Phosphatidylinositol-specific phospholipase C from Bacillus cereus: improved purification, amino acid composition, and amino-terminal sequence

Defining the subcellular distribution and metabolic channeling of phosphatidylinositol

Phosphatidylinositol (PI) is an essential structural component of eukaryotic membranes that also serves as the common precursor for polyphosphoinositide (PPIn) lipids. Despite the recognized importance of PPIn species for signal transduction and membrane homeostasis, there is still a limited understanding of the relationship between PI availability and the turnover of subcellular PPIn pools. To address these shortcomings, we established a molecular toolbox for investigations of PI distribution within intact cells by exploiting the properties of a bacterial enzyme, PI-specific PLC (PI-PLC). Using these tools, we find a minor presence of PI in membranes of the ER, as well as a general enrichment within the cytosolic leaflets of the Golgi complex, peroxisomes, and outer mitochondrial membrane, but only detect very low steady-state levels of PI within the plasma membrane (PM) and endosomes. Kinetic studies also demonstrate the requirement for sustained PI supply from the ER for the maintenance of monophosphorylated PPIn species within the PM, Golgi complex, and endosomal compartments.

Targeting phosphatidylcholine-specific phospholipase C for atherogenesis therapy

Atherosclerosis, a dynamic and progressive vascular disease arising from the combination of endothelial dysfunction and inflammation, is becoming a major killer in the 21st century. Accumulating evidence implicates phosphatidylcholine-specific phospholipase C (PC-PLC) in endothelial dysfunction and several inflammation processes. In addition, in a recent study, we demonstrated that PC-PLC contributed to the progression of atherosclerosis. Considering the important roles of PC-PLC in vascular endothelial cell dysfunction and its proinflammatory properties, we propose that a pharmacological blockade of PC-PLC represents a rational approach to atherosclerosis therapy.

Interaction of two classes of ADP-ribose transfer reactions in immune signaling

CD38 is a bifunctional ectoenzyme predominantly expressed on hematopoietic cells where its expression correlates with differentiation and proliferation. The two enzyme activities displayed by CD38 are an ADP-ribosyl cyclase and a cyclic adenosine diphosphate ribose (cADPR) hydrolase that catalyzes the synthesis and hydrolysis of cADPR. T lymphocytes can be induced to express CD38 when activated with antibodies against specific antigen receptors. If the activated T cells are then exposed with NAD, cell death by apoptosis occurs. During the exposure of activated T cells to NAD, the CD38 is modified by ecto-mono-ADP-ribosyltransferases (ecto-mono-ADPRTs) specific for cysteine and arginine residues. Arginine-ADP-ribosylation results in inactivation of both cyclase and hydrolase activities of CD38, whereas cysteine-ADP-ribosylation results only in the inhibition of the hydrolase activity. The arginine-ADP-ribosylation causes a decrease in intracellular cADPR and a subsequent decrease in Ca(2+) influx, resulting in apoptosis of the activated T cells. Our results suggest that the interaction of two classes of ecto-ADP-ribose transfer enzymes plays an important role in immune regulation by the selective induction of apoptosis in activated T cells and that cADPR mediated signaling is essential for the survival of activated T cells.

Leishmania donovani has distinct mannosylphosphoryltransferases for the initiation and elongation phases of lipophosphoglycan repeating unit biosynthesis

Lipophosphoglycan (LPG) is the predominant surface glycoconjugate of Leishmania promastigotes and plays several roles in the infectious cycle of this protozoan parasite. The salient feature of LPG is the presence of 15-30 copies of a disaccharide-phosphate repeating unit Gal(beta1,4)Man(alpha1-PO4), which is also found on many other secreted molecules (secretory acid phosphatase, phosphoglycan, proteophosphoglycan). This structural diversity suggests that a multiplicity of enzymes mediating repeating unit addition may exist, especially for the mannosylphosphoryltransferases (MPTs), which initiate repeating unit synthesis. This work has taken a combined biochemical-genetic approach to resolve this issue. An lpg- mutant of Leishmania donovani, JEDI, was obtained by antibody selection against cells expressing a repeating unit epitope of LPG. Metabolic and surface labeling experiments revealed that JEDI cells accumulated a truncated form of LPG bearing only a single repeating unit: [Gal(beta 1,4)Man(alpha1-PO4)][Gal(alpha1,6)Gal(alpha1,3)Gal(f)(beta1,3)[Glc(alpha 1-PO4)]Man(alpha1,3)Man(alpha1,4)GlcN(alpha1,6)]-PI. Enzymatic assays of microsomal preparations showed that JEDI lacked MPT activity when tested with a repeating unit acceptor but retained wild-type levels of the MPT activity with an LPG glycan core acceptor. These data indicate that at least two distinct MPT activities are required for LPG repeating unit synthesis: one involved in the 'initiation' of repeating unit synthesis on the LPG core (iMPT), and a second (lacking in JEDI) participating in the 'elongation' phase of repeating unit addition (eMPT), leading to the mature full-length LPG.

