Guanine nucleotides activate cytosolic phospholipase C of ascites tumour cells stimulated by 1-oleoyl-2-acetyl-sn-glycerol.

The hydrolysis of [3H]phosphatidylinositol (PI) and [3H]phosphatidylinositol-4,5-bisphosphate (PIP2) by cytosolic inositide phosphodiesterase (phospholipase C) from Ehrlich ascites tumour cells was determined. Cytosolic fractions were prepared from tumour cells that had been cultivated for two days at low serum level (2%) in the presence of 1-oleoyl-2-acetyl-sn-glycerol (OAG). Cytosols from unstimulated cells (2% serum without OAG) were used for comparison. Phospholipase C acting on PI and PIP2 was significantly inhibited in the cytosol of OAG-stimulated cells. The suppressed enzyme was activated by Ca2+ and also by the guanine nucleotide GTP in a concentration-dependent manner independently of calcium ions. In the presence of Ca2+, GTP exerted a synergistic stimulatory effect. In contrast, GTP and GTP gamma S showed no effect on the phospholipase C activity of unstimulated cells. It is suggested that the suppressed PI- and PIP2-specific enzyme activity can be modulated by its susceptibility to Ca2+ ions and GTP probably via the GTP-binding protein.