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Biochem Biophys Res Commun. 1996 Apr 5;221(1):77-81.

Titration of recombinant aequorin with calcium chloride.

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  • 1Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA.


The photoprotein aequorin emits light in the presence of a trace of Ca2+. The primary structure of the protein indicates the presence of three Ca2+-binding sites, whereas the luminometric titration of heterogeneous natural aequorin with Ca2+ has shown that the light emission takes place by the binding of two Ca2+ ions. In the case of recombinant aequorin, which is more suitable for quantitative studies, the titration with Ca2+ monitored by a Ca2+-sensitive electrode revealed that the photoprotein can bind more than two, most likely three, Ca2+ ions, and the luminometric titration conclusively showed that the luminescence is triggered by the first two Ca2+ ions bound. The affinity of recombinant aequorin for the first two Ca2+ ions, which are essential for light emission, was about 20 times stronger than that for the third Ca2+ ion, which is unrelated to light emission.

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