Bioluminescence in the sea: photoprotein systems

Photoproteins are the primary reactants of the light-emitting reactions of various bioluminescent organisms. A photoprotein emits light in proportion to its amount, like a luciferin, but its light-emitting reaction does not require a luciferase. There are about two dozen types of bioluminescent organisms for which substantial biochemical knowledge is presently available, and about one third of them involve photoproteins. Most photoproteins are found in marine organisms. There are various types of photoproteins: the photoproteins of coelenterates, ctenophores and radiolarians require Ca2+ to trigger their luminescence; the photoproteins of the bivalve Pholas and of the scale worm appear to involve superoxide radicals and O2 in their light-emitting reactions; the photoprotein of euphausiid shrimps emits light only in the presence of a special fluorescent compound; the photoprotein of the millipede Luminodesmus, the only known example of terrestrial origin, requires ATP and Mg2+ to emit light. The Ca2+-sensitive photoproteins of coelenterates have been most frequently studied and most widely used. Therefore, they are overwhelmingly popular compared with other types. All coelenterate photoproteins, including aequorin, halistaurin, obelin and phialidin, have relative molecular masses close to 20 000, contain an identical functional group, and emit blue light in aqueous solution when a trace of Ca2+ is added, in the presence or absence of molecular oxygen. Aequorin contains an oxygenated form of coelenterazine in its functional group. When Ca2+ is added, aequorin decomposes into three parts, i.e., apo-aequorin, coelenteramide and CO2, accompanied by the emission of light. Apo-aequorin can be reconstituted into active aequorin indistinguishable from the original sample, by incubation with an excess of coelenterazine in a buffer containing 5 mM-EDTA and a trace of 2-mercaptoethanol, even at 0 degree C. Thus, aequorin and other coelenterate photoproteins can be luminesced and recharged repeatedly. The regeneration of coelenterate photoproteins in this manner probably takes place in vivo, utilizing stored coelenterazine. The photoproteins of coelenterates, and their chemically modified forms, are useful in measuring and monitoring calcium ions in biological systems, especially in single cells.