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Abstract

1

The terminal phosphate of (γ‐32P)ATP is rapidly incorporated into cardiac sarcoplasmic reticulum membranes (0.7–1.3 μmol/g protein) in the presence of calcium and magnesium. Cardiac sarcoplasmic reticulum membranes catalize an ATP‐ADP phosphate exchange in the presence of calcium and magnesium.
2

Half‐maximum activation of the phosphoprotein formation and ATP‐ADP phosphate exchange is reached at an ionized calcium concentration of about 0.3 μM. The Hill coefficients are 1.3.
3

Transphosphorylation and ATP‐ADP phosphate exchange require magnesium and are maximally activated at magnesium concentrations close to or equal to the ATP concentration.
4

The phosphoprotein level is reduced to about 45% at an ADP/ATP ratio of 0.1. The rate of calcium‐dependent ATP splitting declines, whilst the rate of the calcium‐dependent ATP‐ADP phosphate exchange increases when the ADP/ATP ratio is varied from 0.1 to 1. The sum of both, the rate of ATP splitting and the rate of ADP‐ATP phosphate exchange remains constant.
5

Phosphoprotein formation and ATP‐ADP phosphate exchange are not affected by azide, dinitrophenol, dicyclohexyl carbodiimide and oubain, whilst both activities are reduced by blockade of –SH groups localized on the outside of the sarcoplasmic reticulum membrane.
6

The isolated phosphoprotein is acid stable. The trichloroacetic acid denatured 32P‐labelled membrane complex is dephosphorylated by hydroxylamine, which might indicate that the phosphorylated protein is an acyl‐phosphate.
7

Polyacrylamide gel elctrophoresis (performed with phenol/acetic acid/water) of phosphorylated sarcoplasmic reticulum fractions demonstrates that the 32P‐incorporation occurs into a protein of about 100000 molecular weight.
8

It is suggested that the phosphoprotein represents a phosphorylated intermediate of the calcium‐dependent ATPase which formation occurs as an early step in the reaction sequence of calcium translocation by cardiac sarcoplasmic reticulum similar as in skeletal muscle.