Luciferase activities were determined in triplicates using substrates of decanal and FMNH2 (reduced by Cu+1) [14] and expressed as the bioluminescence peak intensity in arbitrary light unit. To examine the effect of oxygen concentration on luciferase activity, 0.1 mL standard buffer con- taining 0.1 mM FMN and 0.1% decanal was placed in a reaction vial and 1 lL of a saturated CuClO4 solution in acetonitrile was added to reduce the flavin.