Medium chain fatty acid binding to human plasma albumin.

The binding of hexanoic, octanoic, and decanoic acids to defatted human plasma albumin was measured by equilibrium dialysis at 37” in a calcium-free Krebs-Ringer phosphate buffer, pH 7.4. The results were analyzed in terms of multiple stepwise equilibria. For each of the albumin binding sites, the magnitude of the equilibrium constants increased as the chain length of the acid increased: decanoate > octanoate > hexanoate. The first six equilibrium constants ranged from 1.5 x lo4 to 4.7 X 10 for hexanoate, from 3.4 X lo4 to 1.2 X lo3 for octanoate, and from 10” to 3.4 X lo3 for decanoate. In each case, the equilibrium constants occurred in a generally descending order, suggesting that major cooperative binding effects do not occur over the physiologically important range of fatty acid-albumin molar ratios. The equilibrium constants calculated for each of the three acids could not be grouped in a common way in terms of classes of binding sites, indicating that a single, uniform class-site binding model cannot be applied to these medium chain fatty acids. Octanoate binding was relatively insensitive to pH changes over the range of 6.0 to 8.2. Decanoate binding also was similar at pH 6.5 and 7.4. A decrease in octanoate binding occurred when the albumin was acetylated or when the medium contained 6 M urea. Octanoate binding also was decreased when either palmitate or oleate was added to albumin, suggesting that medium chain fatty acid transport may be influenced by changes in the plasma long chain free fatty acid concentration.