the unsaturated organic compounds that constitute a part of natural systems of bioluminescence.

Granules containing luciferins are found in the cytoplasm of the photogenic (luminescent) cells of luminescent organisms. An oxidized form of luciferin, holding a larger energy reserve, forms under the influence of atmospheric oxygen in the presence of special enzymes called luciferases. The transition of the oxidized form from the excited state back to the original state is accompanied by the emission of light quanta. The most widely studied luciferins are heterocyclic compounds (substituted thiazoline carboxylic acid in beetles of the genus Photinus; oxidized tripep-tide in crayfish of the genus Cypridina; an indol derivative in sea feathers of the genus Renilla). Certain others are terpenoids (for example, in the mollusk Latia neritoides). In luminescent bacteria, the luciferins are flavin mononucleotides in combination with fatty aldehydes of the palmitic series. The enzymic oxidation of luciferins in insects occurs in several stages in the presence of adenosine triphosphate and magnesium ions.