Luciferase activity of the intracellular microcrystal of the firefly squid, Watasenia scintillans

The arm light organ of the firefly squid, Watasenia scintillans, emits extremely bright flashes of light, which are caused by a luciferin–luciferase reaction involving ATP, Mg2+ and molecular oxygen. The molecular mechanism underlying the bioluminescence reaction has remained unresolved, because the luciferase could not be identified or isolated. The arm light organ contains numerous rod-like bodies that are 2–6 μm long and 1–2 μm thick. This paper addresses the characterization of the extracted rod-like body. We found that the rod-like bodies emit the light in vitro by the luciferin–luciferase reaction. Furthermore, by using the X-ray powder diffraction method, we confirmed that the rod-like bodies are well-ordered microcrystals.

► The rod-like bodies in the arm light organ of the firefly squid emit light in vitro.
► The rod-like body is well-ordered microcrystals in the X-ray powder diffraction.
► The microcrystal includes luciferin, luciferase and unknown proteins.
► The firefly squid’s luciferase can carry out bioluminescence just in crystal form.