Semi-Micro-Scale Frontal Gel Chromatography of Interacting Systems of a Protein and Small Molecules: Binding of Warfarin, Tryptophan, or FMN to Albumin, and of o-Nitrophenol to Catechol 2,3-Dioxygenase

Frontal gel chromatography is a convenient and accurate method to obtain the free ligand concentration of a protein-ligand mixture. Because a large amount of sample (more than 6 ml) is required for the method, it has been rarely used for binding experiments. We have developed a system to carry out frontal gel chromatography on a semi-micro scale using short gel filtration columns (4.6 mm×50−100 mm); frontal chromatograms could be obtained with small amounts of samples (1−2.5 ml) within 20 min. We used this technique to examine the binding of warfarin, L-tryptophan, or FMN to human serum albumin, the binding of warfarin to bovine serum albumin, and the interaction of catechol 2,3-dioxygen-ase with o-nitrophenol. The data fitted to a binding model in which a protein has one or several independent binding sites. Both human and bovine serum albumin showed the high-affinity bindings of two warfarin molecules. The binding number for L-tryptophan on human serum albumin was confirmed to be one, whereas maximal binding of FMN was 0.6 molecule per albumin molecule. o-Nitrophenol showed high-affinity binding only to holo-catechol 2,3-dioxygenase. The absorption spectrum of the bound o-nitrophenol resembled that of anionic o-nitrophenol. These results demonstrated that frontal gel chromatography on a semi-micro scale is useful for the study of binding systems; the method is rapid and would be easy to automate fully.