In addition to retinol and fatty acids, a wide variety of hydrophobic compounds interact with b-lactoglobulin. These include triglycerides, alkanes, aliphatic ketones, and aromatic compounds (p-nitrophenyl phosphate, toluene). The association constants of these substances with b-lactoglobulin have been calculated, but, in most cases, which region of the protein is involved in the binding is unclear. b-Lactoglobulin possesses one primary binding site per monomer for alkanes and ketones with an association constant of about 103 M-1. The binding site is thought to be located inside a hydrophobic cavity and is unaffected by the state of association of the protein. Modification of the structure of b-lactoglobulin with urea, reduction of disulfide bonds or ethylation reduces binding of these compounds, reflecting the importance of the native structure in determining binding affinities.

Recently, Dai-Dong et al. have verified that bovine b-lactoglobulin has a protecting effect on the thermal denaturation of ascorbic acid in aqueous solution. This observation led to the hypothesis of the existence of a specific interaction between vitamin C and b-lactoglobulin. However, by use of equilibrium dialysis and gel filtration chromatography with radialabelled ascorbic acid, b-lactoglobulin is not able to bind vitamin C. Therefore, the protective effect of b-lactoglobulin on ascorbic acid thermodenaturation could be due to an antioxidant effect because of the presence of reductive thiol groups in the protein. This effect does not necessarily require direct interaction between the protein and vitamin .

The binding of p-nitrophenyl phosphate and other low molecular weight phosphates to b-lactoglobulin inhibits phosphate hydrolysis induced by milk alkaline phosphatase. This observation led to the speculation that b-lactoglobulin may play a regulatory role in mammary gland phosphorus metabolism (23). However, this inhibition seems to be unimportant because, at the normal pH of milk, alkaline phosphatase has a low activity on casein micelles.

Sonicated vesicles containing a complex of b-lactoglobulin and dipalmitoyl phosphatylcholine can be formed, but treatment of the protein with a helix-forming solvent is necessary because the native protein does not complex with the lipid.

b-Lactoglobulin also exhibits considerable adsorption to an artificial phospholipid membrane between pH 1.3 and 4.0, but none occurs at pH between 6 and 7. The existence of an electrostatic mechanism at acid pH has been proposed in which positively charged b-lactoglobulin interacts with negatively charged lipids in the monolayer to form a coating about one molecule thick.


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