Abstract:

Interaction of a drug molecule with human serum albumin (HSA) is usually studied by fluorescence responses of the ligand or/and the single tryptophan residue (Trp-214) of the protein, but qualitative spectral information may lead to multiple conclusions. In this work, we report a study on the interaction of hematoporphyrin monomethyl ether (HMME) with human serum albumin (HSA), using the environment-sensitive spectra of HMME and reaction-induced fluorescence response of Trp-214. Particularly, the single kinetic parameter, the linear slope, was derived from the concentration-dependent absorbance or fluorescence of HMME in a certain solvent. A quantitative change in the slope at [HMME]/[HSA] = 1:1 clearly demonstrated a specific binding of HMME to site I. The microenvironment in site I may be comparable to that in DMSO solvent, because of the similarity of the slope. Linear correlation of the fluorescence to the absorbance of HMME in site I indicates that the energy transfer is not responsible for Trp-214 fluorescence quenching but an electron transfer may be possible. In addition, much higher rate observed for the binding of HMME or 2-taurine-substituted HB (THB) with HSA than that of hypocrellin B was due to the electrostatic attraction under physiological condition.

Key words: Human serum albumin, Photosensitizing agent, Specific binding, Dimensional fitting, Electrostatic driving