Isolation and characterization of cytidine diphosphate diglyceride from beef liver

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Physical Properties and Reactivity of Microdomains in Phosphatidylinositol-Containing Supported Lipid Bilayer

We characterized the size, distribution, and fluidity of microdomains in a lipid bilayer containing phosphatidylinositol (PI) and revealed their roles during the two-dimensional assembly of a membrane deformation protein (FBP17). The morphology of the supported lipid bilayer (SLB) consisting of PI and phosphatidylcholine (PC) on a mica substrate was observed with atomic force microscope (AFM). Single particle tracking (SPT) was performed for the PI+PC-SLB on the mica substrate by using the diagonal illumination setup. The AFM topography showed that PI-derived submicron domains existed in the PI+PC-SLB. The spatiotemporal dependence of the lateral lipid diffusion obtained by SPT showed that the microdomain had lower fluidity than the surrounding region and worked as the obstacles for the lipid diffusion. We observed the two-dimensional assembly of FBP17, which is one of F-BAR family proteins included in endocytosis processes and has the function generating lipid bilayer tubules in vitro. At the initial stage of the FBP17 assembly, the PI-derived microdomain worked as a scaffold for the FBP17 adsorption, and the fluid surrounding region supplied FBP17 to grow the FBP17 domain via the lateral molecular diffusion. This study demonstrated an example clearly revealing the roles of two lipid microregions during the protein reaction on a lipid bilayer.

CDP-diacylglycerol, a critical intermediate in lipid metabolism

The roles of lipids expand beyond the basic building blocks of biological membranes. In addition to forming complex and dynamic barriers, the thousands of different lipid species in the cell contribute to essentially all the processes of life. Specific lipids are increasingly identified in cellular processes, including signal transduction, membrane trafficking, metabolic control and protein regulation. Tight control of their synthesis and degradation is essential for homeostasis. Most of the lipid molecules in the cell originate from a small number of critical intermediates. Thus, regulating the synthesis of intermediates is essential for lipid homeostasis and optimal biological functions. Cytidine diphosphate diacylglycerol (CDP-DAG) is an intermediate which occupies a branch point in lipid metabolism. CDP-DAG is incorporated into different synthetic pathways to form distinct phospholipid end-products depending on its location of synthesis. Identification and characterization of CDP-DAG synthases which catalyze the synthesis of CDP-DAG has been hampered by difficulties extracting these membrane-bound enzymes for purification. Recent developments have clarified the cellular localization of the CDP-DAG synthases and identified a new unrelated CDP-DAG synthase enzyme. These findings have contributed to a deeper understanding of the extensive synthetic and signaling networks stemming from this key lipid intermediate.

Roles of specific lipid species in the cell and their molecular mechanism

Thousands of different molecular species of lipids are present within a single cell, being involved in modulating the basic processes of life. The vast number of different lipid species can be organized into a number of different lipid classes, which may be defined as a group of lipids with a common chemical structure, such as the headgroup, apart from the nature of the hydrocarbon chains. Each lipid class has unique biological roles. In some cases, a relatively small change in the headgroup chemical structure can result in a drastic change in function. Such phenomena are well documented, and largely understood in terms of specific interactions with proteins. In contrast, there are observations that the entire structural specificity of a lipid molecule, including the hydrocarbon chains, is required for biological activity through specific interactions with membrane proteins. Understanding of these phenomena represents a fundamental change in our thinking of the functions of lipids in biology. There are an increasing number of diverse examples of roles for specific lipids in cellular processes including: Signal transduction; trafficking; morphological changes; cell division. We are gaining knowledge and understanding of the underlying molecular mechanisms. They are of growing importance in both basic and applied sciences.

