FLUORESCENCE PROPERTIES OF QUERCETIN IN HUMAN LEUKEMIA JURKAT T-CELLS

We describe the fluorescent properties of the natural flavonoid quercetin in human leukemia Jurkat T-cells. We obtained by spectrofluorimetric measurements that the emission/excitation spectra of intracellular quercetin displayed a dominant emission maximum at similar to 540 nm and two different excitation maxima, at similar to 380 nm and similar to 440 nm, respectively. Our data suggest that quercetin elicits a specific fluorescence upon binding to intracellular proteins and that there are two dominant molecular configurations of these quercetin-bound proteins which are sensitive to the intracellular Ca(2+) concentration. Quercetin fluorescence increased substantially after permeabilization of the cells with digitonin, suggesting that intracellular binding of quercetin may be downregulated by a cytosolic factor which is lost by permeabilization. In addition, we performed spectrofluorimetric measurements of the intracellular Ca(2+) concentration which revealed a strong biphasic calcium release signal after stimulation of intact cells with 50 mu M quercetin. The cytosolic Ca(2+) level increased rapidly from 196 nM to a maximum of 1.7 mu M and then declined bi-exponentially to a level of 426 nM, with time constants of 0.48 min. and 4.5 min., respectively. Quercetin also decreased considerably the intracellular NAD(P)H level in intact cells. Taken together, these data add more evidence for the involvement of Ca(2+) and NAD(P)H in the apoptotic pathway triggered by quercetin in Jurkat cells.