FATTY-ACIDS AND RETINOIDS BIND INDEPENDENTLY AND SIMULTANEOUSLY TO BETA-LACTOGLOBULIN

beta-Lactoglobulin (Blg) binds 1 mol of a fatty acid spin-label analog , 5-doxylstearic acid (5-DSA), per mole of protein with a dissociation constant K-d = 0.8 mu M for the strongest binding site. There are als o several weaker sites for this ligand. Blg saturated with either reti nol or retinoic acid binds 5-DSA with essentially equal affinity (K-d = 0.6 and 1 mu M, respectively). Palmitic acid and SDS displace bound 5-DSA from Blg. However, unlike palmitic acid, 5-DSA binding does not enhance the structural stability of Blg to urea denaturation. The spin -labeled fatty acid also binds to the protein at low pH, presumably at secondary fatty acid binding sites. These results suggest that Blg bi nds at least two different types of hydrophobic ligands simultaneously .