Distinct properties of the two isoforms of CDP-diacylglycerol synthase

CDP-diacylglycerol synthases (CDS) are critical enzymes that catalyze the formation of CDP-diacylglycerol (CDP-DAG) from phosphatidic acid (PA). Here we show in vitro that the two isoforms of human CDS, CDS1 and CDS2, show different acyl chain specificities for its lipid substrate. CDS2 is selective for the acyl chains at the sn-1 and sn-2 positions, the most preferred species being 1-stearoyl-2-arachidonoyl-sn-phosphatidic acid. CDS1, conversely, shows no particular substrate specificity, displaying similar activities for almost all substrates tested. Additionally, we show that inhibition of CDS2 by phosphatidylinositol is also acyl chain-dependent, with the strongest inhibition seen with the 1-stearoyl-2-arachidonoyl species. CDS1 shows no acyl chain-dependent inhibition. Both CDS1 and CDS2 are inhibited by their anionic phospholipid end products, with phosphatidylinositol-(4,5)-bisphosphate showing the strongest inhibition. Our results indicate that CDS1 and CDS2 could create different CDP-DAG pools that may serve to enrich different phospholipid species with specific acyl chains.

The essential roles of cytidine diphosphate-diacylglycerol synthase in bloodstream form Trypanosoma brucei

Lipid metabolism in Trypanosoma brucei, the causative agent of African sleeping sickness, differs from its human host in several fundamental ways. This has lead to the validation of a plethora of novel drug targets, giving hope of novel chemical intervention against this neglected disease. Cytidine diphosphate diacylglycerol (CDP‐DAG) is a central lipid intermediate for several pathways in both prokaryotes and eukaryotes, being produced by CDP‐DAG synthase (CDS). However, nothing is known about the single T. brucei CDS gene (Tb927.7.220/EC 2.7.7.41) or its activity. In this study we show TbCDS is functional by complementation of a non‐viable yeast CDS null strain and that it is essential in the bloodstream form of the parasite via a conditional knockout. The TbCDS conditional knockout showed morphological changes including a cell‐cycle arrest due in part to kinetoplast segregation defects. Biochemical phenotyping of TbCDS conditional knockout showed drastically altered lipid metabolism where reducing levels of phosphatidylinositol detrimentally impacted on glycoylphosphatidylinositol biosynthesis. These studies also suggest that phosphatidylglycerol synthesized via the phosphatidylglycerol‐phosphate synthase is not synthesized from CDP‐DAG, as was previously thought. TbCDS was shown to localized the ER and Golgi, probably to provide CDP‐DAG for the phosphatidylinositol synthases.

Functional analyses of differentially expressed isoforms of the Arabidopsis inositol phosphorylceramide synthase

Sphingolipids are key components of eukaryotic plasma membranes that are involved in many functions, including the formation signal transduction complexes. In addition, these lipid species and their catabolites function as secondary signalling molecules in, amongst other processes, apoptosis. The biosynthetic pathway for the formation of sphingolipid is largely conserved. However, unlike mammalian cells, fungi, protozoa and plants synthesize inositol phosphorylceramide (IPC) as their primary phosphosphingolipid. This key step involves the transfer of the phosphorylinositol group from phosphatidylinositol (PI) to phytoceramide, a process catalysed by IPC synthase in plants and fungi. This enzyme activity is at least partly encoded by the AUR1 gene in the fungi, and recently the distantly related functional orthologue of this gene has been identified in the model plant Arabidopsis. Here we functionally analysed all three predicted Arabidopsis IPC synthases, confirming them as aureobasidin A resistant AUR1p orthologues. Expression profiling revealed that the genes encoding these orthologues are differentially expressed in various tissue types isolated from Arabidopsis.

