Medium-chain fatty acid binding to albumin and transfer to phospholipid bilayers

Temperature-dependent (5-42 degrees C) 13C NMR spectra of albumin complexes with 90% isotopically substituted [1-13C]decanoic acids (3 mol of fatty acid per mol of albumin) showed a single peak at greater than 30 degrees C but three peaks at lower temperatures. The chemical-shift differences result from different ionic and/or hydrogen-bonding interactions between amino acid side chains and the fatty acid carboxyl carbon. Rapid exchange of fatty acid among binding sites obscures these sites at temperatures greater than 30 degrees C. Rate constants for exchange at 33 degrees C were 350 sec-1 for octanoate and 20 sec-1 for decanoate, corresponding to lifetimes in a binding site of 2.8 msec (octanoate) and 50 msec (decanoate). Temperature-dependent data for octanoate showed an activation energy of 2 kcal/mol for exchange. Spectra of albumin complexes with the 12-carbon saturated fatty acid, lauric acid, had several narrow laurate carboxyl peaks at 35 degrees C, indicating longer lifetimes (tau much greater than 66 msec) in the different binding sites. Fatty acid exchange between albumin and model membranes (phosphatidylcholine bilayers) occurred on a time scale comparable to that for exchange among albumin binding sites, following the order octanoate greater than decanoate greater than laurate. The equilibrium distribution of fatty acid between lipid bilayers and protein was measured directly from NMR spectra. Decreasing pH (8.0 to 5.5) increased the relative affinity of fatty acid for the lipid bilayer. The results predict that the relative affinity of octanoic acid for albumin and membranes will be similar to that of long-chain fatty acids (e.g., oleic acid), but the rate of equilibration will be approximately 10(4) faster for octanoic acid.