The Dependence of Photosensitizing Efficacy of Acridine Orange and Toluidine Blue on the Degree of Sensitizer‐cell Interaction

Previous work of photosensitizing action in vivo has commonly been performed under the ‘steady’ conditions where the sensitizing dye is equilibrated with the cells by allowing ample time for the sensitizer‐cell interaction prior to light illumination. The present experiments, by combining a fast mixing technique (20 ms) with short illumination (mostly & < 10 s), intended to measure the change in the efficacy of the sensitizing action on yeast cells during a short period of time starting from the mixing of the two components, sensitizers and cells. We found that the efficacy of the sensitizing action of acridine orange (AO) began to increase within 20 s and continued to do so until at least 10 min. This was true both for the inactivation and the induction of genetic changes. Similar experiments with toluidine blue (TB), which is believed to be normally unpenetrable into the cell, showed no such fast change characterized by a time of less than 60 s, but unexpectedly revealed a small but significant increase in the efficacy after 20–30 min of mixing. This slow change may indicate that TB can penetrate slowly into the hydrophobic region in the membrane, and as a result, it exerts action with a higher efficiency than it remains in the outside medium. The increased induction of genetic changes beyond control level with the fast (& < 60s)TB sensitization in H 2 O and D 2 O medium is discussed in terms of the diffusiveness of 1 O 2 into the cell interior.