Plasmodium CDP-DAG synthase: an atypical gene with an essential N-terminal extension

Cytidine diphosphate diacylglycerol synthase (CDS) diverts phosphatidic acid towards the biosynthesis of CDP-DAG, an obligatory liponucleotide intermediate in anionic phospholipid biosynthesis. The 78kDa predicted Plasmodium falciparum CDS (PfCDS) is recovered as a 50 kDa conserved C-terminal cytidylyltransferase domain (C-PfCDS) and a 28kDa fragment that corresponds to the unusually long hydrophilic asparagine-rich N-terminal extension (N-PfCDS). Here, we show that the two fragments of PfCDS are the processed forms of the 78 kDa pro-form that is encoded from a single transcript with no alternate translation start site for C-PfCDS. PfCDS, which shares 54% sequence identity with Plasmodium knowlesi CDS (PkCDS), could substitute for PkCDS in P. knowlesi. Experiments to disrupt either the full-length or the N-terminal extension of PkCDS indicate that not only the C-terminal cytidylyltransferase domain but also the N-terminal extension is essential to Plasmodium spp. PkCDS and PfCDS introduced in P. knowlesi were processed in the parasite, suggesting a conserved parasite-dependent mechanism. The N-PfCDS appears to be a peripheral membrane protein and is trafficked outside the parasite to the parasitophorous vacuole. Although the function of this unusual N-PfCDS remains enigmatic, the study here highlights features of this essential gene and its biological importance during the intra-erythrocytic cycle of the parasite.

CDP-diacylglycerol synthase from mammalian tissues

CDP-diacylglycerol resides at the branch point of glycerolipid biosynthesis as precursor of both the phosphoinositides and phosphatidylglycerol. The discovery of the phosphoinositide signal transduction pathway and the recognition of its prominent role in intracellular communication has focused new attention on CDP-diacylglycerol synthase. As a rate-limiting step in this pathway, it is a likely target for regulation. Exploration of this possibility will be facilitated by the recent cloning of mammalian CDP-DAG synthase.

CDP-diacylglycerol synthesis in rat liver mitochondria

CDP-diacylglycerol for polyglycerophosphatide biogenesis can be synthesized within rat liver mitochondria. This membrane-associated enzyme was predominantly located in the inner mitochondrial membrane. GTP had a significant effect in activating the microsomal CDP-diacylglycerol synthase, especially if the microsomes were preincubated with GTP in the presence of phosphatidic acid. This stimulatory effect of GTP on the microsomal enzyme was not detected in the mitochondrial fractions. The enzymes could be solubilized from the membrane fractions using CHAPS, and the detergent-soluble activity partially restored by addition of phospholipids. Mitochondrial and microsomal CDP-diacylglycerol synthase activity could be completely separated by anion-exchange column chromatography. The mitochondrial and microsomal CDP-diacylglycerol synthases appear to be two distinct enzymes with different localization and regulatory characteristics.

Isolation of experimental anti-AIDS glycerophospholipids by micro-preparative reversed-phase high-performance liquid chromatography

The experimental anti-AIDS glycerophosphatidic acid: nucleoside (sn-1/sn-2 diacylglycerol:dideoxynucleotide) drugs 3'-azido-3'-deoxythymidine monophosphate diglyceride (AZT-MP-DG) and 2',3'-dideoxycytidine monophosphate diglyceride (ddC-MP-DG) were isolated and purified by reversed-phase high-performance liquid chromatography (HPLC). The chromatographic separation was based on the glycerophospholipid moiety of the drugs and detection of the nucleoside component. The separations were optimized on method development columns packed with the stationary phase to be used in the micro-preparative column and monitored by a UV detector. Fractions were collected and analyzed for purity by analytical-scale HPLC and by thin-layer chromatography (TLC). The purity of the recovered drugs based on UV and light-scattering detection and on TLC was greater than 99%. The purified compounds were isolated for studies on structure confirmation, physical, biophysical and formulation properties and anti-HIV efficacy in culture.

