Soluble Fraction Of Rat Brain Research Articles

Protein purification and cloning of diacylglycerol lipase from rat brain.

Diacylglycerol (DG) lipase, which hydrolyses 1-stearoyl-2-arachidonyl-sn-glycerol to produce an endocannabinoid, 2-arachidonoylglycerol, was purified from the soluble fraction of rat brain lysates. DG lipase was purified about 1,200-fold by a sequential column chromatographic procedure. Among proteins identified by mass spectrometry analysis in the partially purified DG lipase sample, only DDHD domain containing two (DDHD2), which was formerly regarded as a phospholipase A1, exhibited significant DG lipase activity. Rat DDHD2 expressed in Chinese hamster ovary cells showed similar enzymatic properties to partially purified DG lipase from rat brain. The source of DG lipase activity in rat brain was immunoprecipitated using anti-DDHD2 antibody. Thus, we concluded that the DG lipase activity in the soluble fraction of rat brain is derived from DDHD2. DDHD2 is distributed widely in the rat brain. Immunohistochemical analysis revealed that DDHD2 is expressed in hippocampal neurons, but not in glia.

A new method to precipitate myosin V from rat brain soluble fraction.

Myosin can be precipitated from soluble fraction under different assay conditions. This paper describes a new method for precipitating myosin V from rat brain soluble fraction. Brains were homogenized in 50 mM imidazole/HCl buffer, pH 8.0, containing 10 mM EDTA/EGTA, 250 mM sucrose, 1 mM DTT and 1 mM benzamidine, centrifuged at 45000 x g for 40 min and the supernatant was frozen at -20 degrees C. Forty-eight hours later, the supernatant was thawed, centrifuged at 45000 x g for 40 min and the precipitate was washed in 20 mM imidazole buffer pH 8.0. SDS/PAGE analysis showed four polypeptides in the precipitate: 205, 150, 57 and 43 kDa. The precipitate presented high Mg(2+)-ATPase activity, which co-purifies with p205. This polypeptide was recognized by a specific myosin V antibody and was proteolised by calpain, generating two stable polypeptides: p130 and p90. The Mg(2+)-ATPase activity was not stimulated by calcium in both the absence and presence of exogenous calmodulin and the K+/EDTA-ATPase activity represented 25% of the Mg(2+)-ATPase activity. In this work, myosin V from rat brain was precipitated by freezing the soluble fraction and was co-purificated with a 45 kDa polypeptide.

Stereospecific hydrolysis of a phosphoramidate as a model to understand the role of biotransformation in the neurotoxicity of chiral organophosphorus compounds

Calcium-dependent and EDTA-resistant hydrolyses of R and S isomers of O-hexyl O-2,5-dicholorophenyl phosphoramidate (HDCP) were observed in serum and subcellular fractions of liver, kidney and brain from hen, rat and rabbit. In serum, the Ca 2+-dependent hydrolysis was much higher in rabbit than in other species. Liver showed a higher activity than kidney and brain. The S-HDCP isomer was hydrolysed to a higher extent than the other isomer. The fact that this stereospecificity favours the S-isomer is more clearly observed in rabbit serum, and in rat and rabbit liver particulate fractions. In such tissues and species, the EDTA-resistant hydrolysis was not stereospecific. Soluble fractions of rat brain and of hen liver, kidney and brain, showed a lower total activity but with a higher proportion of EDTA-resistant activity and a higher hydrolysis of the R-HDCP isomer. The Ca 2+-dependent stereoselective biodegradation of S-HDCP is dominant in the most active tissues in rabbit and rat. It can therefore be concluded that S-HDCP would be biodegraded faster than R-HDCP. Furthermore, R-HDCP is the isomer that will remain at a higher proportion to be available for interaction with the target of neurotoxicity.

Brain PP2A is modified by thiol-disulfide exchange and intermolecular disulfide formation

The regulation of protein phosphatase 2A (PP2A) activity by thiol-disulfide exchange and resulting formation of an intermolecular disulfide was examined following exposure of a rat brain soluble fraction to a biotinylated derivative of the model disulfide HPDP (HPDP–biotin) which would be expected to label reactive protein thiols with a disulfide-linked biotin. The results show that a low concentration (500 μM) of HPDP–biotin produced substantial inhibition of PP2A activity and promoted the binding of the catalytic subunit of PP2A to an immobilized avidin-affinity column. Both the inhibition of PP2A activity and the binding of PP2A to the avidin column were reversed by treatment with the disulfide reducing agent dithiothreitol (DTT). Furthermore, the specific activity of PP2A was up to 7-fold higher in the DTT-eluted fractions from the avidin-affinity column than in the soluble fraction. These findings demonstrate directly that PP2A is susceptible to reversible inhibitory modification by thiol-disulfide exchange and provide mechanistic support for the emerging view that PP2A is an oxidant-sensitive protein phosphatase.

