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SCIENCE 



[N. S. Vol. XLVI. No. 1184 



two distinct chemical substances, besides water 

 and oxygen, are necessary for light production. 

 One of these is not destroyed by heat and is 

 easily dialyzable; it can be prepared by ex- 

 tracting luminous animals with hot water. 

 This substance has been termed luciferin by 

 Dubois'- and photophelein by myself.^ The 

 second substance is destroyed by heat and does 

 not dialyze; it can be prepared by allowing a 

 water extract of the luminous organ to stand 

 until the light disappears. This has been 

 called luciferase by Dubois^ and pJiotogenin 

 by myself.^ Whenever solutions (non-lumi- 

 nous) of these two substances are mixed, light 

 immediately appears and is brighter the 

 greater the concentration of the solutions. 



According to Dubois, the thermolabile sub- 

 stance, luciferase (photogenin) is an oxidizing 

 enzyme (hence the termination ase), which 

 oxidizes^ the thermostable substance, luciferin 

 (photophelein), which is therefore the source 

 of the light. My own work has led me to be- 

 live that the thermolabile substance is not an 

 enzyme, but is itself the source of the light 

 and I have indicated this by calling it photo- 

 genin (phos, light; gennao, produce). The 

 thermostable substance is, according to my 

 view, a material which assists in the produc- 

 tion of light and I have indicated this by call- 

 ing it photophelein (phos, light; opheleo, as- 

 sist). 



"Which is the soiu'ce of the light, photo- 

 genin (luciferase) or photophelein (luciferin) ? 

 Fortunately the question can be answered 

 by a simple crucial experiment. The two 

 common eastern genera of fireflies produce 

 light of different colors. Photinus emits an 

 orange light, while Photuris emits a green- 

 ish yellow light. The difference in color is 

 especially noticeable when the luminous or- 

 gans of the two species are groimd up in sep- 

 arate mortars. As shown by Ooblentz,' the 



1 Dubois, R., C. B. Soo. Biol., 1885, XXXVII., 

 559, and Ann. de la Soc. Linn, de Lyons, 1913, LX., 

 and 1914, LXI., 161. 



2 Harvey, E. N., Science, N. S., 1916, XLIV., 

 652, and Amer. Jour. Physiol, XLII., 318, 1917. 



3 Coblentz, W. W., Carnegie Inst. Wash. Pub. No. 

 164, 1912. 



difference in color is real; the spectrum of 

 Photinus extending further into the red than 

 that of Photuris. The two light-producing 

 substances can be prepared from each of the 

 two species, and the photogenin of Photinus 

 mised with its own photophelein gives an 

 orange light, while the photogenin of Photuris 

 mixed with its own photophelein gives a green- 

 ish-yellow light, the color characteristic of the 

 species. The two genera may also be " inter- 

 crossed " with respect to the two light-pro- 

 ducing substances, i. e., the photogenin of 

 Photinus gives light with the photophelein of 

 Photuris and vice versa. If the source of 

 light is photophelein (luciferin) as Dubois 

 believes, the light produced by Photinus photo- 

 phelein (luciferin) X Photuris photogenin (lu- 

 ciferase) should be orange, the color ehar- 

 ^acteristic of Photinus. I have found, on the 

 contrary, that the light from this " cross " is 

 greenish yellow. Conversely, the light from a 

 mixture of Photinus photogenin (luciferase) 

 and Photuris photophelein (luciferin) is or- 

 ange. The color of the light in these " crosses " 

 is that characteristic of the animal supplying 

 photogenin (luciferase). The photogenin (lu- 

 ciferase) must, therefore, be the oxidizable 

 substance and the source of the light. 



How does photophelein assist in the pro- 

 duction of light? The process is best studied 

 in the marine ostracod crustacean, Cypridina 

 hilgendorfii. The photogenin and photophe- 

 lein of this animal are secreted into the sea 

 water together, and in time the photophelein 

 is used up and a perfectly clear colorless non- 

 luminous solution of photogenin remains. If 

 we add to such a concentrated solution, photo- 

 phelein or certain specific substances in ex- 

 tracts of non-luminous forms, or fat solvents 

 such as ether, chloroform and higher alcohols, 

 or thymol, saponins, soaps, bile salts, or crys- 

 tals of inorganic salts, such as N'aCl, light 

 appears. Many of these substances are not 

 oxidizable (another proof of the inadequacy of 

 Dubois's theory), but all of them are cytolytic 

 agents. The cytolytic action of these sub- 

 stances on cells is the result of a dissolving 

 action on the cell surface involving an in- 

 creased dispersion of the colloids which results 



