Mn²⁺ transport by Ca²⁺‐ATPase of sarcoplasmic reticulum

Abstract

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 Kd ~ 0.5 mm). Stoichiometries of transported ions per ATP hydrolysed, Vmax values and activation energies are very similar. Altogether, Ca2+‐ATPase appears to use the same mechanism for transporting Mn2+ as that for Ca2+.