Zinc trafficking to apo-Zn-proteins 2. Cellular interplay of proteome, metallothionein, and glutathione

Abstract

Abstract A recent study investigated the impact of glutathione (GSH) on the transfer of zinc (Zn) from proteome to apo-carbonic anhydrase. Here, we probed the requirement of glutathione for zinc trafficking in LLC-PK1 pig kidney epithelial cells. Depletion of GSH by at least 95% left cells viable and able to divide and synthesize Zn-proteins at the control rate over a 48-h period. Loss of GSH stimulated the accumulation of 2.5x the normal concentration of cellular Zn. According to gel filtration chromatography, differential centrifugal filtration, and spectrofluorimetry with TSQ, the extra Zn was distributed between the proteome and metallothionein (MT). To test the functionality of proteome and/or MT as sources of Zn for the constitution of Zn-proteins, GSH-deficient cells were incubated with CaEDTA to isolate them from their normal source of nutrient Zn. Control cells plus CaEDTA stopped dividing; GSH-depleted cells plus CaEDTA continued to divide at ∼40% the rate of GSH deficient cells. Evidently, proteome and/or MT served as a functional source of Zn for generating Zn-proteins. In vitro insertion of Zn bound to proteome into apo-carbonic anhydrase occurred faster at larger concentrations of Zn bound to proteome. These results support the hypothesis that enhanced transport of Zn into cells drives the conversion of apo-Zn-proteins to Zn-proteins by mass action. Similar results were also obtained with human Jurkat T lymphocyte epithelial cells. This study reveals a powerful new model for studying the chemistry of Zn trafficking, including transport processes, involvement of intermediate binding sites, and constitution of Zn-proteins.