Metabolism of [3H]arachidonic acid- and [14C]linoleic acid-labelled chylomicrons in essential fatty acid-deficient rats

Mesenteric duct chylomicrons labelled with [3H]arachidonic (20:4) and [14C]linoleic (18:2) acid were injected intravenously into essential fatty acid-deficient (EFAD) and control rats. The rats were killed after 5, 10, 20 and 240 min, and serum and different organs were analysed for radioactivity of different lipid classes. 3H and 14C in the triacylglycerol (TG) and phosphatidylethanolamine (PE) fractions were cleared from blood faster in the EFAD than in the control rats. The incorporation of [14C]18:2 into liver and heart phospholipids was increased compared to control rats at all time intervals, the increase in the incorporation of [3H]20:4 being less pronounced. Furthermore, the total incorporation of [14C]18:2 was increased in the heart of the EFAD group. The increased incorporation into phosphatidylcholine (PC) and particularly into PE was observed already at 5-20 min, whereas a marked increase in the 14C radioactivity of cardiolipin occurred between 20 and 240 min. The 3H and 14C radioactivity per g white adipose tissue was lower in the EFAD group than in the controls. After 5-20 min there were no differences between the groups in the lipid radioactivity of the stomach and the small and the large intestine. In the upper small intestine, the 3H radioactivity in both groups and the 14C radioactivity in the EFAD group increased markedly between 20 and 240 min. The study demonstrates an increased plasma clearance and an efficient hepatic uptake and initial incorporation into PC and PE of chylomicron [14C]18:2 and [3H]20:4 in EFAD rats. In these rats a marked selective transfer of [14C]18:2 to cardiolipin from other tissue lipids occurred with time. In addition, the study demonstrates a preferential transfer of injected [3H]20:4, and in EFAD rats also of [14C]18:2, into lipids of the upper small intestine, possibly by secretion of [3H]20:4 and [14C]18:2 in bile phospholipids.

The de novo synthesis of molecular species of phosphatidylinositol from endogenously labeled CDP diacylglycerol in alveolar macrophage microsomes

The de novo synthesis of molecular species of phosphatidylinositol (PI) from endogenously labeled CDP diacylglycerol (CDP-DG) and phosphatidic acid (PA), with [14C]-glycerol 3-phosphate, in microsomes of macrophages was studied using a recently developed HPLC technique. Endogenously labeled PA, CDP-DG, and PI were sequentially formed from labeled glycerol 3-phosphate through the addition of CoA, CTP, and then inositol into microsomes. The rate of formation of CDP-DG from endogenously labeled PA was low as compared with those of PA and PI. The low rate of CDP-DG synthesis suggests that it may be the rate-limiting step in the de novo synthesis of PI. Analysis of newly synthesized molecular species of PI by HPLC revealed that large proportions of radioactivity were associated with the 16:0-18:1, 16:0-18:2, 18:1-18:2, and 18:2-18:2 species, and a small amount, 2-3%, of radioactivity was associated with the 18:0-20:4 species. The profiles of newly synthesized PA and CDP-DG species were quite similar to those of PI species. This suggests that the enzymes involved in the formation of PI species from glycerol 3-phosphate show little specificity toward different molecular species of substrates. The results of the present study also suggest that free fatty acid composition in microsomes greatly affect the composition of the molecular species of PI synthesized through the de novo pathway, since the proportion of fatty acids utilized for the de novo synthesis of PI species was similar to that of free fatty acids in the microsomal membrane.

The CMP-stimulated production of diacylglycerol and CDPdiacylglycerol in neuronal nuclei labelled with radioactive arachidonate

A neuronal nuclear fraction (N1), isolated from immature rabbit cerebral cortex, was preincubated with [3H]arachidonate, ATP, CoA, Mg2+ and 1-acyl-sn-glycero-3-phosphocholine or 1-acyl-sn-glycero-3-phosphoinositol. Using the former lysophospholipid, a sizeable incorporation of radioactivity was seen in N1 phosphatidylcholine. In subsequent incubations in the presence of CMP and EGTA, there was a generation of radioactive diacylglycerol in N1 and a corresponding decline in phosphatidylcholine radioactivity. Both these changes could be blocked by the addition of CDPcholine. In incubations using N1 phosphatidylinositol or phosphatidylethanolamine prelabelled with [3H]arachidonate, no evidence was found to support a direct generation of diacylglycerol from these phospholipids. The back reaction of cholinephosphotransferase in N1 is likely the principal source of diacylglycerols bearing arachidonate. Using either lysophospholipid in the preincubations described in the opening sentence, more than half of the incorporated radioactivity derived from [3H]arachidonate was found in N1 phosphatidylinositol. In subsequent incubations with EGTA and CMP there was a production of radioactive CDPdiacylglycerol and a decline in radioactive phosphatidylinositol. Both events could be blocked by the presence of myo-inositol. Radioactive CDPdiacylglycerol, produced in N1 in the presence of CMP and EGTA, was converted back into phosphatidylinositol by the addition of myo-inositol. The production of CDPdiacylglycerol is likely the result of the back reaction of CDPdiacylglycerol:inositol phosphatidate transferase in N1.

