Uptake of Calcium Ions into Microsomes Isolated from Physarum polycephalum

Abstract

Membranous vesicles (microsomes) were isolated from plasmodia of the acellular slime mold, Physarurn polycephalum. The microsomes were about 0.2 μm in diameter, and about 10 nm thick. The main protein component of the vesicles had a molecular weight of 100, 000 daltons.
Calcium ions were taken up by the microsomes only in the presence of Mg²⁺-ATP. The maximum amount of Call ions accumulated in the microsomes was 0.24 μmole/mg protein. The Call uptake was not accelerated by oxalate.
The ATPase [EC 3. 6. 1. 3] activity required Ca²⁺ ions for full activation. The concentra-tion of Ca²⁺ions required for half-maximum activation was about 1 μM. The Km and Vm values were 53 μM and 1.6 pmole/(mg.min), respectively. About 0.2 mole of Ca²⁺ ions was taken up by the microsomes, coupled with the hydrolysis of 1 mole of ATP.
The ATPase activity and Ca²⁺ uptake of the microsomes were not inhibited by sodium azide. Furthermore, electron microscopic examination showed that mitochondrial contamina-tion was slight.
These results suggest that a vesicular calcium transport system, analogous to the sarco-plasmic reticulum in skeletal muscle, is involved in regulation of the Ca²⁺ concentration in plasmodia of Physarum.