|
Cardiolipin is a unique phospholipid containing two phosphatidyl glycerol moieties and four fatty acids per molecule. It
is found exclusively in the mitochondrial inner membrane and at the contact sites between the inner and outer
membranes. The acridine derivative, nonyl-acridine orange (NAO), is a highly specific probe of cardiolipin, with a
binding affinity approximately two orders of magnitude greater than that for binding to other anionic phospholipids. We
recently reported that when NAO is bound in the mitochondria of human prostate cancer PC-3 cells and activated at 488
nm, NAO could transfer fluorescence resonance energy to the phthalocyanine photosensitizer Pc 4. This observation
indicates that one site of Pc 4 binding is very near to NAO and therefore very near to cardiolipin. The average distance
between the two fluorophores was calculated to be 7 nm. In the present study, we have extended the observation to the
endogenously synthesized photosensitizer, protoporphyrin IX, an intermediate in heme biosynthesis that is used for
photodynamic therapy of several types of malignant and non-malignant conditions. Protoporphyrin IX is generated in
the mitochondria but is known to bind to other cellular sites as well, especially the endoplasmic reticulum. The ability of
this molecule to accept resonance energy from NAO in cells is consistent with a localization of at least some of the
molecules in the mitochondria either on the inner membrane, the site of cardiolipin, or within about 10 nm of it. Since
protoporphyrin IX binds with high affinity to the peripheral benzodiazepine receptor, a component of the permeability
transition pore complex that forms at contact sites between the inner and outer membranes, our observations provide
evidence for the close association of several critical molecules for mitochondrial functions and suggest that cardiolipin
may be an early oxidative target during PDT with at least two photosensitizers.
|