1-β-D-Arabinofuranosylcytosine-Phospholipid Conjugates As Prodrugs of Ara-C

The L-, D-, and D,L-isomers of 1-beta-D-arabinofuranosylcytosine 5'-diphosphate-1,2-dipalmitin, new prodrugs of ara-C5, have been evaluated for antitumor activity in L1210 lymphoid leukemic mice. The L-isomer produced significant increase in life span (ILS), and longterm survivors among mice bearing i.p. and i.c. implanted L1210 leukemia and the maximal ILS values found were greater than 543 and greater than 374% with five and four 45-day survivors out of six mice, respectively, at the optimal single doses of 300 mg/kg and 125 mg/kg. The D- and D,L-isomers also displayed significant in vivo antitumor activity against both i.p. and i.c. implanted L1210 leukemia in mice with ILS range of 144-293% at a total dose of 125-250 mg/kg. Significant schedule dependency was not observed when the conjugates were administered i.p. once daily for 5 days, once every 4 days, or as a single dose, but single doses typically produced the best effects. The L-isomer was found to be a more effective prodrug of ara-C than its isomers and other lipophilic prodrugs, 5'-O-palmitoyl-ara-C and N4-acyl-ara-C. Unlike the latter prodrugs, the new conjugates are water soluble by sonication method.

Specific inhibition of phosphatidate cytidylyltransferase from Bacillus subtilis membranes by cytidine monophosphate

In Bacillus subtilis, the phosphatidate cytidylyltransferase was localized exclusively in the membrane fraction prepared by sucrose density gradient fractionation. A single enzyme could synthesize the two liponucleotides: CDPdiacylglycerol and dCDPdiacylglycerol. Kinetic experiments and isotopic exchange reactions suggested a ping-pong mechanism. Among the nucleosides monophosphate, CMP specifically reduced the synthesis of both liponucleotides. This inhibition was non-competitive and might be involved in regulation of phospholipid synthesis.

Inhibitory effect of 1-beta-D-arabinofuranosylcytosine on the synthesis of phosphatidyl-dCMP

The synthesis of phosphatidyl-dCMP in mouse thymocytes is inhibited by the antineoplastic agent 1-beta-D-arabinofuranosyl cytosine (AraCyt) 50% inhibition (ID50) being reached at an AraCyt concentration of 0.18 mM. In the same cells, ID50 for DNA synthesis is 0.03 mM. This inhibition is probably mediated by the phosphorylated derivative of AraCyt (aCTP) since the synthesis of phosphatidyl-dCMP from dCTP using permeabilized thymocytes is inhibited by aCTP (ID50 = 0.11 mM). The incorporation of [3H]AraCyt into the organic phase could also be detected, suggesting that this drug may act as a substrate for the enzyme that catalyzes the transfer of dCTP into phosphatidic acid.

