Mechanism of the calcium pump in the endoplasmic reticulum of liver: phosphoproteins as reaction intermediates.

Microsomal fractions, highly enriched with endoplasmic reticulum of rat and human liver exhibit Ca2+ uptake catalyzed by a Ca2+-pumping ATPase. The mechanism of Ca2+-translocation involves: (i) reversible Ca2+-dependent formation of an acyl-phosphoenzyme intermediate (Mr 116,000 to 118,000) with bound Ca2+, which in the reversed reaction can transphosphorylate its Pi to ADP to re-synthesize ATP; (ii) reversible transition of the ADP-reactive phosphoenzyme into an isomer without bound Ca2+, not further reactive to ADP; (iii) hydrolytic cleavage, stimulated by Mg2+, K+, and ATP of the ADP-unreactive phosphoenzyme with liberation of Pi. By analogy to a mechanism proposed for the Ca2+ pump of sarcoplasmic reticulum, the translocation of Ca2+ to and dissociation from the inner side of the membrane is suggested to occur by a conformational change, coupled with a decrease in Ca2+-affinity of the phosphoenzyme during its transition into the ADP-unreactive isomer. With CaATP as the effective substrate the reactions proceed normally but at a considerably slower rate.