Determination of pKa values of the histidine side chains of phosphatidylinositol-specific phospholipase C from Bacillus cereus by NMR spectroscopy and site-directed mutagenesis

Two active site histidine residues have been implicated in the catalysis of phosphatidylinositol-specific phospholipase C (PI-PLC). In this report, we present the first study of the pKa values of histidines of a PI-PLC. All six histidines of Bacillus cereus PI-PLC were studied by 2D NMR spectroscopy and site-directed mutagenesis. The protein was selectively labeled with 13C epsilon 1-histidine. A series of 1H-13C HSQC NMR spectra were acquired over a pH range of 4.0-9.0. Five of the six histidines have been individually substituted with alanine to aid the resonance assignments in the NMR spectra. Overall, the remaining histidines in the mutants show little chemical shift changes in the 1H-13C HSQC spectra, indicating that the alanine substitution has no effect on the tertiary structure of the protein. H32A and H82A mutants are inactive enzymes, while H92A and H61A are fully active, and H81A retains about 15% of the wild-type activity. The active site histidines, His32 and His82, display pKa values of 7.6 and 6.9, respectively. His92 and His227 exhibit pKa values of 5.4 and 6.9. His61 and His81 do not titrate over the pH range studied. These values are consistent with the crystal structure data, which shows that His92 and His227 are on the surface of the protein, whereas His61 and His81 are buried. The pKa value of 6.9 corroborates the hypothesis of His82 acting as a general acid in the catalysis. His32 is essential to enzyme activity, but its putative role as the general base is in question due to its relatively high pKa.

Phosphatidylinositol-specific phospholipase C: kinetic and stereochemical evidence for an interaction between arginine-69 and the phosphate group of phosphatidylinositol

A new substrate analogue, (2R)-1,2-dipalmitoyloxypropanethiophospho-1-D-myo-inositol (DPsPI), has been used in a new, continuous assay for phosphatidylinositol-specific phospholipase C (PI-PLC). DPsPI is superior to other substrate analogs that have been used for assaying PI-PLC since it is synthesized as a pure diastereomer and maintains both acyl chains of the natural substrate, dipalmitoylphosphatidylinositol (DPPI). The assay that has been developed using this new analogue has allowed us to elucidate detailed kinetic data so far lacking in the field. In addition, several mutants of PI-PLC were constructed and assayed. The results show that Arg-69 is essential for catalysis, since mutations at this position led to a 10(3)- 10(4)-fold decrease in activity with respect that of to the wild-type (WT) enzyme. An alanine mutant of Asp-67, a residue also found at the active site, displays activity similar to that of WT. We have also used nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy to analyze the structural integrity and conformational stability of the mutants. The results show that the overall global conformation of the enzyme is not perturbed by the mutants. The 15N-1H HSQC NMR spectrum of WT PI-PLC is also reported at 600 MHz. The stereoselectivity of the reaction toward the stereoisomers of another analogue, 1,2-dipalmitoyl-sn-glycero-3-thiophospho-1-myo-inositol (DPPsI), was used to probe whether Arg-69 interacts with the phosphate moiety of the substrate. We have calculated that the WT enzyme shows a stereoselectivity ratio of 160000:1 in favor of the Rp isomer versus the Sp isomer. The R69K mutant displayed a significant 10(4)-fold relaxation of stereoselectivity. Our data support the role of Arg-69 in stabilizing the negative charge on the pentacoordinate phosphate in the transition state during catalysis.

