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

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Abstract
Temperature-dependent (5-42.degree.C) 13C NMR spectra of albumin complexes with 90% isotopically substituted [1-13C]octanoic or [1-13C]decanoic acids (3 mol of fatty acid per mol of albumin) showed a single peak at > 30.degree.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 > 30.degree.C. Rate constants for exchange at 33.degree.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, has several narrow laurate carboxyl peaks at 35.degree.C, indicating longer lifetimes (.tau. .mchgt. 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 > decanoate > 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 .apprxeq. 104 faster for octanoic acid.