Homology between the Primary Structures of β-Lactoglobulins and Human Retinol-Binding Protein: Evidence for a Similar Biological Function?

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Abstract
Two types of .beta.-lactoglobulins were identified and isolated from horse colostrum: .beta.-lg I and .beta.-lg II. The amino-acid sequence of some tryptic peptides from the new monomeric .beta.-lactoglobulin II was determined and aligned to the other .beta.-lactoglobulins of known sequence, and to the human plasma retinol-binding protein. The comparison of the primary structures of .beta.-lactoglobulins and human retinol-binding protein shows an unexpectedly high homology of 25%. Thirty-seven identities were found among 149 possible homologous residues. Among them is a tryptophan residue at position 19 of .beta.-lg which might represent the binding site of .beta.-ionone. A common origin of .beta.-lactoglobulin and human retinol-binding protein is suggested. .beta.-Lactoglobulins may be involved in the metabolism of retinol.

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