Bacillus cereus and related species

Bacillus cereus is a gram-positive aerobic or facultatively anaerobic spore-forming rod. It is a cause of food poisoning, which is frequently associated with the consumption of rice-based dishes. The organism produces an emetic or diarrheal syndrome induced by an emetic toxin and enterotoxin, respectively. Other toxins are produced during growth, including phospholipases, proteases, and hemolysins, one of which, cereolysin, is a thiol-activated hemolysin. These toxins may contribute to the pathogenicity of B. cereus in nongastrointestinal disease. B. cereus isolated from clinical material other than feces or vomitus was commonly dismissed as a contaminant, but increasingly it is being recognized as a species with pathogenic potential. It is now recognized as an infrequent cause of serious nongastrointestinal infection, particularly in drug addicts, the immunosuppressed, neonates, and postsurgical patients, especially when prosthetic implants such as ventricular shunts are inserted. Ocular infections are the commonest types of severe infection, including endophthalmitis, panophthalmitis, and keratitis, usually with the characteristic formation of corneal ring abscesses. Even with prompt surgical and antimicrobial agent treatment, enucleation of the eye and blindness are common sequelae. Septicemia, meningitis, endocarditis, osteomyelitis, and surgical and traumatic wound infections are other manifestations of severe disease. B. cereus produces beta-lactamases, unlike Bacillus anthracis, and so is resistant to beta-lactam antibiotics; it is usually susceptible to treatment with clindamycin, vancomycin, gentamicin, chloramphenicol, and erythromycin. Simultaneous therapy via multiple routes may be required.

Purification and characterization of Listeria monocytogenes phosphatidylinositol-specific phospholipase C

We have purified to homogeneity the 33-kDa phosphatidylinositol-specific phospholipase C (PI-PLC) from the culture fluid of Listeria monocytogenes, a facultative intracellular pathogen. The protein was overexpressed, and secretion of PI-PLC was further enhanced by the addition of divalent cations to the culture medium. The basic protein (pI, approximately 9.4) was complexed with anionic proteins in the crude culture fluid. It bound to DEAE-Sepharose and was eluted from Sephacryl S-200 near the void volume in low-ionic-strength buffer, suggesting aggregates of greater than or equal to 150 kDa. Gel filtration chromatography on Sephacryl S-200 in the presence of 1 M ammonium sulfate resulted in disaggregation and complete separation of PI-PLC, which interacted with the column matrix. Amino-terminal sequencing of the pure protein gave results consistent with the previously deduced sequence and showed that the signal cleavage site was between alanine 29 and tyrosine 30. The enzyme was specific for PI and showed no activity with phosphatidylethanolamine, phosphatidylcholine, or phosphatidylserine. It did not cleave PI-4-phosphate or PI-4,5-bisphosphate, but it was active on the membrane form of the variable surface glycoprotein from Trypanosoma brucei, a PI-glycan-anchored protein. When assayed with deoxycholate-mixed micelles of PI, activity was highly dependent on added salt. Activation by salt was also observed with Triton X-100-mixed micelles. The optimal concentration of CaCl2 or MgCl2 was lower than that of KCl or (NH4)2SO4, but activity was not specifically dependent on divalent cations and was not inhibited by addition of EDTA. With deoxycholate, the optimum pH was 7.0. A broader pH optimum ranging from 5.5 to 6.5 was observed with Triton X-100-mixed micelles. These results are consistent with a postulated role for secreted PI-PLC in the acidified primary phagocytic vesicle of infected cells.

Biosynthesis of lipophosphoglycan from Leishmania donovani: characterization of mannosylphosphate transfer in vitro

Lipophosphoglycan (LPG) is the major surface glycoconjugate of Leishmania donovani promastigotes and is composed of a capped polymer of repeating PO4-6Gal(beta 1,4)Man alpha 1 disaccharide units linked via a phosphosaccharide core to a lyso-1-O-alkylphosphatidylinositol anchor. An exogenous acceptor composed of the glycolipid anchor portion of LPG was shown to stimulate the enzymatic synthesis of the repeating phosphorylated disaccharide units of LPG in a cell-free system. Using the exogenous acceptor, GDP-[3H]Man, [beta-32P]GDP-Man, and unlabeled UDP-Gal as substrates, membrane preparations from an LPG-defective mutant of L. donovani that lacks endogenous acceptors catalyzed the incorporation of the doubly labeled mannosylphosphate unit into a product that exhibited the chemical and chromatographic characteristics of LPG. Analysis of fragments generated by mild acid hydrolysis of the radiolabeled product indicated that [3H]mannose-1-[32P]PO4 had been transferred from the dual-labeled sugar nucleotide. These results are consistent with the proposal that the repeating units of the L. donovani LPG are synthesized by the alternating transfer of mannose 1-phosphate and galactose from their respective nucleotide donors.

