Abstract

Binding and co-binding of various 19F-labeled ligands to human serum albumin (HSA) has been studied using 19F NMR. Specifically shifted resonances in slow exchange with the free resonances are detected for many of the ligands. These specifically shifted resonances can be studied to yield accurate estimates of site-specific binding constants and stoichiometries. In addition, the use of two different 19F-labeled ligands can directly reveal competition for a given site or independent binding at different sites. For instance, it is easily shown that both 5-F-L-tryptophan and 5-F-salicylic acid are capable of binding independently to two sites on HSA at the same time, without the need for any curve-fitting or assumptions. These results demonstrate that the concept of "sites" on HSA is not only useful but is necessary. The technique also reveals allosteric interactions between 5-F-L-Trp and warfarin co-bound to HSA. This technique proves to be a powerful methodology for studying ligand and drug binding to HSA that is free from some of the pitfalls associated with more traditional techniques such as equilibrium dialysis.