Pholasin is classified as a photoprotein and comprises apoPholasin (an apoprotein of pholasin) and an unknown prosthetic group as the light-emitting source. The luminescence reaction of pholasin is triggered by reactive oxygen species. Recombinant apoPholasin was recently expressed as a fusion protein of glutathione S-transferase (GST-apoPholasin) and purified from E. coli cells. By incubating non-fluorescent dehydrocoelenterazine (dCTZ, dehydrogenated form of CTZ) with GST-apoPholasin, the complex of GST-apoPholasin and dCTZ (GST-apoPholasin/dCTZ complex) was formed immediately and showed bright yellow fluorescence (lambda(max) = 539 nm, excited at 430 nm). Unexpectedly, the fluorescent chromophore of the GST-apoPholasin/dCTZ complex was identified as non-fluorescent dCTZ. The luminescence intensity of the GST-apoPholasin/dCTZ complex was increased in a catalase-H2O2 system, but not in sodium hypochlorite. (C) 2020 Elsevier Inc. All rights reserved.