Calcium Binding, Quantum Yield, and Emitting Molecule in Aequorin Bioluminescence

Abstract

THE protein aequorin, from the jellyfish Aequorea, emits bluish light in aqueous solution on addition specifically of calcium or strontium; the necessary energy of more than 60 kcalories/mole at the peak wavelength is evidently provided by an intramolecular reaction which is accompanied by a change in absorption spectrum and yields a blue fluorescent reaction product^1,2. The extreme sensitivity to Ca²⁺, much more than to Sr²⁺, provides a basis for quick microdetermination of Ca²⁺ in biological fluids³ and has been used in detecting the relation of Ca²⁺ to contraction of single muscle fibres^4,5 and to activity of mitochondria⁶. The chief properties of aequorin, including the molecular weight (m.w.) of 30,000 by sedimentation, diffusion or 32,000 by amino-acid composition, have been reported⁷. The kinetics of the light-emitting reaction have also been studied⁸.