A fluorescent substrate for the continuous assay of phosphatidylinositol-specific phospholipase C: synthesis and application of 2-naphthyl myo-inositol-1-phosphate

A fluorescent water-soluble substrate for phosphatidylinositol-specific phospholipase C was synthesized. The diacylglycerol moiety of the natural substrate, phosphatidylinositol, was replaced by the fluorescent moiety, 2-naphthol, resulting in the synthetic substrate, racemic 2-naphthyl myo-inositol-1-phosphate. The synthetic substrate provided a continuous fluorometric assay for the phosphatidylinositol-specific phospholipase C from Bacillus cereus. Initial rates of the cleavage of the 2-naphthyl substrate by the phospholipase measured by fluorometry were linear with time and the amount of enzyme added. The specific enzyme activity at pH 8.5 and 25 degrees C was about 0.04 mumol/min mg protein at an initial substrate concentration of 0.8 mM. 31P NMR experiments suggest that, as with phosphatidylinositol itself, cleavage of the fluorescent substrate proceeds in two steps via a myo-inositol-1,2-cyclic phosphate intermediate, and that only the D-isomer is a substrate for the B. cereus phospholipase. The synthetic substrate was stable during long-term storage as a solid in the dark at -20 degrees C. It was also stable for several weeks when stored in the dark frozen in aqueous solution near neutral pH.

Glycosyl-phosphatidylinositol anchored acetylcholinesterase as substrate for phosphatidylinositol-specific phospholipase C from Bacillus cereus

We investigated the enzymatic properties of phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus towards glycosyl-phosphatidylinositol anchored acetylcholinesterase (AChE) from bovine erythrocytes and Torpedo electric organ as substrate. The conversion of membrane from AChE to soluble AChE by PI-PLC depended on the presence of a detergent and of phosphatidylcholine. In presence of mixed micelles containing Triton X-100 (0.05%) and phosphatidylcholine (0.5 mg/ml) the rate of AChE conversion was about 3 times higher than in presence of Triton X-100 alone. Furthermore, inhibition of PI-PLC occurring at Triton X-100 concentrations higher than 0.01% could be prevented by addition of phosphatidylcholine. Ca2+, Mg2+ and sodium chloride had no effect on PI-PLC activity in presence of phosphatidylcholine and Triton X-100, whereas in presence of Triton X-100 alone sodium chloride largely increased the rate of AChE conversion. Determination of kinetic parameters with three different substrates gave Km-values of 7 microM, 17 microM and 2 mM and Vmax-values of 0.095 microM.min-1, 0.325 microM.min-1 and 56 microM.min-1 for Torpedo AChE, bovine erythrocyte AChE and phosphatidylinositol, respectively. The low Km-values for both forms of AChE indicated that PI-PLC not only recognized the phosphatidylinositol moiety of the anchor but also other components thereof.

High-level expression in Escherichia coli and rapid purification of phosphatidylinositol-specific phospholipase C from Bacillus cereus and Bacillus thuringiensis

The construction of four vectors for high-level expression in Escherichia coli of the phosphatidylinositol-specific phospholipase C from Bacillus cereus or Bacillus thuringiensis is described. In all constructs the coding sequence for the mature phospholipase is precisely fused to the E. coli heat-stable enterotoxin II signal sequence for targeting of the protein to the periplasm. In one set of plasmids expression of the B. cereus or B. thuringiensis enzyme is under control of the E. coli alkaline phosphatase promoter, while in a second set of plasmids expression is under control of a lac-tac-tac triple tandem promoter. A simple and rapid procedure for complete purification of the phospholipase C overproduced in E. coli, involving isolation of the periplasmic proteins by osmotic shock followed by a single column chromatography step, is described. The largest quantity of purified enzyme, 40-60 mg per liter culture, is obtained with the plasmid expressing the B. cereus enzyme under control of the lac-tac-tac promoter. Lower quantities are obtained with the plasmids containing the alkaline phosphatase promoter (15-20 and 4-6 mg/liter for the B. cereus and B. thuringiensis enzymes, respectively) and with the plasmid expressing the B. thuringiensis phospholipase under control of the lac-tac-tac promoter (15-20 mg/liter). A comparison of the functional properties of the recombinant phospholipases with the native enzymes isolated from B. cereus or B. thuringiensis culture supernatant shows that they are identical with respect to their catalytic functions, viz., cleavage of phosphatidylinositol and cleavage of the glycosyl-phosphatidylinositol membrane anchor of bovine erythrocyte acetylcholinesterase.

