Mn²⁺ transport by Ca²⁺-ATPase of sarcoplasmic reticulum

Ca²⁺-ATPase of sarcoplasmic reticulum is known to pump Mn²⁺ in addition to Ca²⁺, but whether its transport mechanism is identical to that of Ca²⁺ is ambiguous. To clarify, we examined, by atomic absorption spectroscopy, competition between Mn²⁺ and Ca²⁺ in active transport using vesicles of sarcoplasmic reticulum (SR). Here, we demonstrate that Ca²⁺-ATPase transports Ca²⁺ and Mn²⁺ concomitantly but has a much lower affinity for Mn²⁺ (apparent K_d ~ 0.5 mm). Stoichiometries of transported ions per ATP hydrolysed, V_max values and activation energies are very similar. Altogether, Ca²⁺-ATPase appears to use the same mechanism for transporting Mn²⁺ as that for Ca²⁺.