Calcium measurement is critical for numerous biological investigations. The aequorin complex comprises a 22 kD apoaequorin protein, molecular oxygen and the luminophore coelenterazine. When three Ca2+ ions bind to this complex, coelenterazine is oxidized to coelenteramide, with a concomitant release of carbon dioxide and blue light. The approximately third-power dependence of aequorin's bioluminescence on Ca2+ concentration allows the measurement of Ca2+ concentrations with a broad detection range from ~0.1 µM to >100 µM. Unlike fluorescent Ca2+ indicators, Ca2+-bound aequorin can be detected without illuminating the sample, thereby eliminating interference from autofluorescence. Native coelenterazine and its derivatives confer different Ca2+ affinities and spectral properties on the aequorin complex. Recombinant apoaequorin reconstituted with coelenterazine hcp is reported to have the best luminescence overall, with both a high quantum yield and a fast response time. However, intracellular reconstitution of aequorin from coelenterazine analogs can be relatively slow. Aequorins containing the cp, f or h form of coelenterazine exhibit 10–20 times stronger luminescence than that of apoaequorin reconstituted with native coelenterazine. Coelenterazines h and cp has been used in HTS screening assay for GPCRs.
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