Zinc metabolism in Ehrlich cells: properties of a metallothionein-like zinc-binding protein

Abstract

Host zinc deficiency halts the proliferation of the mouse Ehrlich ascites tumor. The major site of measurable cellular zinc depletion is a cytosolic zinc binding protein. This protein is characterized as a metallothionein on the basis of its presence as two isoproteins which behave on DEAE-Sephadex and in polyacrylamide gel electrophoresis like metallothioneins, the lack of protein absorbance at 280 nm, its sulfhydryl/zinc ratio of 3.5, and its reactivity in zinc transfer to apocarbonic anhydrase. Finally, the protein exhibits cross-reactivity with a known rat metallothionein in a radioimmunoassay. Coupled with the similarity in structure and antigenicity of rat and mouse metallothioneins, this adds strong support to the identification of the zinc-binding protein as a metallothionein. In zinc-deficient cells this metallothionein-like protein appears to exist as an apoprotein. When small amounts of dietary zinc stimulate the deficient cells to divide, zinc is not observed in metallothionein. Larger concentrations of dietary zinc support both proliferation and the steady state presence of zinc in this protein. It is demonstrated that metallothionein is the principal donor of zinc to apocarbonic anhydrase added to Ehrlich cytosol. These results are used to construct a model of zinc metabolism in which zinc metallothionein is a labile depot of zinc in the Ehrlich cell which can be mobilized under zinc-deficient conditions.