Aequorin is a calcium-sensitive bioluminescent protein from the jellyfish Aequorea victoria that has been used extensively as a microinjectable calcium indicator in cells. The aequorin complex comprises a 22,000-dalton apoaequorin protein, molecular oxygen and the luminophore coelenterazine (Figure 1). When three Ca2+ ions bind to this complex, coelenterazine is oxidized to coelenteramide, with a concomitant release of carbon dioxide and blue light (Figures 2 and 3). The approximately third-power dependence of aequorin's bioluminescence on Ca2+ concentration gives it a broad detection range, allowing the measurement of Ca2+ concentrations from ~0.1 µM to >100 µM.
In addition to native coelenterazine (C-2944), we offer five derivatives of coelenterazine designated cp, f, h, hcp and n by Shimomura and colleagues that confer different Ca2+ affinities and spectral properties to the aequorin complex (Table 1). Like native coelenterazine, these five derivatives can be used to reconstitute the aequorin complex both in vivo and in vitro. Aequorin reconstituted with coelenterazine hcp shows very favorable characteristics, including a fast response to binding Ca2+ and the highest luminescence quantum yield of these five coelenterazine derivatives. Aequorins containing the cp, f or h form of coelenterazine are reported to exhibit relative intensities that are 10–20 times that of apoaequorin reconstituted with native coelenterazine. Knight and co-workers reported that aequorin reconstituted with coelenterazine h is more sensitive to Ca2+ than is the native complex, thus providing a valuable tool for measuring small changes in Ca2+ concentrations. Coelenterazine n is reportedly the most useful low-sensitivity coelenterazine, producing an apoaequorin/coelenterazine complex that exhibits 10,000-fold lower luminescent intensity than the apoaequorin/coelenterazine hcp complex.
|C2944||423.47||FF,D,LL,AA||MeOH||429||7500||see Notes||pH 7||1, 2, 3|
|C6780||407.47||FF,D,LL,AA||MeOH||437||9500||see Notes||MeOH||1, 2|