Identification of major Ca 2+/calmodulin-dependent protein kinase phosphatase-binding proteins in brain: biochemical analysis of the interaction

Ca 2+/calmodulin-dependent protein kinase phosphatase (CaMKP) is a unique protein phosphatase that specifically dephosphorylates and regulates multifunctional Ca 2+/calmodulin-dependent protein kinases (CaMKs). To clarify the physiological significance of CaMKP, we identified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and fructose bisphosphate aldolase as major binding partners of CaMKP in a soluble fraction of rat brain using the two-dimensional far-Western blotting technique, in conjunction with peptide mass fingerprinting analysis. We analyzed the affinities of these interactions. Wild type CaMKP–glutathione S-transferase (GST) associated with GAPDH in a GST pull-down assay. Deletion analysis suggested that the N-terminal side of the catalytic domain of CaMKP was responsible for the binding to GAPDH. Further, anti-CaMKP antibody coimmunoprecipitated GAPDH in a rat brain extract. GAPDH was phosphorylated by CaMKI or CaMKIV in vitro; however, when CaMKP coexisted, the phosphorylation was markedly attenuated. Under these conditions, CaMKP significantly dephosphorylated CaMKI and CaMKIV, which had been phosphorylated by CaMK kinase, whereas it did not dephosphorylate the previously phosphorylated GAPDH. The results suggest that CaMKP regulates the phosphorylation level of GAPDH in the CaMKP–GAPDH complex by dephosphorylating and deactivating CaMKs that are responsible for the phosphorylation of GAPDH.

RNA interference suggests a primary role for monoacylglycerol lipase in the degradation of the endocannabinoid 2-arachidonoylglycerol.

The endogenous cannabinoid 2-arachidonoylglycerol (2-AG) is produced by neurons and other cells in a stimulus-dependent manner and undergoes rapid biological inactivation through transport into cells and catalytic hydrolysis. The enzymatic pathways responsible for 2-AG degradation are only partially understood. We have shown previously that overexpression of monoacylglycerol lipase (MGL), a cytosolic serine hydrolase that cleaves 1- and 2-monoacylglycerols to fatty acid and glycerol, reduces stimulus-dependent 2-AG accumulation in primary cultures of rat brain neurons. We report here that RNA interference-mediated silencing of MGL expression greatly enhances 2-AG accumulation in HeLa cells. After stimulation with the calcium ionophore ionomycin, 2-AG levels in MGL-silenced cells were comparable with those found in cells in which 2-AG degradation had been blocked using methyl arachidonyl fluorophosphonate, a nonselective inhibitor of 2-AG hydrolysis. The results indicate that MGL plays an important role in the degradation of endogenous 2-AG in intact HeLa cells. Furthermore, immunodepletion experiments show that MGL accounts for at least 50% of the total 2-AG-hydrolyzing activity in soluble fractions of rat brain, suggesting that this enzyme also contributes to 2-AG deactivation in the central nervous system.

Differential regulation of brain protein phosphatase activities by nitric oxide and nitroxyl anion

