title: The pH-dependent distribution of the photosensitizer chlorin e6 among plasma proteins and membranes: a physico-chemical approach.
abstract: Decrease in interstitial pH of the tumor stroma and over-expression of low density lipoprotein (LDL) receptors by several types of neoplastic cells have been suggested to be important determinants of selective retention of photosensitizers by proliferative tissues. The interactions of chlorin e6 (Ce6), a photosensitizer bearing three carboxylic groups, with plasma proteins and DOPC unilamellar vesicles are investigated by fluorescence spectroscopy. The binding constant to liposomes, with reference to the DOPC concentration, is 6 x 10(3) M(-1) at pH 7.4. Binding of Ce6 to LDL involves about ten high affinity sites close to the apoprotein and some solubilization in the lipid compartment. The overall association constant is 5.7 x 10(7) M(-1) at pH 7.4. Human serum albumin (HSA) is the major carrier (association constant 1.8 x 10(8) M(-1) at pH 7.4). Whereas the affinity of Ce6 for LDL and liposomes increases at lower pH, it decreases for albumin. Between pH 7.4 and 6.5, the relative affinities of Ce6 for LDL versus HSA, and for membranes versus HSA, are multiplied by 4.6 and 3.5, respectively. These effects are likely driven by the ionization equilibria of the photosensitizer carboxylic chains. Then, the cellular uptake of chlorin e6 may be facilitated by its pH-mediated redistribution within the tumor stroma.
authors.affiliation: Laboratoire de Biophysique Moléculaire Cellulaire and Tissulaire (BIOMOCETI) CNRS UMR 7033, Université Pierre and Marie Curie, Genopole Campus 1, 5 rue Henri Desbruères, 91030 EVRY cedex, France.
authors.forename: Halina
authors.initials: H
authors.lastname: Mojzisova
authors.forename: Stephanie
authors.initials: S
authors.lastname: Bonneau
authors.forename: Christine
authors.initials: C
authors.lastname: Vever-Bizet
authors.forename: Daniel
authors.initials: D
authors.lastname: Brault
mesh_terms: D001798:Blood Proteins / Q000737:chemistry / Q000378:metabolism*; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism*; D002735:Chlorophyllides; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010713:Phosphatidylcholines / Q000737:chemistry / Q000378:metabolism; D011166:Porphyrins / Q000737:chemistry / Q000378:metabolism*; D011838:Radiation-Sensitizing Agents / Q000737:chemistry / Q000378:metabolism*; D012709:Serum Albumin / Q000737:chemistry / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D053835:Unilamellar Liposomes / Q000737:chemistry / Q000378:metabolism
chemical_list: D001798:Blood Proteins; D002735:Chlorophyllides; D010713:Phosphatidylcholines; D011166:Porphyrins; D011838:Radiation-Sensitizing Agents; D012709:Serum Albumin; D053835:Unilamellar Liposomes; C062985:phytochlorin; C017251:1,2-oleoylphosphatidylcholine