Kinetic characterization of calcium uptake by the rat liver Golgi apparatus

We carried out a kinetic characterization of the Ca(2+)active transport in the rat liver Golgi Apparatus (GA) membrane. Calcium accumulation by vesicles of a GA enriched fraction was found to be a function of both Ca(2+)and ATP-Mg concentrations, it was inhibited by 2 microm thapsigargin but not stimulated by 3 microm calmodulin. The kinetic parameter values obtained for the GA Ca(2+)pump were: J(max)of 3.96 nmol/mg min, K(m)for Ca(2+)of 0.150 microm and two K(m)'s for ATP of 1.14 microm and 519 microm. These results were almost identical to those obtained for the endoplasmic reticulum (ER) fraction, indicating that the GA Ca(2+)pump is a sarco/endoplasmic reticulum (SERCA) P-type, analogous-if not identical-to that present in the ER.