Coupling Ratio for Ca²⁺ Transport by Calcium Oxalate Precipitation

Abstract

The SERCA isoform 1a is constructed to transport 2 Ca²⁺ ions across the sarcoplasmic reticulum membrane coupled to the hydrolysis of one molecule of MgATP. However, observed coupling ratios for Ca²⁺ transported/ATP hydrolzyed are usually less than 2:1, since part of the Ca²⁺ accumulated at high intravesicular concentrations by the active transport of Ca²⁺ leaks out of the vesicles because of Ca²⁺-induced Ca²⁺ exchange. However, in the presence of a high concentration of oxalate (5 mM) Ca²⁺ will precipitate as Ca-oxalate inside the vesicles and thereby be prevented from leaking out and, in addition, this treatment will reduce the intravesicular free concentration of Ca²⁺ to a level where optimal coupling ratios of 2:1 can be achieved.