A calcium-activated polyphosphoinositide phosphodiesterase in the plasma membrane of human and rabbit erythrocytes

Haemoglobin-free human erythrocyte ghosts that were prepared in the presence of EDTA and were then exposed to Ca²⁺ showed a substantial loss of phosphatidylinositol phosphate and phosphatidylinositol diphosphate, measured either chemically or by loss of ³²P from the lipids of prelabelled membranes. At the same time there was, as reported previously (Allan, D. and Michell, R.H., (1976) Biochim. Biophys. Acta 455, 824-830), an approximately equivalent rise in the diacylglycerol content of the membranes. Analysis of the ³²P-labelled water-soluble material released during this process showed that the major products were inositol diphosphate and inositol triphosphate. No change was seen in the phosphatidylinositol or phosphatidate content of the membranes, and there was no Ca²⁺-activated loss of ³²P from the phosphatidate of prelabelled membranes: this suggests that Ca²⁺ did not activate phosphoinositide phosphomonoesterases or phosphatidate phosphomonoesterase in human erythrocyte membranes. It is concluded that human erythrocyte membranes contain at their cytoplasmic surface a Ca²⁺-activated phosphodiesterase that is active against both phosphatidylinositol phosphate and phosphatidylinositol diphosphate. Rabbit erythrocytes also contained this enzyme, but in these cells there was also evidence for the presence of a Ca²⁺-activated phosphatidate phosphomonoesterase.