The existence of multiple transport systems for Mn in has been demonstrated in this study. Mn (supplied as MnCI) was accumulated by at all Mn concentrations examined (25 nM-1 mM) but a log-log plot of uptake rates and total amounts accumulated revealed the existence of at least two Mn concentration-dependent transport systems. Over a low Mn concentration range (25-1000 nM), high-affinity Mn uptake occurred with a value of 0.3 μM, while transformation of kinetic data obtained over the concentration range 5-200 μM revealed another system with a of 62 μM. Meaningful kinetic analyses were not possible at higher Mn concentrations because of toxicity: only about 30% of cells remained viable after 30 min incubation with 1000 μM MnCI. Release of K accompanied Mn accumulation and this increased with increasing Mn concentration. However, even in non-toxic Mn concentrations, the ratio of Mn uptake to K release greatly exceeded electroneutral stoichiometric exchange. In 50 μM MnCI, the ratio was 1: 123 and this increased to 1:2670 in 1000 μM MnCI, a toxic concentration. External Mg was found to decrease Mn accumulation at all concentrations examined, but to differing extents. Over the low Mn concentration range (5-200 μM), Mg competitively inhibited Mn uptake with a half-maximal inhibitory concentration, , of 5.5 μM Mg. However, even in the presence of a 50-fold excess of Mg, inhibition of Mn uptake was of the order of 72% and it appears that the cellular requirement for Mn could be maintained even in the presence of such a large excess of Mg. Over the high Mn concentration range (5-200 μM), the for Mg was 25.2 μM. At low Mn concentrations, Zn and Co, but not Cd, inhibited Mn uptake, which indicated that the high-affinity Mn uptake system was of low specificity, while at higher Mn concentrations, where the lower-affinity Mn transport system operated, inhibition was less marked. However, competition studies with potentially toxic metal cations were complicated due to toxic effects, particularly noticeable at 50 μM Co and Cd.


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