MODIFICATION OF THE PHOTODYNAMIC-ACTION OF DELTA-AMINOLEVULINIC-ACID (ALA) ON RAT PANCREATOMA CELLS BY MITOCHONDRIAL BENZODIAZEPINE RECEPTOR LIGANDS

We have shown that addition of exogenous delta-aminolaevulinic acid (A LA) to rat pancreatoma AR4-2J cells in culture leads to the increased production of porphobilinogen (PEG) and the accumulation of photoactiv e protoporphyrin IX (PPix) in these cells. Exposure to light (lambda>4 00 nm) at an intensity of 0.2 mW cm(-2) for 8 min resulted in an ALA d ose-dependent cytolysis of the cells, with an EC(50) of 6.6 +/- 0.7 mu M. This cytolytic effect was light intensity dependent, with greater cell destruction after exposure to light at an intensity of 0.47 mW cm (-2) than at 0.2 mW cm(-2); it was also dependent on the duration of i llumination, cell survival decreasing with increasing illumination tim es. The photodestruction of the AR4-2J cells following exposure to ALA can be attributed to the production of endogenous PPix, a photoactive porphyrin that we have shown to generate singlet oxygen upon illumina tion, whereas ALA itself does not. Further investigation of the molecu lar mechanisms underlying the photodynamic action of ALA demonstrated the involvement of the mitochondrial (peripheral) benzodiazepine recep tor (MBR), a high-affinity recognition site for dicarboxylic porphyrin s, and especially PPix. The centrally acting benzodiazepine compounds clonazepam and flumazenil, which have negligible affinities for the MB R, bad no effect on ALA-mediated phototoxicity. In contrast, bath the isoquinoline carboxamide PK11195 and the benzodiazepine Ro 5-4864 liga nds, displaying a high affinity for the MBR, did affect ALA-mediated p hototoxicity, each markedly increasing the EC(50) for cell photodestru ction and thus exerting a photoprotective effect. It is concluded that the MBR may play an important role in the expression of ALA-mediated PPix phototoxicity and that MBR ligands, by diminishing the actions of endogenous PPix, have the potential to rescue cells from porphyrin-in duced photolysis.