Stereospecific synthesis and enzyme studies of CDP-diacylglycerols

The fatty acid specificity of two enzymes that metabolize CDPdiacylglycerol, CDPdiacylglycerol hydrolase (EC 3.6.1.26) and CDPdiacylglycerol: inositol phosphatidyltransferase (EC 2.7.8.11), has been examined in guinea pig brain. Mixed CDPdiacylglycerols were stereospecifically synthesized by the following sequence: (i) hydrolysis of a homodiacyl lecithin to 1-acyl lysoPC by action of snake venom phospholipase A2, (ii) reacylation with the anhydride of the desired second fatty acid and dimethylaminopyridine, (iii) hydrolysis of the resultant heterodiacyl lecithin to phosphatidate with cabbage phospholipase D, and (iv) reaction of phosphatidate with CMPmorpholidate to give CDPdiacylglycerol. CDPdiacylglycerol: inositol phosphatidyltransferase showed the following rates of conversion of 40-microM suspensions of CDPdiacylglycerol in 0.15% Triton X-100 to phosphatidylinositol relative to the 1-stearoyl-2-oleoyl derivative (100%): dipalmitoyl, 70%; distearoyl, 38%; diarachidonoyl, 9%; 1-arachidonoyl-2-stearoyl, 6%; 1-stearoyl-2-arachidonoyl, 4%. These results indicate that the composition of isolated phosphatidylinositol and related lipids is not explained by the fatty acid specificity of the biosynthetic enzymes and supports the intervention of a deacylation-reacylation sequence. The rates of hydrolysis of the synthetic CDPdiacylglycerols at 76 microM, in 0.3% Triton X-100, by the CDPdiacylglycerol hydrolase relative to the 1-stearoyl-2-oleoyl derivative (100%) were: dipalmitoyl, 70%; distearoyl, 32%; 1-arachidonoyl-2-stearoyl, 30%; 1-stearoyl-2-arachidonoyl, 28%; diarachidonoyl, 22%. Inhibition of this enzyme by AMP was shown to be non-competitive, with a Ki of 40 microM. The lysosomal localization of the mammalian hydrolase was confirmed.

Synthesis of phosphatidyl-dCMP in human lymphocytes and its enhancement by chlorpromazine

When phytohemagglutinin-stimulated lymphocytes are labelled with [3H]deoxycytidine (dCyd), a compound soluble in organic solvents is formed. Several analytical procedures such as thin-layer chromatography and alkaline hydrolysis of the product obtained after labeling of stimulated lymphocytes with [14C]glycerol and [3H]dCyd, suggest that the product synthesized is phosphatidyl-dCMP. This compound is synthesized in much smaller amounts than when permeabilized lymphocytes and [3H]dCTP as a precursor are used. The synthesis of phosphatidyl-dCMP is highly enhanced by chlorpromazine, phosphatidic acid and arachidonic acid, whereas addition of inositol diminishes the amount of detected phosphatidyl-dCMP. It is suggested that this compound would be a precursor of phosphatidylinositol.

Properties of a CDP-diglyceride hydrolase from guinea pig brain

Enzymatic hydrolysis of the pyrophosphate bond of CDP-diglyceride (CDP-DG), previously shown to occur in bacteria, is demonstrable in mammalian tissues. Activity was enriched in a lysosomal fraction obtained from guinea pig cerebral cortex and was purified 92-fold relative to the homogenate by a combination of XM-300 ultrafiltration and DEAE-cellulose column chromatography. When incubated with CDP-dipalmitin, the purified enzyme produced stoichiometric amounts of CMP and phosphatidate. dCDP-DG served as a substrate, while ADP-DG was an inhibitor, as were 5'-AMP and 5'-dAMP. CDP-DG hydrolysis was not affected by the presence of excess amounts of CDP-choline, CDP-glycerol, sodium pyrophosphate, or cyclic 3',5'-AMP.

Purification and characterization of nucleoside diphosphatase from rat-liver microsomes. Evidence for metalloenzyme and glycoprotein

