Mechanisms of Manganese Transport Across Cell Membranes

Abstract

The uptake of manganese (Mn) by human fibroblast cells was studied to determine the chemical form of Mn used by the cells. These experiments tested the uptake of free Mn and Mn bound to the plasma protein, transferrin. Mn solutions were added to C-1474 human fibroblasts at varying concentrations and incubation times. The Mn concentrations ranged from 100 - 800 µmolar. At a two hour time point, uptake at 800 µmolar was 4-fold higher than at 100-400 µmolar and 2-fold higher than uptake at 600 µmolar. This suggested an inefficient system that requires high concentrations of Mn to trigger uptake. Removal of the albumin from the incubation medium and addition of EDTA (Ethylenedinitrilo-tetraacetic acid) to harvesting washes resulted in a reversal of the uptake seen at 800 µmolar. This suggested that Mn was not penetrating the cell in a free form. Next, the uptake of a transferrin (Tf)-Mn complex was tested. The K-562 erythrocyte tumor cell line was used to study uptake of this complex. Tf was used as a carrier protein because it has been shown to bind Mn (Scheuhammer et al. 1985). Complexes of Tf-Mn were formed by oxidizing Mn with ceruloplasmin. Initial studies showed 3-fold higher incorporation of free Mn than Tf-Mn complex. Iron (Fe) was also substituted for Mn to test the integrity of the Tf molecule. Under the same protocol, free Fe was incorporated ten times more readily than Tf-Fe. Adjustments with incubation medium and complex formation time are needed to explore the possibilities of Mn uptake via Tf.