Abstract— Stentorin acts as the photoreceptor for the step‐up photophobic and negative phototactic responses in Stentor coeruleus. The chromophore of stentorin appears to be hypericin which is linked to apoprotein. In addition to the photomovement responses of the organism, S. coeruleus was found to be photodynamically sensitive to light absorbed by the hypericin chromophore, as the apparent action spectrum for the photodynamic killing matches the absorption spectrum of stentorin. The protective effect of β‐carotene and crocetin on the photodynamic killing of S. coeruleus suggests that singlet oxygen generated by the stentorin‐sensitization plays an important role, according to the so‐called Type II mechanism of photosensitization. The generation of singlet oxygen via hypericin triplet was confirmed by in vitro photooxidation of tryptophan as a substrate. The photodynamic killing was more effective in deuterium oxide than in H2O in both the photosensitization by stentorin (endogenous) and added hypericin (exogenous). These results are consistent with the involvement of singlet oxygen in the photodynamic killing of S. coeruleus.

decreased flexibility provides a basis to explain the low affinity of ONC towards nucleotides and, more generally, its lower catalytic activity. The results also suggest the basis of the unusually high thermal stability of the enzyme.[1] [1] Merlino A., et al., J. Biol. Chem., accepted for publication.