Rapid Kinetic Studies of the Light Emitting Protein Aequorin

SHIMUMURA et al.1,2 isolated from luminescent jelly fish (Aequora forskalea) a protein (aequorin) which on addition of Ca2+ emits light. In contrast to other bioluminescent compounds, aequorin luminescence requires neither oxygen nor FMNH2, ATP nor long chain aldehydes, but only Ca2+. Recently, other bioluminescent proteins reacting only with Ca2+ have been isolated and termed Ca2+-activated photoproteins3,4. Since the intensity of the emitted light varies with the Ca2+ concentration, aequorin and related proteins can serve as useful tools for detecting small changes in Ca2+ concentration in biological systems5,6. Little is known, however, about the mechanism of this reaction. Since light emission from aequorin occurs within a few milliseconds of rapid mixing with Ca2+, kinetic studies are possible only with rapid mixing instruments, stopped and continuous flow. Hastings et al. studied the kinetics of aequorin using stopped flow and double stopped flow apparatuses7. Because both have a dead time of about 2 ms, which is in the same range as the rise time of the light emission from aequorin, they were unable to establish a Ca2+ dependence for the rate of rise of the light emission. Ca2+ dependence of this rate might be very important for delineation of the reaction mechanism.