106 Papers from the Department of Marine Biology. 
by trypsin,as it is difficult to obtain a tryptic digest of protein which 
does not give the biuret reaction,and the work of Fischer and Abder- 
halden has shown that certain artificial polypeptids are not digested 
by pure activated pancreatic juice. We have, then, three possibilities: 
luciferin is (1) either a natural proteose not attacked by trypsin, or 
(2) if attacked by trypsin, its decomposition products (presumably 
amino acids) still contain the group oxidizable with light production, 
or (3) it is not a protein at all. I believe that luciferin has too many 
protein characteristics to conform to the last possibility. I have been 
unable to oxidize with light production various mixtures of amino acids 
(from beef and casein) by means of luciferase and consequently am led 
to believe that luciferin is a natural proteose soluble in absolute alcohol 
and not digestible by trypsin. 
Dubois believes Pholas luciferin to be a natural albumin with acid 
properties. Cypridina luciferin could not possibly be regarded as an 
albumin, but it is very likely that the luciferins of different species of 
luminous animals differ in certain characteristics. Just as in the case 
of the luciferases, we know that the luciferins are not identical sub- 
stances,and only future work can determine in what particulars they 
differ. 
SUMMARY. 
Under the term photophelein I have previously included two different 
substances. One of these is a body, found only in Cypridina hilgen- 
dorfiit, which oxidizes spontaneously without light production in the air 
and corresponds to the luciferin found by Dubois in Pholas dactylus. 
It is dialyzable, thermostable, and may be called Cypridina luciferin. 
In the presence of non-dialyzing thermolable Cypridina luciferase 
(corresponding to my old term photogenin) it oxidizes with light pro- 
duction. Cypridina luciferin differs from Pholas luciferin in that it can 
not be oxidized with light production by KMnO,, H2O», with or without 
hemoglobin, or similar oxidizing agents. The other substance is found 
in many non-luminous animals, is also thermostable, and assists in pro- 
moting the luciferin-luciferase reaction. For this substance I propose 
to retain my old term photophelein. Cypridina luciferin will give no 
light with Pholas luciferase and Pholas luciferin will give no light with 
Cypridina luciferase. 
When Cypridina luciferin is oxidized, nofundamental splitting of the 
molecule occurs, because the product, which I propose to call oxy- 
luciferin, can be readily reduced to luciferin again. This reduction is 
brought about under conditions similar to those necessary for the reduc- 
tion of dyes such as methylene blue. Indeed, the change luciferin—> 
oxyluciferin appears to be very similar to the change leuco-methylene 
blue— methylene blue. Oxyluciferin can be reduced to luciferin, which 
will again give light with luciferase by the reductases of muscle tissue, 
