Bioluminescence: Chemical Aspects

Glowworms: a review of Arachnocampa spp. and kin

The term 'glowworm' is used in connection with the flightless females of lampyrid fireflies and to describe the luminescent larvae of certain fungus gnats that belong to the subfamilies Arachnocampinae, Keroplatinae and Macrocerinae of the dipteran family Keroplatidae. This review focuses on the luminescent larval fungus gnats. The weakly luminescent species of the Holarctic feed mainly on fungal spores, but some, such as Orfelia fultoni, have turned to a carnivorous diet. Larval Australian and New Zealand Arachnocampa spp. produce brighter in vivo (but not necessarily in vitro) lights, live in cool, damp and dark places and are exclusively predatory. They lure their prey (usually small flying insects) with the help of their blue-green light emissions towards snares consisting of vertical silk threads coated with sticky mucus droplets. Fungus gnats with similar 'fishing lines' are found in the Neotropics, but they are not luminescent. The larval stage is longest in the life cycle of Arachnocampa, lasting up to a year, depending on climatic conditions such as temperature and humidity as well as food supply. In A. luminosa, but not the Australian A. flava, female pupae and even female imagines are luminescent. However, it remains to be demonstrated whether it is the light of the female, a pheromone or both that attract the males. Light organs and the chemical reactions to produce light differ between the holarctic and the Australian/New Zealand species. Prey is attracted only by the glowworm's light; odours of the fishing lines or the glowworms themselves are not involved. Recognition of the prey by the glowworm involves mechano- and chemoreception. The eyes of both larval and adult glowworms are large and functional over a spectral range covering UV to green wavelengths. Adults are poor fliers, live only for a few days, have degenerate mouth parts and do not feed. Maintenance of glowworms in captivity is possible and the impact of tourism on glowworms in natural settings can be minimized through appropriate precautions.

Kinetic characteristics of calcium-dependent, cholinergic receptor controlled ATP secretion from adrenal medullary chromaffin cells

Adrenal chromaffin cells secrete catecholamines (CA) and ATP in response to acetylcholine (ACh) and high [K+]o. The release process is relatively fast making it difficult to measure the early phase of the secretory response. Recently we were able to resolve the time course of the secretory response by measuring the release of ATP using luciferin-luciferase included in the extracellular medium. For the three secretagogues studied, ACh, nicotine and high [K+]o, the early phase of release followed a complex kinetics. Allowing for an initial delay of the secretory response, the kinetics could be described as the sum of two power exponential processes. Increasing the temperature from 23 to 37 degrees C induced a marked decrease in the two time constants needed to fit the early time course of the ATP secretion. The activation energies, estimated from Arrhenius plots, were approx. 20 and 16 kcal/mol for both phases of ATP release induced by either cholinergic agonists or high [K+]o. These results suggest that cholinergic receptor activation and membrane depolarization induce ATP (and CA) secretion through a common pathway. The initial delay in the onset of the secretory response decreased with increasing doses of secretagogue and with temperature. We propose that the delay preceding the actual onset of ATP release represents the time required for generation of intracellular second messengers. The effective concentration attained by these messengers depend apparently on both receptor occupancy by the agonist and the ensuing Ca2+ channel activation.

Mn(II)-EPR measurements of cation binding by aequorin

Cation binding at 5 degrees C by aequorin, a bioluminescent protein from the jellyfish Aequorea victoria, was examined by means of Mn(II) EPR. The bioluminescence of aequorin is triggered by Ca(II), as well as by trivalent lanthanides, and is inhibited by Mg(II) and Mn(II). Three EF-hand Ca(II)-binding domains have been identified in the aequorin amino acid sequence. In the work reported here, active native aequorin was found to have a single tight binding site for Mn(II) with an association constant of 0.566 microM-1. Ca(II) and La(III) competed for the Mn(II) site with association constants of 1.92 microM-1 and 1.38 microM-1, respectively. The affinity of Ca(II) and La(III) for their two other (presumed) sites on aequorin was an order of magnitude less than their affinity for the Mn(II) site. Mg(II) competed for the Mn(II) site as well but with a much smaller association constant of 0.0109 microM-1. Ca(II)-independent discharged aequorin did not bind Mn(II) to a significant degree. Conjectures on the location of the Mn(II) site in the aequorin amino acid sequence and on the relationship between the binding parameters of the cations and their influence on aequorin activity are given.

Photochemically-induced dynamic nuclear polarization proton-NMR of aequorin discharged by calcium-independent light emission

Photochemically-induced dynamic nuclear polarization was used to identify exposed amino-acid residues and to assign resonances in the 1H-NMR spectrum of Ca(II)-independent discharged (inactivated) aequorin. A previous nuclear magnetic resonance, circular dichroism and fluorescence study [Ray, B.D., Ho, S., Kemple, M. D., Prendergast, F. G. and Nageswara Rao, B.D. (1985) Biochemistry 24, 4280-4287] indicated that as the Ca(II)-activated bioluminescent protein aequorin from jellyfish spontaneously emits light in the absence of Ca(II), it changes from a rigid, fully active form to a discharged form in which a number of amino-acid residues are more mobile than in the native protein. Laser-photochemically-induced dynamic nuclear polarization experiments identified tryptophan and tyrosine residues, but not histidine residues, in Ca(II)-independent discharged aequorin to be accessible to the flavin dye used. These exposed residues are also among the mobile residues of the Ca(II)-independent discharged protein. Resonances of all the protons (including the alpha protons) of the accessible tryptophan and tyrosine residues were assigned with the aid of two-dimensional photochemically-induced dynamic nuclear polarization J-correlated spectroscopy. The oxidized chromophore, from which light is emitted in aequorin, was not accessible to the dye in the Ca(II)-independent discharged protein. No exposed residue was detected in the photochemically-induced dynamic nuclear polarization spectrum of Ca(II)-independent discharged aequorin from which the oxidized chromophore was removed, corroborating the previous finding that in this apo-discharged form the protein partially refolds and thereby loses some of the mobility acquired in the formation of the Ca(II)-independent discharged protein.