Nucleoside diphosphatase was purified from rat liver microsomes more than 3000-fold with a 16% yield using a procedure including concanavalin-A--Sepharose and phenyl-Sepharose column chromatography. The purified enzyme had a specific activity of 2500 units/mg protein and appeared homogeneous by gel electrophoresis. The enzyme had a sedimentation coefficient of 6.5 S by sucrose-density gradient centrifugation. The enzyme had a sedimentation coefficient of 6.5 S by sucrose-density gradient centrifugation, and a Stokes' radius of 4.8 nm was estimated by the gel filtration technique. Its molecular weight is 130,000, but only one single band of Mr 65,000 was detected after sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The native enzyme seems thus to be composed of two identical subunits. The purified enzyme was confirmed to be a glycoprotein containing approximately 9% carbohydrates. The enzyme had a pH optimum of 7.5, an isoelectric point of 4.85 and a Km of 2.5 mM for UDP. On the basis of direct measurement of metal content in the native enzyme, the rat liver nucleoside diphosphatase was found to be a metalloenzyme containing 0.9 mol zinc and 0.1 mol manganese/mol 65,000-Mr subunit. Metal-free nucleoside diphosphatase has been prepared. The activity of the metal-free enzyme was restored by the addition of several divalent cations, zinc being the most effective.

The stimulation of rat liver microsomal CTP: phosphatidate cytidylyltransferase activity by guanosine triphosphate

GTP has been found to markedly enhance the formation of CDPdiacylglycerol in rat liver microsomes. The magnitude of the GTP stimulatory effect was influenced by the fatty acid composition of the phosphatidic acid substrate. Heat or proteolytic treatment of microsomal membranes prevented GTP from stimulating liponucleotide formation. Treatment of microsomes with low levels of detergent reduced the stimulatory action of the GTP. The increased synthesis of CDPdiacylglycerol obtained by the addition of GTP to the incubation medium is due to an increase in CTP:phosphatidate cytidylyltransferase activity, rather than to a reduction in the breakdown of either the substrates or product of the reaction. The results suggest that some other protein or enzyme mediates the effect of GTP upon CTP:phosphatidate cytidylyltransferase activity.

Cytotoxic liponucleotide analogs. II. Antitumor activity of CDP-diacylglycerol analogs containing the cytosine arabinoside moiety

Among events limiting the effectiveness of cancer chemotherapy are the general lack of preferential uptake of anticancer drugs by tumor cells and the occurrence of drug resistance. An approach has been undertaken to explore whether or not such events can be favorably altered or circumvented therapeutically by development of a new class of anticancer molecules, cytotoxic liponucleotide analogs. The design of cytotoxic liponucleotide analogs encompasses both biochemical and biophysical aspects of liponucleotide and glycerophospholipid structure and metabolism. Several cytotoxic liponucleotide analogs of cytidine diphosphate diacylglycerol (CDPdiacylglycerol/dCDPdiacylglycerol), containing the 1-beta-D-arabinofuranosyl moiety, were tested for antitumor activity. Multispecies ara-CDPdiacylglycerol (1-beta-D-arabinofuranosylcytosine 5'-diphosphate diacylglycerol), which contains egg lecithin-derived mixed fatty acyl chains, was more active than 1-beta-D-arabinofuranosylcytosine (ara-C), a clinically used anticancer drug, against leukemia L5178Y and P388 ascites cells in mice. At identical single doses (50 mg/kg per day times 4) administered intraperitoneally, ara-CDPdiacylglycerol prolonged the life spans of L5178Y tumor-bearing mice 93%, while ara-C prolonged life by 18%. Ara-CDPdiacylglycerol increased life spans of P388 tumor-bearing mice by 357% at doses of 50 mg/kg per day times 4; the maximum increase with ara-C was 159% (85 mg/kg per day times 4). Against a P388 ara-C-resistant cell line (P/Ara-C, kinase deficient) in mice, ara-CDPdiacylglycerol prolonged survival times by 34% at a dose of 50 mg/kg per day times 4 and by 55% at 75 mg/kg per day times 4; the drug was not active against two other ara-C-resistant murine leukemia mutants (CA 55, CA5b). With cell line-derived human colon carcinoma HCT-15 grown in mice immunosuppressed with anti-thymocyte serum, ara-CDPdiacylglycerol at a single daily dose of 50 mg/kg per day times 4 significantly reduced tumor weights to 21% of the controls; the same dose schedule of ara-C caused no observable reduction of tumor weights. Results of these preliminary antitumor evaluations indicate that cytotoxic liponucleotide analogs should be investigated further to determine their potential as antineoplastic molecules.

