Taurine Esculin
I believe that in these experiments Dubois has been working with an oxidation product of luciferin, what I have called oxyluciferin, rather than a pro-substance. The mode of preparation of Pholas preluciferin and Pholas co-luciferase is such as could be used in the preparation of Cypridina oxyluciferin, and it seems more logical to look for the presence of Pholas oxyluciferin in one or both of Dubois' extracts rather than believe that luciferin can be formed from both taurine and esculin. When the co-luciferase solution stands with the preluciferin solution we would in reality have not the formation of luciferin from preluciferin, but the formation of luciferin from oxyluciferin, by some reducing agent in the mixture. Indeed, in a very recent paper Dubois (1919c) takes the
view that his co-luciferase is a reducing enzyme which forms luciferin by reduction (presumably from oxidized luciferin) and no mention is made of preluciferin.
It is, of course, obvious that when luciferin oxidizes, some oxidation products must be formed. Most observers have assumed the oxidation products of luciferin to be relatively simple and to represent a rather complete breaking down of the luciferin molecule. Carbon dioxide was mentioned by Phipson (1872) as being formed. We have just seen that no carbon dioxide is formed during the oxidation of Cypridina luciferin and there is evidence that no fundamental change at all occurs. It is for this reason that I have called the oxidation product of luciferin oxyluciferin.[7] As we shall later see, the change luciferin oxyluciferin is to be compared to the oxidation of colorless dyes (leuco-compounds) to the colored dye. The chemical properties of oxyluciferin are similar to those of luciferin and the oxyluciferin can be readily reduced to luciferin again.
[7] is unfortunate that Dubois (1918 b) has used the term oxyluciferine in a quite different sense from the present use. He regards oxyluciferine as a substance still capable of giving light by autoöxidation, and represents the steps in luminescence as follows:
| "Co-luciférase | + preluciférine | = luciférine. |
| Luciférase | + luciférine | = oxyluciférine. |
| Oxyluciférine | + oxygéne | = lumiérè." |
I should represent them as follows:
Luciferin + oxygen ⇆ oxyluciferin.
The reaction proceeds to right with light production only in presence of luciferase.