While nitric oxide (NO) is a physiological messenger molecule, increasing evidence suggests that its one‐electron reduction product, nitroxyl anion (NO–), may be particularly cytotoxic. We have examined the differential regulation of redox‐sensitive protein phosphatase activities in a rat brain soluble fraction by the nitric oxide donor dethylamine NONOate and the nitroxyl anion donor Angeli's salt using p‐nitrophenylphosphate (p‐NPP) as a nonspecific substrate. We report that micromolar NONOate but not Angeli's salt produced a 2–3‐fold increase in a calcineurin (CaN) phosphatase activity, defined as Ni2± dependent, calmidazolium‐sensitive, p‐NPPase activity. Activation of CaN by NONOate was (i) enhanced by carboxy‐PTIO, which promotes formation of the S‐nitrosylating agent N2O3 (ii) mimicked by S‐nitrosoglutathione (iii) partially reversed by the sulfhydryl reductant DTT, and (iv) accompanied by protein S‐nitrosylation. NONOate also produced a modest, largely DTT‐reversible, inhibition of the vanadate‐sensitive fraction of p‐NPPase that was attributed to protein tyrosine phosphatase (PTP) activity. We propose that regulation of protein phosphatase activities by reversible sulfhydryl modulation may contribute to the physiological actions of nitric oxide. In contrast to NONOate, Angeli's salt produced a marked and irreversible inhibition of PTP activity consistent with a possible higher order oxidation of an active‐site cysteine residue. Notably, Angeli's salt was at least a 10‐fold more potent inhibitor of PTP activity than was hydrogen peroxide. We propose that PTP inactivation by nitroxyl anion may promote excessive and neurotoxic protein tyrosine phosphorylation.

Na+,K+-ATPase interaction with a brain endogenous inhibitor (endobain E)

Na +,K +-ATPase activity of rat brain synaptosomal membranes was evaluated in the presence of an inhibitory fraction II-E (termed endobain E), isolated by gel filtration and anionic exchange HPLC of a rat brain soluble fraction. We studied endobain E aging, analyzed its inhibitory potency in the absence or presence of ouabain as well as its ability to block high affinity [ 3H]ouabain binding to cerebral cortex membranes. Similar loss of endobain E activity was observed when samples were stored either dried or in solution. Endobain E fraction inhibited synaptosomal membrane Na +,K +-ATPase activity in a concentration-dependent manner and the slope of the corresponding curve strongly resembled that of ouabain. Assays performed in the presence of endobain E and ouabain indicated that the inhibitory effect was additive or less than additive, depending on their respective concentrations during preincubation and⧹or incubation. High affinity [ 3H]ouabain binding to cerebral cortex membranes proved concentration-dependent from 0.10 to 0.50 mg protein per ml; binding inhibition by endobain E was independent of protein concentration within the above range. [ 3H]ouabain binding inhibition by endobain E was concentration-dependent over a 10-fold range, an effect similar to that found for Na +,K +-ATPase inhibition. The extent of endobain E effect on Na +,K +-ATPase inhibition was much higher (90–100%) than that on [ 3H]ouabain binding blockade (50%). Findings suggest some type of interaction between endobain E and ouabain inhibitory mechanisms and favour the view that the former behaves as an endogenous ouabain.

Isolation and properties of a novel phospholipase A from rat brain that hydrolyses fatty acids at sn-1 and sn-2 positions.

A Ca(2+)-independent phospholipase A that releases various fatty acids from sn-1 and sn-2 positions was partially purified from rat brain soluble fraction. The enzyme showed an approximate molecular mass of 300 kDa on gel filtration column chromatography. Its enzymatic properties are distinct from those of well characterized phospholipase A2 enzymes; by using a series of synthetic phosphatidylcholines, the enzyme cleaved oleic, linoleic, and arachidonic acids like phospholipase A2, and released palmitic and stearic acids like phospholipase A1. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidic acid were hydrolysed with almost equal efficiencies by this enzyme. These results indicate that the enzyme isolated is a novel Ca(2+)-independent intracellular phospholipase A that might be responsible for production of various fatty acids from membrane phospholipids.

Brain contains inhibiting factors specific to the large T-antigen gene

SV40 large T-antigen (LTa) gene-induced immortalized rat dopamine-producing nerve cells (1RB 3AN 27), which produce LTa protein and divide in vitro, do not divide and do not produce LTa protein when transplanted into striatum of adult rats. This suggests the presence of LTa gene-inhibiting factors in brain. Here we report that rat brain soluble fraction (SF) contains factors which specifically inhibit LTa gene activity in vitro. The brain SF inhibited LTa protein levels and the growth of 1RB 3AN 27 cells and 2RSG cells (LTa gene-induced immortalized rat parotid acinar cells) in vitro, but it stimulated the growth of spontaneously immortalized human parotid acinar cells (2HPC8) and had no effect on the proliferation of murine neuroblastoma cells (NBP 2) and rat glioma cells (C-6) in culture. In contrast, the liver SF inhibited the growth of all cell lines tested at varying degrees and thus lacked specificity with respect to LTa gene activity. The presence of specific LTa gene-inhibiting factors in the brain and general tumor growth-inhibiting factors in the liver may provide some of the mechanisms of protection against in vivo carcinogenesis.

