The Amino-Acid Sequence of β-Lactoglobulin II from Horse Colostrum(Equus caballus,Perissodactyla): β-Lactoglobulins are Retinol-Binding Proteins

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Abstract
.beta.-Lactoglobulin isolated from horse colostrum is heterogenous and contains 2 components: .beta.-lactoglobulin I and .beta.-lactoglobulin II. These 2 proteins are monomeric and show differences in their electrophoretic mobilities, chain lengths and primary structures. The complete amino-acid sequence of .beta.-lactoglobulin II was determined by automated Edman degradation of the intact protein and of the peptides derived from these by digestion with trypsin or chymotrypsin and by chemical cleavage with cyanogen bromide. Unlike other .beta.-lactoglobulins which contain 162 amino acids, horse .beta.-lactoglobulin II is unique in that it contains 166 amino acids. The additional 4 amino acids represent an insertion between positions 116 and 117 of other .beta.-lactoglobulins so far sequenced, including horse .beta.-lactoglobulin I. Sequence comparison of .beta.-lactoglobulins I and II from horse colostrum reveals 48 amino acid substitutions (30%). Such a diversity between members of the .beta.-lactoglobulin gene family has not been encountered before. Sequence comparison with bovine .beta.-lactoglobulin A shows 85 amino acid replacements accounting for 53% of the residues. The structural homology with human retinol-binding protein may reveal similar biological functions and clues to the origin of milk proteins.

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