The fluorescent pH probe carboxy-seminaphtorhodafluor-1 (C-Snarf-1) has been used for
laser microspectro-fluorometric assays of intracellular pH in 3T3 mouse fibroblasts treated with
hypocrellin A. These results are compared to those previously obtained with the structurally related hydroxylated polycyclic
quinone,
hypericin (Sureau et al., J. Am. Chem. Soc. 118, 9484-9487, 1996). A mean local intracellular pH drop of 0.6 units has been observed in the presence of 1 microM
hypocrellin A after 90 s of exposure to 0.1 microW of
laser irradiation at 514.5 nm. The time evolution of the cytoplasm acidification for
hypocrellin A-treated cells is faster than that for cells treated by
hypericin. Thus, release of
protons from an excited state of
hypocrellin A appears to be more efficient than that from
hypericin. In addition, the pH dependence of the quenching of C-Snarf-1 fluorescence in 3T3 cells under continuous irradiation has been observed. It is shown here that under continuous illumination, a pH decrease is able to induce a modification of the intracellular binding equilibrium of C-Snarf-1 that results in an increase of C-Snarf-1 fluorescence intensity. This latter observation suggests that the
protons generated upon the photoexcitation of
hypericin or its analogs may be involved in the production of other photoreactive species. Finally, we suggest that, just as for
hypericin, this pH drop may be involved in the
antiviral and antitumor activity of
hypocrellin A.