Aequorin luminescence: relation of light emission to calcium concentration--a calcium-independent component

Light emission from the calcium-sensitive bioluminescent protein aequorin was measured at calcium ion concentrations of 10(-9) to 10(-2) molar. At very low Ca2+ concentations, light emission is independent of calcium ion concentration. The maximum slope of the log-log plot of light as a function of calcium ion concentration is about 2.5. The complete relation is well described by a two-state model involving three calcium-binding sites.