gel filtration on 4% Agarose in the presence of 6 m guanidine hydrochloride. The fully denatured enzyme was found to have a molecular weight between 40,000 and 44,000, values in good agreement with a molecular weight of 42,900, which was estimated for the native enzyme by sedimentation in sucrose density gradients. These molecular weight measurements suggest that the enzyme is not composed of subunits. Amino acid and carbohydrate analyses indicate that the enzyme is a glycoprotein with about 12% by weight carbohydrate.