While I have not studied the properties of oxyluciferin as fully as those of luciferin, so far as I can judge, both substances give the same general reactions and possess identical properties. Both crude luciferin and crude oxyluciferin solution are yellow in color, but I do not believe that either pure luciferin or oxyluciferin are yellow in color, because an ether or benzine extract of Cypridina is also yellow, although luciferase, luciferin, and oxyluciferin are insoluble in ether and benzine. The yellow pigment which can be observed to make up part of the luminous gland of Cypridina is not luciferin or luciferase. It may be a pigment related to urochrome.
When tests are applied and precipitating reagents are added to crude luciferin and crude oxyluciferin solution, they give identical results in each case. A more complete account of the chemistry of luciferin has been given in this chapter, and there is no need of duplicating these statements regarding oxyluciferin. Like luciferin, the oxyluciferin will pass porcelain filters, dialyze through parchment or collodion membranes, and is undigested by salivary diastase, pepsin HCl, Merck's pancreatin in neutral solution, and erepsin. The salivary diastase and the pancreatin (containing amylopsin, trypsin, and lipase) were allowed to digest for four days at 38° C. without showing any evidence of digestive action.
As luciferin is so easily oxidizable a substance, we should expect to find that it will reduce just as glucose will reduce. However, a concentrated solution of luciferin has no reducing action on Fehling's (alkaline Cu), Barfoed's (acid Cu), Nylander's (alkaline Bi) or Knapp's (alkaline Hg) reagent. Glucose will reduce methylene blue in alkaline (not in neutral solution), but luciferin will not reduce methylene blue in alkaline or neutral solution. It would seem, then, that luciferin must contain no aldehyde group. If so, we should expect to obtain reduction of some of the above reagents. Just what group is concerned in the oxidation is unknown at the present time, and in the absence of more experimental data, speculation regarding it can be of little value.
SUMMARY
In summing up we may say that the luminescence of at least three groups of luminous animals, the beetles, Pholas, and Cypridina, has been definitely shown to be due to the interaction of two substances, luciferin and luciferase, in presence of water and oxygen. Luciferin and luciferase have quite different properties and may be easily separated from each other by various chemical procedures. As the luciferins and luciferases from different luminous animals have somewhat different properties, they may be designated by prefixing the generic name of the animal from which they are obtained.
Cypridina luciferin differs from Pholas luciferin in that it can not be oxidized with light production by KMnO4, H2O2, with or without hæmoglobin, or similar oxidizing agents. Cypridina luciferase differs from Pholas and firefly luciferase in that it is not readily
destroyed by the fat-solvent anæsthetics, such as chloroform, ether, etc.
When Cypridina luciferin is oxidized, no fundamental splitting of the molecule occurs, because the product, oxyluciferin, can be readily reduced to luciferin again. This reduction is brought about under conditions similar to those necessary for the reduction of dyes, such as methylene blue. Oxyluciferin can be reduced to luciferin, which will again give light with luciferase, by the reductases of muscle tissue, liver, etc., or by bacteria; by Schardinger's enzyme of milk; by H2S; by the nascent hydrogen from the action of acetic acid on magnesium or of water or NaOH on aluminium, zinc or magnesium; and by palladium black and sodium hypophosphite, all well-known reducing methods. Reduction of oxyluciferin no doubt occurs even in presence of luciferase if oxygen is absent, and reduction of oxyluciferin no doubt occurs in animals which burn luciferin within the cell, thus tending for conservation of material. Dilute alkali favors oxidation and dilute acid favors the reduction. Light favors the reduction of oxyluciferin.
Apparently luciferin and oxyluciferin have identical chemical properties. Neither is digested by the enzymes: malt diastase, ptyalin, yeast invertase, pepsin, trypsin, steapsin, amylopsin, rennin, erepsin, urease or enzymes occurring in the water extracts of dried spleen, kidney, or liver. Luciferase is destroyed only by pepsin (probably), trypsin, erepsin, and something in spleen and liver extract.