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J. Biol. Chem., Vol. 265, Issue 16, 9201-9207, 06, 1990

Divalent cation regulation of phosphoinositide metabolism. Naturally occurring B lymphoblasts contain a Mg2(+)-regulated phosphatidylinositol-specific phospholipase C

MM Chien and JC Cambier
Department of Pediatrics, National Jewish Center for Immunology and Respiratory Medicine, Denver, Colorado 80206.

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.
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