| Abstract: | Details concerning the establishment of the complete primary structure of human milk lysozyme (previously published in a preliminary note) are presented. The chymotryptic peptides obtained from the reduced alkylated enzyme were purified and their amino acid sequences determined chiefly by the ‘Edman-dansylation’; procedure, and in two cases by partial acid or peptic hydrolyses. The tryptic peptides are alined into a single chain containing 129 amino acid residues, on the basis of overlapping peptides. Two labile glutamine residues easily converted into glutamic acid residues were characterized. Human milk lysozyme is compared with other human lysozymes (from normal and leuchaemic individuals) prepared by our group. The structure proposed is identical with the sequence of human leuchaemia lysozyme (from the urine of a patient with chronic monocytic leuchaemia) reported by Canfield. Human milk lysozyme is also near by related to several bird egg-white lysozymes (and bovine α-lactalbumin): Identical positions of Cys and Trp residues and of the residues essential for the catalytic activity or involved in some hydrogen bonds; several identical regions, especially in the β-sheet region; between 71 and 77 identical amino acid residues. It is suggested that by an insertion and a deletion in the sequence of human milk lysozyme, sequences homologous to those of bird lysozymes can be obtained. |
