An antagonist of ATP-regulated potassium channels, the guanidine derivative U-37883A, stimulates the synthesis of phosphatidylserine in rat liver endoplasmic reticulum membranes

Abstract

The guanidine derivative U-37883A has been found to stimulate in vitro synthesis of phosphatidylserine in endoplasmic reticulum membranes, catalyzed exclusively by a serine-specific base exchange enzyme. The stimulation of the enzyme activity by the drug was concentration-dependent, with EC 50 of 54 μM, while the biologically inactive analog of U-37883A, U-42069, was without effect. The stimulation caused by U-37883A was enhanced under the conditions when active transport of Ca 2+ into the lumen of microsomal vesicles was induced, whereas it was inhibited by a calcium ionophore, A23187, and by a specific inhibitor of Ca 2+-ATPase, thapsigargin. On the other hand, a potassium ionophore, valinomycin, had no effect on phosphatidylserine synthesis. U-37883A did not affect the K m of the base exchange enzyme for serine, but greatly reduced the EC 50 value of the enzyme for calcium. Furthermore, Ca 2+ uptake by endoplasmic reticulum vesicles has been found to increase in the presence of U-37883A. These observations suggest that U-37883A enhances phosphatidylserine synthesis indirectly by acting on calcium transport, thus affecting calcium concentration within the lumen of endoplasmic reticulum membranes. Alternatively, the effect of the drug could be propagated via the mechanism by which phospholipid flip-flop movement, known to regulate the serine-specific base exchange reaction, is modulated.