Effects of scavengers for active oxygen species on photoresponse and photodynamic damages to a pigmented protozoan, Blepharisma

In the absence of O2, the step-up photophobic response of Blepharisma japonicum is suppressed. To evaluate the possibility that reactivated oxygen species (ROS) produced by blepharismin photosensitization are involved in the primary transduction for the step-up photophobic response of B. japonicum, the effects of OH radical scavengers and the singlet oxygen quencher on the photophobic response were examined. The reagents used in the present assays hardly suppressed the step-up photophobic response, even though OH radical scavengers were rescued from the photodynamic killing of the cells. Alternatively, in the presence of an electron acceptor, the latency of the step-up photophobic response was prolonged. In addition, oxyblepharismin fluorescence was quenched slightly in the presence of free amino acids. These results support a hypothesis proposed by Lenci's research group, that electron transfer from the first excited singlet state of blepharismin (or oxyblepharismin) to associated proteins may be involved in the primary phototransduction for the step-up photophobic response.