Electron paramagnetic resonance of spin-labeled aequorin

Aequorin is a Ca-activated bioluminescent protein from jellyfish. This protein contains two sulfhydryl groups, one of which is essential for its bioluminescence. Little information concerning the structure of and relationship between the metal binding sites of aequorin and the sulfhydryl group(s) is known. Aequorin was modified by attachment of either a maleimide spin-label [studied by electron paramagnetic resonance (EPR)] or the fluorescent label Acrylodan at the essential sulfhydryl in order to gain such information. These modifications caused destabilization of the chromophore of aequorin. Both of the attached labels showed considerable freedom of motion. The spin-label was quite accessible to the solvent, and the fluorescent label was less so. In addition the metal binding properties of the spin-labeled aequorin were studied by Mn(II) EPR. One tight Mn(II) binding site per spin-labeled aequorin was found. The distance between the Mn(II) binding site and the spin-label is at least 20 A. Furthermore, the relative affinity of spin-labeled aequorin for various metal ions was found to be in the order Pr(III) greater than Mn(II) greater than Ca(II) greater than Mg(II).

Influence of deuterium oxide on calcium transients and myofibrillar responses of frog skeletal muscle

The influence of substituting D2O for H2O on calcium transients and on contraction was studied in intact single skeletal muscle fibres injected with aequorin and in mechanically skinned fibres from frogs. Most experiments were carried out at 10 degrees C. Experiments performed in vitro established that the calcium concentration-effect curve for aequorin is depressed and the rate of change of aequorin light emission after sudden changes of [Ca2+] is decreased when D2O is substituted for H2O. D2O substitution greatly reduces the amplitude of the aequorin signals of intact fibres both in twitches and in tetani. This is partly the result of the influence of D2O on aequorin, but the amplitude of the calcium transients is reduced as well. In both H2O and D2O Ringer solutions, the amplitude and time course of the sarcoplasmic calcium transient during a tetanus vary greatly with the stimulus frequency. In H2O, frequencies high enough to produce mechanical fusion normally produce cytoplasmic calcium concentrations high enough to saturate the myofibrils, with the result that stimulus frequency has very little influence on the mechanical response. Saturation does not occur in D2O, and the amplitude and form of fully fused tetani are greatly influenced by the frequency of stimulation. Aequorin was used as a calcium indicator to estimate the influence of D2O substitution on the apparent equilibrium association constant for the calcium-EGTA complex. The constant was reduced by more than one order of magnitude in D2O at pD = pH = 7.0. Experiments on mechanically skinned muscle fibres showed that D2O substitution decreased the apparent sensitivity of the myofibrils to calcium, but increased the force of contraction at saturating calcium concentrations. The latter effect is probably responsible for the potentiation of tetanic tension that is observed in certain D2O/H2O mixtures or in H2O solutions immediately after a switch from D2O. In intact muscle fibres the potentiation of twitch or tetanic tension after a switch from D2O to H2O declines with a half-time of about 25 s; this probably reflects the time course of exchange of intracellular D2O for H2O. The effects of D2O on the intracellular calcium transient appeared and disappeared with a half-time of less than 1 s; this time is of the same order of magnitude as that calculated for the exchange of H2O and D2O in the T-tubules. We conclude that D2O affects calcium release at a site in the T-tubule which is more accessible to the extracellular space than to the intracellular space.

Spectral characteristics of the bioluminescence induced in the marine fish, Porichthys notatus by Cypridina (ostracod) luciferin

Specimens of Porichthys notatus, which are naturally luminous along the coast of California, are non-luminous in Puget Sound. However, luminescence capability may be induced in the adult Puget Sound Porichthys by the administration of purified Cypridina (ostracod) luciferin, synthetic Cypridina luciferin, or Cypridina organisms. The bioluminescence emission spectra produced by the Puget Sound fish following induction is similar, if not identical, to that of the naturally luminous Porichthys notatus from California waters (maxima: 485 and 507 nm).

Renilla luciferin as the substrate for calcium induced photoprotein bioluminescence. Assignment of luciferin tautomers in aequorin and mnemiopsin

A study was made of the effects of pH and protic and aprotic solvents on the spectral properties of Renilla (sea pansy) luciferin and a number of its analogs. The results have made possible the assignment of two tautomeric forms of Renilla luciferin, one which absorbs maximally at 435 nm and another which exhibits an absorption maximum at 454 nm. Furthermore the results provide an explanation for the visible absorption characteristics of the photoproteins aequorin (lambda-max 454 nm) and mnemiopsin (lambda-max 435 nm). In addition a Renilla-like luciferin can be extracted from both of these photoproteins. This luciferin produces light with Renilla luciferase, at a rate dependent upon the concentration of dissolved oxygen, and in other respects is indistinguishable from Renilla luciferin in this bioluminescent reaction. The results suggest that the native chromophore in both photoproteins is Renilla luciferin (or a nearly identical derivative). The results also suggest that a hydroperoxide intermediate probably exists in photoproteins, on energetic grounds, and to account for the oxygen concentration independency of the rate of photoprotein reactions. This hydroperoxide may be attached initially to an amino-acid side chain (possibly indolyl-OOH, imidazoyl-OOH, or -SOOH) rather than to the luciferin chromophore.