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Magnesium Transport in Escherichia coli

INTERFERENCE BY MANGANESE WITH MAGNESIUM METABOLISM

From the ¹ From the Department of Biology, Washington University, St. Louis, Missouri 63130

Escherichia coli has a specific active transport system for magnesium which can be studied with the radioisotope ²⁸Mg. The accumulation of ²⁸Mg is temperature-dependent and inhibited by cyanide, dinitrophenol, and m-chlorophenyl carbonylcyanide hydrazone. The initial rate of uptake of ²⁸Mg shows typical saturation kinetics with a K_m = 18 µm magnesium and a V_max = 0.56 µmole per min per 10¹² cells at 25° in Tris medium. Neither calcium nor strontium compete with, inhibit, or stimulate the transport of ²⁸Mg. However, manganese shows complex effects on cellular magnesium metabolism. (a) Manganese appears to be a competitive inhibitor for the magnesium accumulation system with a K_i = 0.5 mm manganese in Tris. (b) The addition of high (5 to 10 mm) manganese causes the rapid loss of ²⁸Mg from previously loaded cells and a net loss of magnesium, as if mangenese were displacing internal bound magnesium which then leaves the cells via the magnesium transport system.

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