Divalent cation regulation of phosphoinositide metabolism. Naturally occurring B lymphoblasts contain a Mg2(+)-regulated phosphatidylinositol-specific phospholipase C.
Membranes isolated from normal murine B lymphocytes were found to contain a novel phosphatidylinositol (PtdIns)-specific phospholipase C (PLC) which becomes activated as the Mg2+ concentration is raised from 30 to 1000 microM. This activity, which has not been described previously in any tissue, is restricted to naturally occurring B cell blasts, i.e. it was not detected in quiescent B cells, B lymphomas, or plasmacytomas. As seen in other cell systems, B cell membranes were found to contain Mg2(+)-stimulated inositol 1,4,5-trisphosphate phosphatase activity. Although neither the inositol 1,4,5-trisphosphate phosphatase nor the PtdIns PLC activities were affected by Ca2+, B cell membranes were found to contain a Ca2(+)-stimulated phosphatidylinositol 4,5-bisphosphate (PtdInsP2) PLC activity which is activated by [Ca2+] greater than 100 nM. Based on several characteristics, it appears that the Mg2(+)- and Ca2(+)-regulated PLCs are distinct species. First, they have distinct specificity for PtdIns and PtdInsP2, respectively. Second, they have distinct tissue distribution while the Ca2(+)-regulated activity was detected in all B cells, the Mg2(+)-regulated activity is restricted to low density, natural B blasts. Third, the kinetics of activation of the enzymes is distinct; the Mg2(+)-regulated enzyme exhibits slower and less transient activation kinetics. Fourth, the activities exhibit absolute specificity in terms of activation by Mg2+ and Ca2+, i.e. the PtdIns PLC is activated only by Mg2+ and the PtdInsP2 PLC is activated only by Ca2+. Data are consistent with the possibility that Mg2+ mobilization which follows ligation of certain receptors, may play an important role in the regulation of levels of the second messenger diacylglycerol.
Chien
M M
MM
Department of Pediatrics, National Jewish Center for Immunology and Respiratory Medicine, Denver, Colorado 80206.
Cambier
J C
JC
D000818:Animals; D001402:B-Lymphocytes / Q000201:enzymology*; D002118:Calcium / Q000494:pharmacology*; D002413:Cations, Divalent; D002462:Cell Membrane / Q000201:enzymology; D004789:Enzyme Activation / Q000187:drug effects; D006868:Hydrolysis; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D007295:Inositol Phosphates / Q000378:metabolism; D000072181:Inositol Polyphosphate 5-Phosphatases; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D008968:Molecular Conformation; D019269:Phosphatidylinositol 4,5-Diphosphate; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D010716:Phosphatidylinositols / Q000378:metabolism*; D054801:Phosphoinositide Phospholipase C; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D010766:Phosphorylation; D013379:Substrate Specificity
D002413:Cations, Divalent; D007295:Inositol Phosphates; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols; D015544:Inositol 1,4,5-Trisphosphate; D010744:Phosphoric Monoester Hydrolases; D000072181:Inositol Polyphosphate 5-Phosphatases; D010727:Phosphoric Diester Hydrolases; D054801:Phosphoinositide Phospholipase C; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D008274:Magnesium; D002118:Calcium