Purification and Characterization of Rab GDIβ from Rat Brain

A new Rab GDI was purified from the synaptic soluble fraction of rat brain by several column chromatographies as a protein that inhibited the dissociation of [3H]GDP from Rab3A but was not recognized by an anti-Rab GDIα antibody. The partial amino acid sequence analysis revealed that it was identical with rat Rab GDIβ. Purified Rab GDIβ showed the kinetic properties similar to those of Rab GDIα, including the inhibitory effect on the dissociation of GDP from Rab3A, the substrate specificity, the requirement of the post-translational lipid modifications of Rab3A, the stoichiometric interaction with the GDP-bound form of Rab3A, the inhibitory effect on the binding of Rab3A to erythrocyte ghosts, and the stimulatory effect on the dissociation of Rab3A from the membrane.

Ca 2+-binding proteins in rat synaptic fractions surveyed by the 45Ca 2+ overlay method

Ca 2+-binding proteins in the synaptic and subsynaptic fractions (P 2, synaptosome, synaptic plasma membrane, and postsynaptic density [PSD]-enriched fractions) and soluble fraction of rat brain were surveyed by a 45Ca 2+ overlay method. The PSD-enriched fraction from cerebral cortex contained two major Ca 2+-binding proteins (55,000 M r and 19,000 M r) and a distinct group (in 140,000 M r region)m and two minor ones (66,000 M rand16,000 M r); and the fraction from cerebellum contained two (55,000 M rand19,000 M r). The proteins with 55,000 M rand19,000 M r were identified as tubulin and calmodulin, respectively, and present in all the fractions investigated. The Ca 2+-binding proteins of 140,000 M r region were found only in the PSD-enriched fraction isolated from cerebral cortex: neither the PSD-enriched fraction isolated from cerebellum nor other subcellular fractions prepared from cerebral cortex and cerebellum contained the proteins. The 140,000 M r Ca 2+-binding proteins were the substrates for the Ca 2+/calmodulin-dependent protein kinase II associated with PSD, and no change in the Ca 2+-binding was detected by the 45Ca 2+ overlay method after phosphorylation of the proteins by the protein kinase. The 16,000 M r Ca 2+-binding protein might be the β-subunit of calcineurin. Calretinin and calbindin-D 28k were also detected as Ca 2+-binding proteins in the soluble fractions of both cerebral cortex and cerebellum.

Isolation and characterization of the epsilon subspecies of protein kinase C from rat brain.

The epsilon subspecies of protein kinase C (epsilon PKC) was purified to near homogeneity from the soluble fraction of rat brain by successive chromatographies on DEAE-cellulose, threonine-Sepharose, phenyl-5PW, Mono Q, heparin-5PW, and hydroxyapatite columns. The enzyme from COS-7 cells that were transfected with an epsilon PKC cDNA expression plasmid showed the same elution profile. The purified enzyme from the brain was a double (96 and 93 kDa) on SDS/PAGE. Both the doublet proteins were recognized by antibodies raised against several oligopeptides that were parts of the deduced amino acid sequence of the rat brain epsilon PKC. When treated with potato acid phosphatase, both doublet proteins disappeared with the concomitant appearance of a single protein at 90 kDa, suggesting that epsilon PKC exists in the tissue as phosphorylated forms. The physiological significance of this phosphorylation is unknown. The enzymes from the rat brain and COS-7 cells were indistinguishable from each other in their kinetic and catalytic properties. Unlike alpha-, beta I-, beta II-, and gamma PKC, epsilon PKC was independent of Ca2+ but absolutely required phosphatidylserine and diacylglycerol for its activation; a tumor-promoting phorbol ester could replace diacylglycerol. epsilon PKC showed enzymological properties similar to those of delta PKC, except that epsilon PKC but not delta PKC was greatly activated by free arachidonic acid. Immunoblot analysis revealed that, in marked contrast to delta PKC, epsilon PKC is expressed predominantly in the brain tissue and only in trace amounts in heart, lung, spleen, thymus, and testis.

