The Complete Amino‐Acid Sequence of Human α‐Lactalbumin

Abstract

α‐Lactalbumin was isolated from human milk in a yield of 1.8 mg/ml of milk. The purification procedure involved ammonium sulphate fractionation (30% to 80% saturation) and pH‐4.0 precipitation, followed by gel filtration with Sephadex G‐100. A final purification stage using DEAE‐cellulose was necessary in some preparations. Peptides derived from the reduced, S ‐aminoethylated protein by treatment with cyanogen bromide and digestion of these CNBr fragments with trypsin, chymotrypsin or thermolysin, were purified by gel filtration, ion exchange chromatography and high‐voltage paper electrophoresis. From the sequences of these peptides it has proved possible to deduce unambiguously the complete primary structure of the protein. Comparison with bovine α‐lactalbumin shows an identity in 72% of the residues with a further 6% being chemically similar amino acids. The corresponding figures for the human α‐lactalbumin/human lysozyme comparison, are 39% and 12%, respectively. The significance of some of the amino acid replacements is discussed.