The primary structure of monomeric beta-lactoglobulin I from horse colostrum (Equus caballus, Perissodactyla).

beta-Lactoglobulin-like proteins were detected in horse colostrum and normal milk using immunological techniques. In contrast to the beta-lactoglobulins sequenced so far these proteins are monomeric and genetically not homogenous. In this paper we report the first primary structure of a monomeric beta-lactoglobulin from horse colostrum. By means of an automatic liquid-phase sequenator the sequence of peptides obtained by tryptic digestion and by cyanogen bromide cleavage was determined. A limited tryptic digestion and hydrolysis with chymotrypsin provided the necessary overlapping peptides. The horse beta-lactoglobulin I consists of 162 amino acids, among these four cysteine, six methionine residues and one tryptophan residue. Homologous comparison with bovine beta-lactoglobulin A shows an unexpectedly great difference of 72 amino acids (or 44%). Thirteen of these exchanges are explained as two-point mutations. We found that the free thiol group, localized at position 121 or in equal amounts at positions 119 and 121 in bovine beta-lactoglobulin, is absent in beta-lactoglobulin I from horse colostrum. In position 121 a tyrosine substitution for cysteine was found. The amino-acid exchanges of the horse beta-lactoglobulin I as compared to the other beta-lactoglobulins are discussed.