Active Transport of Calcium in Membrane Vesicles from Mycobacterium phlei

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Abstract
Active transport of calcium ions has been demonstrated in inside‐out membrane vesicles from Mycobacterium phlei mediated by respiratory linked substrates as well as by ATP hydrolysis. The uptake of calcium exhibited an apparent Km of 80 μM and V of 16.6 nmol calcium uptake × min−1× mg protein−1. A fortyfold concentration gradient for calcium ions was calculated for both the ATP‐induced and the respiration‐induced transport of calcium. Removal of coupling‐factor‐latent ATPase resulted in the complete loss of ATP‐driven Ca2+ transport whereas the respiration‐driven uptake was reduced by 40–50%. The uptake of calcium was inhibited by the proton conducting ionophores carbonylcyanide m‐chlorophenylhydrazone and Gramicidin‐D. The accumulated calcium was freely exchangeable with external calcium and was rapidly released by the addition of inhibitors of energy transduction, proton‐translocating uncouplers or the ionophore A23187. The uptake of the weak base, methylamine, upon the oxidation of respiratory‐linked substrates or the hydrolysis of ATP showed the generation of a proton gradient (inside acidic) which was partially collapsed on the addition of calcium ions. These results suggest that a Ca2+/H+ antiport mechanism may be responsible for the transport of calcium.

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