Purification and properties of membrane and cytosolic phosphatidylinositol-specific phospholipases C from human spleen

Cytosolic phospholipase C activity: I. Evidence for coupling with cytosolic guanine nucleotide-binding protein, Gi alpha

In a previous report we showed that glucocorticoid inhibition of cytosolic PLC activity correlated with a reduction in cytosolic Gi alpha levels, suggesting that there may be a functional relationship between cytosolic PLC and cytosolic Gi alpha. In order to establish the nature of the coupling between cytosolic Gi alpha and cytosolic PLC we examined the effects of G-protein activators, and inhibitors on cytosolic PLC activity from rat splenocytes and the rat lymphoma cell line Nb 2, with [3H] PI and [3H]PIP2 as substrates. 1) Neither GTP nor its nonhydrolyzable analogue, GTP gamma S, at 100 microM had any effect on the calcium stimulated as well as the basal PLC activity. 2) However, affinity purified antibodies to Gi alpha 1 and Gi alpha 2 inhibited soluble PLC activity, by 85% and 55%, respectively, with PI as substrate; with PIP2 as substrate, soluble PLC activity was inhibited 50-70% by antibodies to Gi1, whereas antibodies to Gi2 had little effect. 3) Administration of Gi alpha 1 antisense oligonucleotides to splenocytes for 48 h produced 25-40% decrease in cytosolic Gi alpha 1 levels compared to control. The soluble PLC activity with both PI and PIP2 as substrates was also reduced by 25-50% compared to control conditions. This suggest that cytosolic Gi alpha is associated with the activation of splenocyte soluble PLC. 4) Pertussis toxin administered in vivo significantly reduced cytosolic Gi alpha immunoreactivity and soluble PLC activity when PI was used as substrate, providing additional evidence that cytosolic Gi alpha is associated with the activation of soluble PLC. 5) Another agent that has been used extensively to define G-protein coupled processes is NaF/AlCl3. NaF (5 mM; with or without AlCl3) inhibited soluble PLC activity with PIP2 as substrate, in contrast to the stimulatory effect that has been reported in the activation of membrane PLC. 6) Because NaF can act as a protein phosphatase inhibitor, we also tested the effects of trifluoperizine (50 microM, TFP), an inhibitor of protein phosphatase 2B; TFP (50 microM) significantly inhibited soluble PLC activity when PI was used as substrate. These results suggest a direct involvement of cytosolic Gi alpha in the activation of soluble PLC from splenocytes. Other questions pertaining to the functional significance, the nature, and possible substrate preference of the splenocyte Gi alpha coupled PLC is addressed in the second paper.

Characteristics of phospholipase C present in membranes of Xenopus laevis oocytes. Stimulation by phosphatidic acid

1. Phospholipase C activity present in the membranes of Xenopus laevis oocytes has been studied. 2. These membranes contain an activity capable of hydrolyzing phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. 3. Hydrolysis of PtdIns 4,5-P2 is absolutely dependent on the presence of Ca2+, however, the hydrolysis of PtdIns occurs in the absence of Ca2+ but addition of the cation stimulates the reaction. 4. Spermine, spermidine and polylysine cause significant stimulation of the phospholipase C activity. 5. Phosphatidic acid causes approximately a 2-fold stimulation of the hydrolysis of both PtdIns and PtdIns 4,5-P2. With PtdIns as substrate, this stimulatory effect of phosphatidic acid is specific and reaches a maximum at a 400 microM concentration.

Detection and partial characterization of glycosylphosphatidylinositol-specific phospholipase activities from Fasciola hepatica and Schistosoma mansoni

The tegument of trematodes serves as a dynamic host-parasite interface where surface antigens are shed in a process of immune evasion. Phospholipases, which could provide an enzymatic mechanism for release of glycosylphosphatidylinositol (GPI)-anchored proteins, were detected in detergent extracts of adult worms of Fasciola hepatica and cercaria and adult worms of Schistosoma mansoni. The enzymatic activities were partially characterized from both adult worm species and demonstrated a preference for [3H]GPI substrate over [3H]PI. Lipase activities from both species were sensitive to sulfhydryl-modifying reagents and the detergents CHAPS and n-octylglucoside. The presence of 1 M ammonium sulfate increased the enzyme activity in adult worms of both species by 8-11-fold and in cercaria by 146-fold, whereas other conditions of high ionic strength were inhibitory. Such stimulation suggested dissociation of a negative inhibitor which is prominent in the cercarial stage. The schistosome extract, which was partially sensitive to cation chelators and o-phenanthroline, contained a GPI-phospholipase D activity. In contrast, the F. hepatica extract contained a cation-independent phospholipase C activity which was partially purified and shown by gel filtration to have a molecular mass of 30,000-80,000.

PI-specific phospholipase C "alpha" from sheep seminal vesicles is a proteolytic fragment of PI-PLC delta

Phosphatidylinositide-specific phospholipase C enzymes (PLCs) catalyze the conversion of the phosphoinositides to biologically important signal transducing molecules. These enzymes may be grouped into "families" which share similar structures and modes of regulation. The existence of a structurally distinct family of PLC termed "alpha" has been recently called into question. In the current paper we show by immunoblotting experiments that PLC "alpha" from sheep seminal vesicles is recognized by monoclonal antibodies raised against the delta 1 isoform of bovine brain PLC, and appears to be derived from a higher molecular weight band at 85 kDa. We also show that antibodies raised against PLC alpha efficiently immunoprecipitate the 85-kDa PLC delta 1 isoform from bovine brain and Chinese hamster lung fibroblasts. These data provide strong evidence that the PLC alpha from sheep seminal vesicles is a proteolytic fragment of PLC delta 1. Thus, there is still no conclusive evidence for a separate "alpha" class of PLC.