Calcium-dependent hydrolyses of polyphosphoinositides in human erythrocyte membranes

Incubation of erythrocytes with [32P]phosphate resulted in a linear incorporation of the label into PtdIns(4,5)P2 (phosphatidylinositol 4,5-bisphosphate), PtdIns4P (phosphatidylinositol 4-monophosphate), and PA (phosphatidic acid) over a period of 2 h at 37 degrees C. Exposure of 32P-labelled erythrocyte ghosts to calcium caused a loss of label from PtdIns(4,5)P2 and PtdIns4P, but not PA. The concentration of calcium required for half-maximal hydrolyses of both polyphosphoinositides was about 1 microM. Strontium, at higher concentrations, stimulated the hydrolyses of both polyphosphoinositides but barium, up to 1 mM, had little effect. Intact erythrocytes incubated in the presence of Ca-EGTA buffers and the ionophore A23187 did not show marked losses of [32P]PtdIns(4,5)P2 or [32P]PtdIns4P, but rather exhibited a dramatic increase in the level of [32P]PA. In contrast, cells which had been depleted of their ATP lost significant amounts of [32P]PtdIns(4,5)P2 and [32P]PtdIns4P and had less change in their levels of [32P]PA relative to intact cells. The calcium activation curve and the time course for [32P]PA synthesis in intact cells were similar to the calcium activation curve and the time course for the hydrolyses of [32P]PtdIns(4,5)P2 and [32P]PtdIns4P in ATP-depleted erythrocytes. These results strongly support a link between Ca2+-dependent polyphosphoinositide breakdown and PA synthesis in human erythrocytes.