Effect of phosphate on phosphatidylserine-mediated calcium transport

Summary

Experiments were performed to study the effect of inorganic phosphate (Pi) on phosphatidylserine-mediated Ca²⁺ transport utilizing two- and three-compartment lipid-aqueous phase model systems. When the three-compartment model was used, the rate of Ca²⁺ transport from an aqueous donor compartment to an aqueous receiver compartment, separated by a nonaqueous phospholipid phase, was determined. This experiment showed that the rate of Ca²⁺ transport was proportional to the phosphatidylserine concentration and the pH. Pi modulated the rate of Ca²⁺ transport; even when the Pi concentration of the donor aqueous phase was low, there was a marked enhancement of transport. To determine whether the Pi-mediated rise in the Ca²⁺ transport rate was due to an increase in the uptake of Ca²⁺ into the lipid phase, or to an increase in the ability of the lipid phase to release Ca²⁺, a two-compartment model was used. It was found that the ability of the phosphatidylserine phase to take up Ca²⁺ increased as the Pi concentration of the aqueous donor phase was raised. With the increase in Ca²⁺ uptake there was a concomitant elevation in the rate of Ca²⁺ transport into an aqueous receiver phase. However, Pi did not stimulate Ca²⁺ release from the phosphatide. Thus it was concluded that Pi enhanced the interaction between Ca²⁺ and phosphatidylserine, possibly by forming a Ca-phospholipid-Pi complex. Once this interaction had taken place, Ca²⁺ release into the aqueous receiver compartment was independent of the Pi concentration.