Uptake of Thallium, a Toxic Heavy-Metal, in the Cyanobacterium Synechococcus・R-2(Anacystis nidulans, S.Leopoliensis) PCC 7942

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Uptake of the toxic heavy-metal, thallium, was studied in the cyanobacterium Synechococcus R-2(PCC7942) using clinically available ^<201>Tl^+. Thallium was found to distribute across the plasmalemma passively, and so the accumulation ratio of the ion([Tl^+]_i/[Tl^+]_0) could be used to calculate the apparent membrane potential(ΔΨ_<i, 0>) of the cells(^ETl_<i, 0>^+=ΔΨ_<i, 0>). The permeability of the plasmalemma to Tl^+(^PTl^+≈1 to 5 nm s^<-1>) is higher than that of K^+. Valinomycin does not increase the permeability of Tl^+. Transient changes in the ΔΨ_<i, 0> of cells, because of electrogenic transport of ions, could be detected from its effects upon the uptake rate of Tl^+. HCO_3^- hyperpolarized Synechococcus cells, whereas NH_4^+, CH_3NH_3^+, and K^+ led to depolarization. The use of Tl^+ as a reporter of ΔΨ_<i, 0> has some inherent limitations. Tl^+ is toxic at very low concetrations(inhibitory effects are apparent after about 6 h at concentrations as low as 1 mmol m^<-3>). The rate of equilibration is slow(t_<1/2>≈5 to 20 min). Equilibration of Tl^+ takes about 2 h, which limits its value as a membrane potential probe. Large amounts of Tl^+ bind to the surface of the cells making the method impracticable for measuring accumulation ratios of less than about 10(ΔΨ_<i, 0> values smaller than about -60 mV). Cultures continuously exposed to Tl^+(10 mmol m^<-3>) eventually become Tl^+ resistant by actively extruding Tl^+(ΔμTl_<i, 0>^+=-3±0.2 kJ mol^<-1>) and so thallium cannot be used as a ΔΨ_<i, 0> probe in such cells.
日本植物生理学会の論文

Uptake of Thallium, a Toxic Heavy-Metal, in the Cyanobacterium Synechococcus・R-2(Anacystis nidulans, S.Leopoliensis) PCC 7942

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