The calcium pump in rat liver endoplasmic reticulum. Demonstration of the phosphorylated intermediate.

Rat liver microsomal fractions enriched with smooth-surfaced vesicles possess an ATP-supported Ca2+ transport activity, which is stimulated by oxalate and half-maximally activated at 3.5 X 10(-7) M free Ca2+. Catalysis of Ca2+ transport involves transient covalent binding of the terminal phosphate from ATP by the vesicles, resulting in the formation of a Mr 118,000 phosphopeptide, which is acid precipitable and unstable in the presence of hydroxylamine, which may be characteristic for an acylphosphate. Phosphorylation of the Mr 118,000 peptide requires the presence of Ca2+, while dephosphorylation is markedly accelerated by Mg2+. In the presence of Ca2+ and Mg2+ phosphorylation proceeds much faster than dephosphorylation, indicating that the latter may be rate limiting for the hydrolysis of ATP. The Mr 118,000 peptide is estimated to represent about 2% of total smooth-surfaced endoplasmic reticulum membrane protein. Comparative studies with sarcoplasmic reticulum from rat skeletal muscle suggest extensive homology of the Ca2+ transport ATPases.