Ehrlich
ascites-tumour cells normally contain a large concentration of
Zn-metallothionein. When cells are placed in
culture media, containing or pretreated with the
metal-ion-chelating resin Chelex-100, they stop growing, remain viable and lose
zinc specifically from the
metallothionein (MT) pool. The kinetics of loss of
zinc are first-order and are very rapid, having a rate constant of greater than or equal to 0.6 h-1. MT
protein labelled with 35S is biodegraded with a rate constant of 0.07-0.014 h-1 in control cells, 0.08 h-1 in cells exposed to the
zinc-deficient medium and 0.12-0.18 h-1 in cells treated directly with
Chelex. Over the 6 h period in which
zinc is totally lost from Zn-MT there is relatively little decrease in MT-like
protein as measured by
cadmium-binding to the 10,000-Mr
protein fraction. Other pools of
zinc and 35S-labelled
protein turn over more slowly. There is no loss of
zinc from rat liver Zn-MT that is dialysed against
Chelex to model the possible reaction of the resin with Ehrlich-cell Zn-MT. However,
Chelex does compete slowly for MT-bound
zinc when resin and MT are directly mixed. Analysis of the known and possible pathways of
zinc metabolism in cells in relationship to these rate constants shows that biodegradation of MT
protein cannot account for the rate of loss of
zinc from Zn-MT.