Preparation and application of an affinity matrix for phosphatidylinositol-specific phospholipase C

A non-hydrolyzable phosphonate analogue of phosphatidyl inositol, racemic myo-inosityl-(1)-5-oxa-16-trifluoroacetamidohexadecyl phosphonate, was synthesized. This phosphonate inhibited the activity of phosphatidyl inositol-specific phospholipase C (PI-PLC) from Bacillus cereus with an IC50 of approximately 10 mM. Removal of the trifluoroacetyl blocking group followed by covalent binding of the phosphonate to cyanogen bromide activated Sepharose 4B via the amino group produced an affinity matrix specific for the PI-PLC from B. cereus. This affinity matrix was used to purify the phospholipase C from a complex mixture of proteins in a single step. Competition experiments with myo-inositol in the elution medium indicated that specific binding of the enzyme to the matrix most likely involves the enzyme active site. The inositol phosphonate derivatized matrix was stable over several months in neutral and alkaline media and was used repeatedly without loss of binding capacity. These results show that affinity matrices employing myo-inositol phosphonate ligands are useful for isolation and binding studies of PI-PLC and possibly of other enzymes interacting with phosphoinositides or myo-inositol phosphate derivatives.

Inhibition of the phosphatidylinositol-specific phospholipase C from Bacillus cereus by a monoclonal antibody binding to a region with sequence similarity to eukaryotic phospholipases

Bacterial phosphatidylinositol-specific phospholipases C (PI-PLC) display similar substrate specificity as their eukaryotic counterparts involved in signal transduction of insulin and Ca2(+)-mobilizing hormones, and are used in the study of the novel glycosylphosphatidylinositol-protein anchors (GPI-anchors). For the investigation of structure-function aspects of the PI-PLC secreted from Bacillus cereus cells, a panel of murine monoclonal antibodies was generated and shown to be specific for the PI-PLC polypeptide in enzyme-linked immunosorbent assays and Western blots. Two of the monoclonals inhibited reactions catalyzed by the bacterial enzyme in vitro: hydrolysis of phosphatidylinositol and the release of bovine erythrocyte acetylcholinesterase from its GPI-anchor. At saturating concentrations of inhibitory antibody only a few percent of the enzyme activity remained. The epitope recognized by one of the inhibitory antibodies, A72-24, was mapped by proteolytic digestion, protein sequencing, and Western blotting of the generated fragments. The data indicate that at least part of the epitope resides within an 8 kDa-stretch of the bacterial PI-PLC (Gln-45 - Lys-122). Essentially the same segment of the bacterial polypeptide has previously been shown to display limited amino acid sequence similarity with several eukaryotic PI-specific phospholipases C (Kuppe, A., Evans, L.M., McMillen, D.A. and Griffith, O.H. (1989) J. Bacteriol. 171, 6077-6083). The results reported here suggest that the conserved peptide of these enzymes may contain functionally important residues.

Phosphatidylinositol-specific phospholipase C from Bacillus cereus combines intrinsic phosphotransferase and cyclic phosphodiesterase activities: a 31P NMR study