Complex Interactions Between Polyamines and Calpain-Mediated Proteolysis in Rat Brain

Polyamine synthesis is induced by various extracellular signals, and it is widely held that this biochemical response participates in cell growth and differentiation. Certain of the triggers for synthesis in brain tissues also increase the breakdown of high-molecular-weight structural proteins, apparently by activating calcium-dependent proteases (calpains). The present experiments tested the possibility that calpain activity is modulated by polyamines. Spermine, spermidine, and putrescine all increased calcium-dependent proteolysis of [14C]casein by soluble fractions of rat brain. The order of potency was spermine greater than spermidine greater than putrescine, with apparent affinities of 30, 300, and 6,000 microM, respectively. Each of the three polyamines at physiological concentrations also potentiated the calcium-dependent breakdown of two endogenous high-molecular-weight structural proteins known to be substrates of calpain, in both supernatant and membrane fractions. The thiol protease inhibitor leupeptin, a known calpain inhibitor, also inhibited calcium-dependent proteolysis in the presence and absence of polyamines. The polyamines did not increase the activity of purified calpain I or calpain II determined with either [14C]casein or purified spectrin as the substrate, nor did they interfere with the inhibitory effects of calpastatin, an endogenous inhibitor of calpain. However, polyamines potentiated the stimulation of endogenous but not purified calpain activity produced by an endogenous calpain activator. These results suggest a role for polyamines in protein degradation as well as protein synthesis.

Purification, properties, and phosphorylation by protein kinase C of two phosphoinositidase C isozymes from rat brain.

Two forms of phosphoinositidase C have been purified from the soluble fraction of rat brain. The purification scheme included gel filtration followed by chromatography on cellulose phosphate, phenyl-Sepharose, and Mono Q. Gradient sodium dodecyl sulphate-polyacrylamide gel electrophoresis gave apparent molecular masses of 151 kDa and 147 kDa. Western blotting with monoclonal antibodies showed that the isozymes corresponded to PLC-beta-1 and PLC-gamma of bovine brain. With both enzymes phosphatidylinositol 4,5-bisphosphate was a better substrate than phosphatidylinositol at neutral pH and low calcium ion concentrations. Both enzymes produced a proportion of inositol 1:2-cyclic phosphates from each substrate, particularly at acid pH. Some GTPase activity was seen in the early stages of purification, but was separated from PLC-beta-1 and PLC-gamma on Mono Q. Purified rat brain protein kinase C phosphorylated PLC-gamma but not PLC-beta-1. Incubation with the kinase increased the activity of both enzymes however, possibly by phosphorylation of another protein in the preparations.

Intracellular Substrates for Extracellular Signaling: Characterization of a Ubiquitous, Neuron-Enriched Phosphoprotein (Stathmin)

Abstract We previously identified (Sobel, A., and Tashjian, A. H., Jr. (1983) J. Biol. Chem. 258, 10312-10324) a group of cytoplasmic proteins whose phosphorylation could be related to the regulation by extracellular effectors of cells as different as pituitary and muscle cells. Among these phosphoproteins, proteins 7 and 8 (Mr approximately 19,000, pI approximately 5.8-6.0), that we now designate P1 and P2, are very abundant in rat brain. Partial purification of these proteins was therefore achieved after 100 degrees C precipitation of a rat brain-soluble fraction and further fractionation of the supernatant by ion exchange chromatography. Several related non-phosphorylated (N1, N2) and phosphorylated (P3) proteins were also identified in the heat-resistant supernatant. Antisera raised against P2 extracted from nitrocellulose blots of semipreparative two-dimensional gels recognized all the proteins N1, N2, P1, P2, and P3, confirming that they belong to the same protein family, and suggesting that they are likely various forms of a single protein core. The same protein could be detected biochemically and immunologically at various concentrations in all the tissues or cell types from diverse mammalian and nonmammalian species tested. Together with our previous data relating its phosphorylation to the regulation of the proliferation, differentiation, and/or the functions of the cells considered, this observation leads us to suggest that it might be an ubiquitous regulatory phosphoprotein playing the role of an intracellular relay for extracellular signals, after their binding to specific membrane receptors and the generation of second messengers. We propose to name this protein stathmin, from the greek stathmos (relay).

