Fluorescence Spectroscopic Study of Hypericin‐photosensitized Oxidation of Low‐density Lipoproteins

Abstract

By means of UV-VIS absorption and fluorescence spectroscopy, we demonstrate that the photosensitizer hypericin (Hyp) interacts nonspecifically with low-density lipoproteins (LDL), most probably with the lipid fraction of LDL. The molar ratio of monomeric Hyp binding to nonoxidized LDL and mildly oxidized LDL is 30:1. Increasing the Hyp concentration further leads to the formation of Hyp aggregates inside the LDL molecule. We also demonstrate that photoactivated Hyp oxidizes LDL in a light dose and excitation wavelength dependent manner. The level of oxidation of LDL depends on the amount of Hyp inside the LDL molecule. The maximum of the photosensitized oxidation of the LDL by Hyp is achieved for a 30:1 molar ratio, which corresponds to the maximum concentration of monomeric form of Hyp in LDL.

Solubilization of poorly soluble photosensitizer hypericin by polymeric micelles and polyethylene glycol

“…This fact is well known and is attributed to the formation of non-fluorescent Hyp aggregates in aqueous solutions (Falk and Meyer 1994;Kascakova et al 2005). The fluorescence spectra of monomeric Hyp are represented by the spectra of Hyp dissolved in DMSO and in the complex with LDL (Hyp/LDL = 10:1) (Kascakova et al 2005). The solubilization activity of PEG-PE is manifested by the fact, that the fluorescence intensity of Hyp in the presence of PEG-PE is almost identical to those intensities observed in DMSO and LDL, where Hyp is readily soluble (Fig.…”

“…It means that the quantum yield of the fluorescence of Hyp incorporated in PEG-PE is similar to those for Hyp in LDL or dissolved in various polar organic solvents (e.g. DMSO), where the quantum yield of Hyp fluorescence has been determined to be ~0.2 (Falk and Meyer 1994;Hadjur et al 1996;Kascakova et al 2005).…”

“…Hyp exhibits no fluorescence in aqueous PBS. This fact is well known and is attributed to the formation of non-fluorescent Hyp aggregates in aqueous solutions (Falk and Meyer 1994;Kascakova et al 2005). The fluorescence spectra of monomeric Hyp are represented by the spectra of Hyp dissolved in DMSO and in the complex with LDL (Hyp/LDL = 10:1) (Kascakova et al 2005).…”

“…This compound is sparingly soluble in non-polar organic solvents and oil, and in aqueous environment forms insoluble non-fluorescent aggregates (Falk and Meyer 1994;Falk 1999). The formation of aggregates significantly suppresses Hyp photodynamic activity (Falk and Meyer 1994;Kascakova et al 2005;van de Putte et al 2006). Moreover, this aqueous insolubility makes intravenous injection of Hyp problematic and restricts its medical applications.…”

“…The study of Hyp distribution in human plasma has shown that Hyp molecules are predominantly associated with low-density lipoproteins (LDL) and only relatively small amounts are bound to high-density lipoproteins (HDL) and proteins (mainly serum albumin) (Chen et al 2001). In the presence of the plasma (lipo)proteins Hyp aggregates can redissociate resulting in fluorescent monomeric and biological active form of this molecule (Falk and Meyer 1994;Lavie et al 1995;Huygens et al 2005;Kascakova et al 2005). Our group has published several articles about the properties of Hyp/LDL complexes (Kascakova et al 2005;Mukherjee et al 2008;Gbur et al 2009;Huntosova et al 2010;Buriankova et al 2011).…”