The inositol phosphate products formed during the cleavage of phosphatidylinositol by phosphatidylinositol-specific phospholipase C from Bacillus cereus were analyzed by 31P NMR. 31P NMR spectroscopy can distinguish between the inositol phosphate species and phosphatidylinositol. Chemical shift values (with reference to phosphoric acid) observed are 0.41, 3.62, 4.45, and 16.30 ppm for phosphatidylinositol, myo-inositol 1-monophosphate, myo-inositol 2-monophosphate, and myo-inositol 1,2-cyclic monophosphate, respectively. It is shown that under a variety of experimental conditions this phospholipase C cleaves phosphatidylinositol via an intramolecular phosphotransfer reaction producing diacylglycerol and D-myo-inositol 1,2-cyclic monophosphate. We also report the new and unexpected observation that the phosphatidylinositol-specific phospholipase C from B. cereus is able to hydrolyze the inositol cyclic phosphate to form D-myo-inositol 1-monophosphate. The enzyme, therefore, possesses phosphotransferase and cyclic phosphodiesterase activities. The second reaction requires thousandfold higher enzyme concentrations to be observed by 31P NMR. This reaction was shown to be regiospecific in that only the 1-phosphate was produced and stereospecific in that only D-myo-inositol 1,2-cyclic monophosphate was hydrolyzed. Inhibition with a monoclonal antibody specific for the B. cereus phospholipase C showed that the cyclic phosphodiesterase activity is intrinsic to the bacterial enzyme. We propose a two-step mechanism for the phosphatidyl-inositol-specific phospholipase C from B. cereus involving sequential phosphotransferase and cyclic phosphodiesterase activities. This mechanism bears a resemblance to the well-known two-step mechanism of pancreatic ribonuclease, RNase A.

Inhibition of phosphatidylinositol-specific phospholipase C by phosphonate substrate analogues

Non-hydrolysable analogues of phosphatidylinositol were synthesized and tested as inhibitors of phosphatidylinositol-specific phospholipase C from Bacillus cereus. In these molecules, the phosphodiester bond of phosphatidylinositol hydrolyzed by the phospholipase was replaced by a phosphonate linkage and a simpler hydrophobic group replaced the diacylglycerol moiety. One of the phosphonates also contained a carboxylate functional group suitable for matrix attachment. All three synthetic phosphonates inhibited the phospholipase C activity in a concentration-dependent manner, with the analogue most closely resembling the structure of the natural substrate, phosphatidylinositol, being the most potent inhibitor. The data indicate that phosphonate analogues of phosphatidylinositol may be useful for study of phospholipase C and other proteins interacting with myo-inositol phospholipids.

Phosphatidylinositol-specific phospholipase C of Bacillus cereus: cloning, sequencing, and relationship to other phospholipases

The phosphatidylinositol (PI)-specific phospholipase C (PLC) of Bacillus cereus was cloned into Escherichia coli by using monoclonal antibody probes raised against the purified protein. The enzyme is specific for hydrolysis of the membrane lipid PI and PI-glycan-containing membrane anchors, which are important structural components of one class of membrane proteins. The protein expressed in E. coli comigrated with B. cereus PI-PLC in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, as detected by immunoblotting, and conferred PI-PLC activity on the host. This enzyme activity was inhibited by PI-PLC-specific monoclonal antibodies. The nucleotide sequence of the PI-PLC gene suggests that this secreted bacterial protein is synthesized as a larger precursor with a 31-amino-acid N-terminal extension to the mature enzyme of 298 amino acids. From analysis of coding and flanking sequences of the gene, we conclude that the PI-PLC gene does not reside next to the gene cluster of the other two secreted phospholipases C on the bacterial chromosome. The deduced amino acid sequence of the B. cereus PI-PLC contains a stretch of significant similarity to the glycosylphosphatidylinositol-specific PLC of Trypanosoma brucei. The conserved peptide is proposed to play a role in the function of these enzymes.

Improved purification and biochemical properties of phosphatidylinositol-specific phospholipase C from Bacillus thuringiensis

Monophosphatidylinositol inositol phosphohydrolase (phosphatidylinositol-specific phospholipase C. PtdIns-PLC. EC 3.1.4.10) has been purified from a Bacillus thuringiensis culture supernatant and from the cellular fraction of a recombinant Escherichia coli clone containing the PtdIns-PLC gene from B. thuringiensis. The two-step purification procedure involved ion-exchange chromatography on DEAE-Sepharose followed by separation on a Mono-Q/FPLC-column with yields of 32% and 50%, respectively. The molecular mass was determined to be 34 kDa by SDS/PAGE. The isoelectric point of the enzyme was 5.15. The amino-terminal sequences were shown to be identical for the enzymes purified from both organisms. PtdIns-PLC was inhibited by divalent cations using mixed micelles of Triton X-100 and pure phosphatidylinositol. PtdIns-PLC activity was detectable on polyacrylamide gels by activity staining on phosphatidylinostiol-containing agarose.