Invariance of Stoichiometry of the Sarcoplasmic Reticulum Calcium Pump at Physiological Calcium Concentrations – a Reevaluation

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Abstract
The decline of the transport ratio of the sarcoplasmic Ca pump observed in a recent study results from the retardation of calcium oxalate precipitation at low Ca/protein ratios. The prevailing high internal Ca level supports a rapid Ca backflux and a compensatory ATP hydrolysis during net Ca uptake which reduces the transport ratio. The determined Ca backflux does not fully account for the decline of the transport ratio. A supposed modulation of the stoichiometry of the pump by external Ca (0.1 .mu.M) is at variance with results of previous studies showing a constant transport ratio of 2 in the same Ca concentration range.