J. W. HASTINGS AND W. D. McELROY 263 



DuBois ( 1887 ) described lucif erin as a substance in the heat stabile 

 fraction of Pholas extracts which produced hght when an unheated 

 dark fraction containing the enzyme was added. McEboy and Harvey 

 (1951) showed that more than one substance could produce this 

 "luciferin" reaction in fireflies. Harvey (1920) defined luciferin ex- 

 tracted from Cypridum as "the heat resistant dialyzable substance 

 which takes up oxygen and oxidizes with light production in the 

 presence of . . . luciferase." Although he emphasized the chemical 

 dissimilarities of luciferins extracted from different organisms, Cypri- 

 dina luciferin serves as a model for the general mechanism of biolumi- 

 nescent reactions, where the overall reaction is written: 



LH2 + iOs -> L* + HoO 

 L* ^ L + light 



Luciferin in this scheme (LHo) embraces at least two features: (1) 

 it is (or forms) the Hght-emitting molecule, and (2) it is the reduced 

 compound which is split in the terminal reaction with oxygen, releas- 

 ing sufficient energy for light emission. 



Although no compound with a completely analogous role has yet 

 been demonstrated in either the firefly or bacterial systems, the essen- 

 tial feature of the above scheme is the designation of luciferin as the 

 substrate in the terminal oxidation. Studies by McElroy and colleagues 

 indicate that in the firefly system the compound as isolated is more 

 strictly a proluciferin and must undergo preliminary dark reactions, 

 possibly involving energy transfer from ATP, before its final oxidative 

 reaction. Similar reactions may also occur in the bacterial system. In 

 addition, it is possible that luciferin might not be directly involved in 

 light emission. That is to say, the energy from the oxidative split of 

 luciferin might be transferred to and activate another molecule in the 

 system, which would itself be designated as the light emitter. 



These ideas are in accord with the fact that it has not been pos- 

 sible to reverse the luminescent oxidation of luciferin to reform active 

 luciferin. That the reaction involves a rather drastic split of luciferin 

 might be expected on energetic grounds, since energy sufficient for 

 the emission of a quantum of light is derived from the reaction. It 

 would therefore be expected that, if the general mechanism of bio- 

 luminescence involves the irreversible oxidation of luciferin, the total 



