The Nature of Animal Light

by E. Newton Harvey

[10] Light disappears quickly because of the effect of the acidity on the luciferase.

It is possible that the action of bacteria (which produces CO₂), muscle tissue (which contains lactic acid), milk (in which lactic acid may be formed by bacteria), or Mg + acid, in forming luciferin, is not the result of their reducing power but of their acidity. Fortunately we can test this matter by the use of reducing fluids which are not acid. If they also form luciferin from oxyluciferin, a reduction must occur. Nascent H can be generated by the action of NaOH on Al, or when finely divided Mg or Zn

or Al is placed in water. With Mg the water becomes only slightly alkaline from formation of almost insoluble Mg(OH)₂. If we add some Al powder and dilute NaOH to an oxyluciferin solution, H is given off and luciferin is formed. As oxyluciferin cannot be formed by the addition of alkali alone we must have in this experiment a reduction of oxyluciferin in alkaline medium by the nascent H produced. Luciferin can also be formed by merely adding Al or Zn or Mg dust to an oxyluciferin solution. Methylene blue can also be readily reduced to its leuco-base by Zn dust or Al + NaOH.

Indeed, if one adds some Al or Zn or Mg powder to a solution of luciferase, light will appear whenever the solution is shaken. Luciferase solution must always contain the oxidation product of luciferin, oxyluciferin. In presence of nascent H this is reduced to luciferin, and since the reaction of the medium is alkaline and luciferase is present this is oxidized with light production, when, by shaking, air is dissolved. The light can never become very bright except at the surface because of the deficiency of oxygen in the solution. It would seem, then, that the action of bacteria, yeast, muscle cells, etc., on oxyluciferin must be due not entirely to their acid reaction but to their reducing power as well.

The above experiment is a very striking and instructive one. Given a test tube of luciferase solution containing, as it does, oxyluciferin, add some Zn dust or Mg powder, and the evolution of hydrogen begins. Conditions are now favorable for the reduction of oxyluciferin and this occurs. Shake the contents of the tube to dissolve oxygen and light appears. Allow the tube to stand and the light soon disappears. Shake again and the light reappears. The lumi

nescence reduction and oxidation process can be demonstrated many times.

A similar experiment can be performed with luciferase and oxyluciferin solution by addition of NH₄SH. This will serve also as another example of the reduction of oxyluciferin in an alkaline medium. Whenever we shake a tube of luciferase, oxyluciferin and NH₄SH, light will appear. When the tube is at rest it becomes dark. Even the merest touch is sufficient to agitate the tube contents, cause solution of oxygen and appearance of light. It is just as if we stimulate the tube to produce light and I believe the phenomenon has a deeper significance and a more fundamental similarity to the phenomena of stimulation than may at first appear. What more simple means of controlling a process can we think of than by admission or withdrawal of oxygen? The firefly turns on its light by stimulation through nerves of the luminous organ. Noctiluca flashes on stimulation of any kind, even the slightest agitation causing a brilliant emission of light. If the stimulation process means merely the admission of oxygen to the photogenic cells we have a mechanism in the cell itself for automatically producing the light. The admission of oxygen results in aërobic conditions and luciferin in presence of luciferase can then oxidize to oxyluciferin with luminescence. When the oxygen is used up, the light ceases, anaërobic conditions prevail, and the oxyluciferin is reduced to luciferin again. Thus, luciferin is reformed during the rest period of Noctiluca or between the flashes of the firefly. What more efficient type of light than this is to be desired?

Again, methylene blue offers an interesting parallel to

oxyluciferin. A little NH₄SH added to methylene blue solution will reduce (decolorize) it to the leuco-base. If the tube is now shaken the blue color returns. On standing reduction again occurs. The process can be repeated a number of times, the reaction going in one or the other direction, depending on the oxygen content of the mixture.

As methylene blue contains no oxygen, its reduction consists in the addition of two atoms of hydrogen. When leuco-methylene blue oxidizes, water is formed by the union of these two atoms of hydrogen with oxygen, thus: