Bioluminescence in scale-worm photosomes: the photoprotein polynoidin is specific for the detection of superoxide radicals.

Photosomes are the characteristic organelles of the luminous epithelium in the elytral appendages of polynoïd annelids. They are paracrystals of endoplasmic reticulum and emit a flash of bioluminescence in response to stimulation. The series of flashes in response to repetitive stimulation begins with a period of facilitation because the number of reacting photosomes increases in each photogenic cell. Reacting photosomes are coupled to the plasma membrane by dyad junctions which are established under stimulation and dedifferentiate in the resting system. The calcium influx of an action potential propagated through the conducting elytral epithelium triggers the luminous reaction. This reaction is based on a membrane photoprotein, polynoidin, which is specifically triggered by superoxide radicals. These oxy radicals result from the oxydation of riboflavin, which is present in a compartment of the photosomes. Polynoidin proved to be an interesting probe in the detection of superoxide radicals produced by activated white blood cells. Its potential applications are discussed.