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K. H. Valkonen*, N. Marttinen*, H.-L. Malinen*, V.-P. Jaakola** and
T. Alatossava*
* Biotechnology Laboratory, REDEC of Kajaani, University of Oulu, FIN-88600
Sotkamo, Finland
** Department of Biological and Environmental Science, University of Jyvskyl,
PL 35, FIN-40351 Jyvskyl, Finland
Introduction
β-lactoglobulin (βLG) is a major whey protein found in the milk of cows
and other ruminants, deer, bison, and buffalo. βLG is also found in some
nonruminants, such as pigs,1 horses,2 dogs, dolphins,3 cats,4 and whales.
However, βLG is not found in human milk.5, 6, 7, 8 βLG is a glycoprotein
which exists at the normal pH of bovine milk as a dimer with a molecular
weight of 36,000, and consists of two monomeric subunits with molecular
weight of 18,000 (162 residues). Several genetic variants of βLG have
been detected,9 of which the bovine phenotype A and phenotype B are most
predominant. The bovine βLG A variant differs from the βLG B variant by
only two amino acids: aspartate-64 (Asp) and valine-118 (Val). These amino
acids are substituted by glycine (Gly) and alanine (Ala) in the B variant.
All variants contain five cysteine residues, four of which are involved
in forming intrachain disulfide bridges.
The function of βLG is not yet clear, although binding and transport
of retinol, small hydrophobic ligands and fatty acid in postnatal animals,
have been suggested as possible functions, since its three-dimensional
structure is essentially similar to that of human retinol-binding protein
in serum.10, 11, 12, 13, 14, 15 βLG
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