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. The first primary structure of a monomeric .beta.-lactoglobulin from horse colostrum is reported. 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 4 cysteine, 6 methionine residues and 1 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 2-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.