BERNARD L. STREHLER 243 



Regarding the question as to which substance among a number 

 necessary for luminescence is to be regarded as luciferin, I would like 

 to quote from a paper by Harvey and Tsuji, entitled "Luminescence 

 of Cypridina luciferin without luciferase, together with an appraisal 

 of the term, luciferin," now in press in the Journal of Cellular and 

 Comparative Physiology: 



Rather than placing the emphasis on a limiting factor, or on heat stability 

 or dialyzability or even oxidizability [as an indication of luciferinl, as has 

 been done previously, light emission should be the criterion. In the case 

 of luminous organisms requiring dissolved molecular oxygen for lumines- 

 cence, luciferin may properly be defined as the oxidizable substance sup- 

 plying molecules capable of absorbing enough excess emergy to emit in the 

 visible region. Such a definition implies that some form of luciferin molecule 

 — either free base or acid, either dissociated anion or cation, in reduced or 

 oxidized form, either free or combined with protein, like a prosthetic en- 

 zyme group — can pick up the energy of the oxidative reaction in which it 

 is involved. Such a definition does not mean that luciferin is the same sub- 

 stance in different luminous animals, nor does it necessarily designate 

 luciferin molecules themselves as the ones which emit, but it does imply 

 that a related molecule, such as a luciferin-luciferase combination, or an 

 oxidized luciferin molecule, or a molecule of an intermediate step, is the 

 emitter. The molecule actually emitting might be referred to as the photo- 

 gen. ... It has long been recognized that a substance, whose molecules 

 are readily excited to fluoresce by the energy of radiation, is most likely 

 to be chemiluminescent from the energy of a chemical reaction. . . . 

 Therefore it seems most logical to regard the reduced form of the fluorescent 

 flavin as bacterial luciferin rather than the non-fluorescent aldehyde. Al- 

 though riboflavin and FMN are not fluorescent in the reduced form, the 

 oxidized flavins fluoresce yellow green over a wide pH range, from pH 1 to 

 11. An analogous situation is to be observed among the ctenophores, where 

 a striking phenomenon is the fluorescence of the luminous organ after, but 

 not before bioluminescence has occurred, as if the final product of lumi- 

 nescence was a fluorescent molecule (Harvey, 1925). 



Reduced FMN is comparable to Cypridina luciferin in that it under- 

 goes spontaneous oxidation by dissolved oxygen without luminescence 

 and only emits light in the presence of what may be called bacterial 

 luciferase. 



References 



Harvey, E. N. 1925. Studies on bioluminescence XVII. Fluorescence and 

 inhibition of luminescence in ctenophores by ultraviolet light. /. Gen. 

 Physiol, 7, 331-39. 



