Aufnahme von Cobalt,Blei und Cadmium durch Bäckerhefe

The toxicity for and the uptake by Saccharomyces cerevisiae of the essential trace element Co²⁺ and the non essential elements Cd²⁺ and Pb²⁺ were compared. Inhibition of yeast growth is observed 4 hrs after addition of Co at concentrations higher than 10^?4 M. Cd is about 100 times more toxic than Co and inhibits growth immediately after addition. No toxicity of Pb is observed with concentrations up to 5 · 10^?4 M. For uptake experiments the radioactive isotopes ⁶⁰Co, ^115mCd and ²¹⁰Pb were used. The yeast samples were washed with carrier solution in order to remove the isotopes adsorbed only by the cell wall and measured by γ-scintillation counting. Co uptake by yeast is strongly dependent on glucose both under aerobic and anaerobic conditions. There is no efflux of Co once taken up. Dead yeast does not accumulate Co at all. Co already taken up is released from cells when killed, indicating that no irreversible binding in the cell interior takes place. Co transport follows biphasic saturation kinetics with the MICHAELIS constants K₁ = 10^?4 and K₂ = 8 · 10^?4 M and the maximal velocities ν¹ = 9.5 and ν₂ = 63 μ moles/g dry weight · hr. The Q₁₀ for Co uptake from 10^?4 M solution is 2.3. Uptake of Cd resembles that of Co. The absorption of Cd is also glucose-dependent, but independent of the presence of air, no Cd is taken up by dead cells, and Cd previously taken up is released during killing the cells. There is a considerable Pb uptake irrespective of the presence of glucose, and even by dead yeast. Killing of the yeast during uptake does not lead to loss of Pb. It is concluded that the essential trace element Co is accumulated by an energy-dependent, probably active, transport system in the membrane. Cd is similarly transported, probably because of its chemical similarity to Zn. In contrast, Pb appears to be taken up only by diffusion, but subsequently to be trapped by binding in the cell interior. In this way, the accumulation of Pb is similar in amount to those of Co or Cd although the mechanisms differ widely.