[The mechanisms of uptake of zinc by baker's yeast]

The uptake of labelled Zn by baker's yeast after exhaustion of the intracellular substrates for energy metabolism has been investigated. Without addition of glucose (substrate), equilibrium is reached rapidly. Binding of Zn is attributed to cell wall components. The amount bound approaches a saturation value at a Zn concentration in solution of the order of 10(-2) M. Initially, the Zn is bound reversibly, but it gradually changes into a firmly bound form. Substrate-independently, killed cells bind more Zn than living cells; apparently more binding sites become accessible. After addition of substrate, however, living cells take up additional Zn irreversibly over long periods. It is suggested that the Zn absorbed substrate-dependently enters the interior of the cell. The process does not depend strongly on the presence of air. It follows Michaelis-Menten kinetics, and the temperature coefficients (Q 10), at different concentrations and temperatures, are 2-2,7. No efflux of Zn from the interior of the cell is observed. Reasons are given for considering the substrate-dependent uptake of Zn as an active process.