Studies in bioluminescence. IV. Properties of luciferin from Pholas dactylus

A novel yellow fluorescent protein of recombinant apoPholasin with dehydrocoelenterazine

Pholasin is classified as a photoprotein and comprises apoPholasin (an apoprotein of pholasin) and an unknown prosthetic group as the light-emitting source. The luminescence reaction of pholasin is triggered by reactive oxygen species. Recombinant apoPholasin was recently expressed as a fusion protein of glutathione S-transferase (GST-apoPholasin) and purified from E. coli cells. By incubating non-fluorescent dehydrocoelenterazine (dCTZ, dehydrogenated form of CTZ) with GST-apoPholasin, the complex of GST-apoPholasin and dCTZ (GST-apoPholasin/dCTZ complex) was formed immediately and showed bright yellow fluorescence (λ_max = 539 nm, excited at 430 nm). Unexpectedly, the fluorescent chromophore of the GST-apoPholasin/dCTZ complex was identified as non-fluorescent dCTZ. The luminescence intensity of the GST-apoPholasin/dCTZ complex was increased in a catalase-H₂O₂ system, but not in sodium hypochlorite.

K/Na-triggered bioluminescence in the oceanic squid Symplectoteuthis oualaniensis

A distinctive type of luminescent system present in the large dorsal luminous organ of the oceanic squid Symplectoteuthis oualaniensis is described. The organ produces an intense blue flash of light followed by a rapid decay in light intensity. Luminescence originates from numerous oval granules present in the luminous organ. The essential light-emitting components are membrane bound. Intact granules or washed homogenates of the granules are triggered to emit light by monovalent cations such as, in decreasing order of effectiveness, potassium, rubidium, sodium, cesium, ammonium, and lithium. Calcium, magnesium, and strontium ions do not trigger light emission. Analysis of the kinetics of the decay of light intensity suggests that two light-emitting components are involved, one decaying faster than the other. The light-emitting reaction has an absolute requirement for molecular oxygen. The optimum KCl or NaCl concentration is approximately 0.6 M and the optimum pH is approximately 7.8. A free sulfhydryl group is essential for activity.

Bioluminescence: from chemical bonds to photons

The biological transformation of chemical to photic energy involves an enzyme-mediated chemiluminescent reaction, in which one of the products exists in an electronically excited state, emitting a photon as it returns to the ground state. The colour of bioluminescence differs in different organisms, ranging from the deep blue (460 nm) of certain crustacea, through the bluish green (490 nm) of some bacteria, the green (530 nm) of mushrooms to the red (about 600 nm) of the railroad worm. In one case, energy transfer has been demonstrated from the enzyme system to material that emits light with a longer wavelength. The energies involved range from about 165 to 250 kJ/einstein (40 to 60 kcal/einstein). Boyle first showed that air was involved in bioluminescence in 1668 in his experiments with an air pump. Over the past 100 years, it has become clear that most if not all bioluminescent systems require molecular oxygen. The recent isolation and characterization of an oxygen-containing (peroxide) enzyme intermediate from the bacterial system is described and a reaction mechanism is postulated. This scheme is compared with other hypothetical mechanisms, in particular those involving a four-membered ring intermediate, a dioxetane, in which the simultaneous cleavage of two bonds leaves one product in an excited state. I shall discuss the special role of luciferases in bioluminescence, especially in flashing mechanisms involving 'precharged' intermediates.

Peroxide-dependent and -independent lipid peroxidations catalyzed by chelated iron

Oxidation of linoleic acid (LA) in tetradecyltrimethylammonium bromide micelles was induced by ferrous- and ferric-chelates in the presence of linoleic acid hydroperoxide (LOOH). Ferrous-chelates also induced lipid peroxidation in the presence of H2O2, but ferric-chelates did not, thought they could generate OH-radicals in the presence of H2O2, resulting in deoxyribose degradation. Of the chelators tested, nitrilotriacetic acid (NTA) chelated with iron showed the highest activity for induction of H2O2- and LOOH-dependent lipid peroxidations and H2O2-dependent deoxyribose degradation. NTA with ferrous ion, but not with ferric ion, also initiated oxidation of LA after a short lag period in the absence of peroxides such as H2O2 and LOOH, but other chelators with ferrous ion did not. The peroxide-independent lipid peroxidation and associated oxidation of ferrous-NTA to ferric-NTA progressed in two steps: an induction step in a lag period and then a propagation step. Ferrous ion complexed with NTA was autoxidized pH-dependently and synchronously with oxygen uptake. The rates of both reactions increased with increase of pH, but were not related to the length of the lag period, which was also dependent on pH, and was shortest at pH 4.2. The EPR spectrum of the ferric-NTA complex prepared directly from ferric salt was different from that of the complex prepared from ferrous salt, confirming that some ferric-type active oxygen participated in induction of peroxide-independent lipid peroxidation. From these results, we propose a possible mechanism of lipid peroxidation induced by ferrous-NTA without peroxides. The finding that iron-NTA had the highest activity for induction of the oxidations of LA and deoxyribose is discussed in relation to the carcinogenic and nephrotoxic effects of this chelating agent.

Biological diversity, chemical mechanisms, and the evolutionary origins of bioluminescent systems

A diversity of organisms are endowed with the ability to emit light, and to display and control it in a variety of ways. Most of the luciferins (substrates) of the various phylogenetically distant systems fall into unrelated chemical classes, and, based on still limited data, the luciferases (enzymes) and reaction mechanisms are distinctly different. Based on its diversity and phylogenetic distribution, it is estimated that bioluminescence may have arisen independently as many as 30 times in the course of evolution. However, there are several examples of cross-phyletic similarities among the substrates; some of these may be accounted for nutritionally, but in other cases they may have evolved independently.

Characterization and properties of Pholas luciferase as a metalloglycoprotein

The luciferase of the bioluminescent boring mollusc, Pholas dactylus, has been purified by a new method which includes centrifugation in cesium chloride gradients. Homogeneous preparations have been obtained and molecular weight determinations and subunit analysis support the idea that this preparation is an oxyluciferin-luciferase complex. The preparation catalyzes oxidation of ascorbic acid in presence of H2O2, and this peroxidase activitity has been used for characterization (thermal and pH stabilities, activity as a function of pH, isoelectric point, turnover number). The existence of two atoms of copper has been established and their involvement in the peroxidase activity indicated. Chemical analyses have shown that Pholad luciferase is a glycoprotein and the existence of glucosamine, fucose, mannose, and galactose residues has been demonstrated. The apparent buoyant dentisty (1.340), the sedimentation coefficient (10.7 S), the Stoke's radius (83 A), the partial specific volum (0.707), and the molecular weight (350,000) have been determined. The frictional ratio (flf0 = 1.8) derived from the Stoke's radius indicates that the molecule is asymmetric. The quaternary structure has been examined. Subunits of molecular weight 150,000 and 46,000 have been observed. The latter has electrophoretic properties identical with luciferin or oxyluciferin.