| Abstract: | In order to improve calibration of firefly luciferase signals obtained by injecting the enzyme into single, isolated heart and liver cells we have investigated why the luminescence from cells is greatly depressed compared with in vitro (in mammalian ionic milieu) and why the decay of the intracellular signal is remarkably slow. We have shown that inorganic pyrophosphate greatly depresses the signal in vitro and that micromolar concentrations of inoragnic pyrophosphate, comparable with that in cytoplasm, reverse this inhibition and stabilize the signal, eliminating its decay. Higher concentrations of pyrophosphate depress the signal by inhibiting ATP-binding to luciferase. Luciferse-injected cells exposed to extracellular luciferin concentrations above about 100 μmol/1 (corresponding to a cytoplasmic level of c. 5–10 μmol/1 because of a transplasmalemmal gradient) show a gradual, irreversible loss of signal. We attribute this phenomenon (which is not seen in vitro) to the gradual accumlation of a luminescently inactive, irreversible, luciferase-oxyluciferin complex. At low luciferin levels this complex is prevented from forming by cytoplasmic pyrophosphate. Above c. 100μmol/1 extracellular luciferin, the pyrophosphate level in the cytoplasm fails to fully prevent the complex forming. In vitro this phenomenon does not occur because the luciferase concentrations and hence oxyluciferin levels are orders of magnitude lower than in cells injected with concentrated luciferase solutions, which have a cytoplasmic luciferase concentration of approximately 2-4 μmol/1. |