Ca 2+/Calmodulin-sensitive inositol 1,4,5-trisphosphate 3-kinase in rat and bovine brain tissues

Inositol 1,4,5-trisphosphate (Ins P3) 3-kinase catalyzes the ATP-dependent phosphorylation of Ins P3 to Inositol 1,3,4,5-tetrakisphosphate (Ins P4). Ca2+/calmodulin (CaM)-sensitivity of Ins P3 3-kinase was measured in the crude soluble fraction from rat brain and different anatomic regions of bovine brain. Kinase activity was inhibited in the presence of EGTA (free Ca2+ below 1 nM) as compared to Ca2+ (10 microM free Ca2+) or Ca2+ (10 microM free Ca2+) and CaM (1 microM). Ca2+-sensitivity was also seen for the cAMP phosphodiesterase measured under the same assay conditions, but was not for the Ins P3 5-phosphatase. DEAE-cellulose chromatography of the soluble fraction of rat brain or bovine cerebellum resolved a Ca2+/CaM-sensitive Ins P3 3-kinase (maximal stimulation at 1 microM Ins P3 substrate level was 2.0-3.0 fold).

Purification and properties of inositol-1,4-bisphosphate 4-phosphohydrolase from rat brain

Inositol-1,4-bisphosphate 4-phosphohydrolase (inositol-1,4-bisphosphatase) was highly purified from a soluble fraction of rat brain. On SDS-polyacrylamide gel electrophoresis, the purified enzyme gave a single protein band and its molecular weight was estimated to be 42 000. The isoelectric point of the enzyme was 4.3. The enzyme specifically hydrolyzed the 4-phosphomonoester linkage of inositol 1,4-bisphosphate. The K m value for inositol 1,4-bisphosphate was 30 μM, and it required Mg 2+ for activity. Ca 2+ was a competitive inhibitor with a K i value of 60 μM as regards the Mg 2+ binding. Li +, which is known to be a strong inhibitor of inositol 1-phosphatase (EC 3.1.3.25), inhibited the enzyme activity and caused 50% inhibition at a concentration of 1 mM ( IC 50 = 1 mM ). Li + was an uncompetitive inhibitor of substrate binding with a K i value of 0.6 mM. These inhibitory parameters of Li + were quite similar to those for inositol 1-phosphatase ( IC 50 = 1 mM , K i = 0.3 mM ). Thus, the effect of Li + on decreasing the free inositol level with a subsequent decrease in agonist-sensitive phosphoinositides, is caused by its inhibition of multiple enzymes involved in conversion of inositol 1,4-bisphosphate to inositol.

Three distinct forms of rat brain protein kinase C: Differential response to unsaturated fatty acids

Although the three distinct forms of protein kinase C isolated from rat brain soluble fraction are structurally very similar, they respond differently to free unsaturated fatty acids such as arachidonic acid to exhibit their catalytic activity. Type I enzyme encoded by γ-sequence, as predicted by cDNA clone analysis, responds to these fatty acids only slightly, whereas Type III enzyme determined by α-sequence is activated by free unsaturated fatty acids in the presence of Ca 2+ in a comparable manner to phosphatidylserine plus diacylglycerol. Type II, a mixture of two enzymes encoded by βI- and βII-sequence, resulting from alternative splicing, shows properties in between those of Type I and Type III. Some of these forms of protein kinase C may function at a relatively later phase of cellular responses when large amounts of unsaturated fatty acids and Ca 2+ are mobilized.

The metabolism of inositol 4-monophosphate in rat mammalian tissues

Rat brain soluble fraction contains an enzymatic activity that dephosphorylates inositol 1,4-bisphosphate (Ins(1,4)P 2). We have used anion exchange h.p.l.c. in order to identify the inositol monophosphate product of Ins(1,4)P 2 hydrolysis (i.e. Ins(1)P 1, Ins(4)P 1 or both). When [ 3H]Ins(1,4)P 2 was used as substrate, we obtained an inositol monophosphate isomer that was separated from the co-injected standard [ 3H]Ins(1)P 1. This suggested an Ins(1,4)P 21-phosphatase pathway leading to the production of the inositol 4-monophosphate isomer. The dephosphorylation of [ 32P]Ins(4)P 1 was measured in rat brain, liver and heart soluble fraction and was Li +-sensitive. Chromatography of the soluble fraction of a rat brain homogenate on DEAE-cellulose resolved a monophosphate phosphatase activity that hydrolyzed both [ 3H]Ins(1)P 1 and [4- 32P]Ins(4)P 1 isomers.