Fatty acid-specific fluorescent probes and their use in resolving mixtures of unbound free fatty acids in equilibrium with albumin.

We report the first measurements for profiling mixtures of unbound free fatty acids. Measurements utilized fluorescent probes with distinctly different response profiles for different free fatty acids (FFA). These probes were constructed by labeling site-specific mutants of the rat intestinal fatty acid binding protein (rI-FABP) with acrylodan. The probes were produced and screened by high-throughput methods, and from more than 30 000 such probes we selected six that together have sufficient specificity and sensitivity for resolving the profile of unbound FFA (FFAu) in mixtures of different FFAu. We developed analytical methods to determine the FFAu profile from the fluorescence (ratio) response of the different probes and used these methods to determine FFAu profiles for mixtures of arachidonate, linoleate, oleate, palmitate, and stearate in equilibrium with bovine serum albumin (BSA). Measurements were performed using mixtures with a range of total FFAu concentrations, including 0.9 nM, which is similar to normal plasma levels. We also measured single FFA binding isotherms for BSA and found that binding was described well by six to seven sites with the same binding constants (Kd). The Kd values for the FFA (4-38 nM) were inversely related to the aqueous solubility of the FFA. We constructed a model with these parameters to predict the FFAu profile in equilibrium with BSA and found excellent agreement between the profiles measured using the FFA probes and those calculated with this model. These results should lead to a better understanding of albumin's role in buffering FFAu and to profiling FFAu in intra- and extracellular biological fluids.