Biosynthesis of phosphatidylinositol in Crithidia fasciculata

Microsomal preparations from the protozoan (Crithidia fasciculata were shown to incorporate myo-[2-3H]inositol into phosphatidylinositol by both the CDPdiacylglycerol:myo-inositol phosphatidyltransferase reaction and by a myo-inositol exchange reaction. Non-ionic detergent and Mg2+ were necessary for the measurement of transferase activity. Untreated preparations could not be saturated with Mg2+, even at very high concentrations (50-75 mM). However, low concentrations of EGTA (75 micro M) both stimulated the activity 3-fold and reduced the Mg2+ required for saturation to 15-20 mM. EGTA also increased the apparent Km for CDPdiacylglycerol while increasing the sensitivity to substrate inhibition above 1 mM. The transferase activity was inhibited by relatively low concentrations of Ca2+ (50 micro M). This and the EGTA effect suggest a possible role for Ca2+ in the modulation of phosphatidylinositol synthesis. The myo-inositol exchange activity required Mn2+, was insensitive to Ca2+ inhibition and was only slightly stimulated by detergents and EGTA. This activity was preferentially inactivated by heating at 50 degrees C in the presence of Triton X-100. In a detergent solubilized preparation the exchange activity but not the transferase exhibited a non-specific requirement for phospholipid. The differences in properties of the two activities suggest the presence of a separate exchange enzyme.

Characterization of the forward and reverse reactions catalyzed by CDP-diacylglycerol:inositol transferase in rabbit lung tissue

CDPdiacylglycerol:inositol transferase activity in rabbit lung tissue has been characterized and the optimum conditions for assaying this enzyme in vitro were determined. Rabbit lung tissue CDPdiacylglycerol:inositol transferase activity was found primarily in the microsomal fraction. The pH optimum of the enzyme activity was between 8.8 and 9.4, and the reaction was dependent on either Mn2+ or Mg2+. Detergents and Ca2+ inhibited the activity of the enzyme. The apparent Km values of the enzyme for CDPdioleoylglycerol and myoinositol were 0.18 mM and 0.10 mM, respectively. The reversibility of the reaction catalyzed by CDPdiacylglycerol:inositol transferase in microsomes prepared from rabbit lung tissue was demonstrated by the synthesis of [3H]CMPdiacylglycerol when [3H]CMP and phosphatidylinositol were present in the incubation mixture. The reverse reaction was characterized and its importance in the regulation of the acidic phospholipid composition of surfactant during lung development is discussed. The pH optimum for the reverse reaction was 6.2, and the reverse reaction was also dependent on Mn2+ or Mg2+. The apparent Km value of CDPdiacylglycerol:inositol transferase for CMP was found to be 2.8 mM.

A phospholipid derivative of cytosine arabinoside and its conversion to phosphatidylinositol by animal tissue

We have synthesized an analog (ara-CDP-DL-dipalmitin) of cytidine diphosphate diglyceride (CDP-diglyceride) in which the antitumor drug, cytosine arabinoside, is substituted for the cytidine moiety. Enzymes in rat and human liver convert this analog to phosphatidylinositol, thereby releasing cytosine arabinoside-5'-monophosphate, an obligatory intermediate in the activation of cytosine arabinoside. Unlike cytidine diphosphate diglyceride, however, ara-CDP-DL-diapalmitin is not an efficient substrate for phosphatidylglycerophosphate synthesis in liver or phosphatidylserine in Escherichia coli. The antitumor activity of ara-CDP-DL-dipalmitin in mice bearing L5178Y leukemia is described.

Isolation and characterization of the liponucleotides of Saccharomyces cerevisiae

The liponucleotide fraction of Saccharomyces cerevisiae was isolated from cells grown on tritiated uracil and identified as CDPdiacylglycerol on the basis of (1), its behavior as a single compound upon DEAE-cellulose and thin-layer chromatography; (2), its extreme lability to mild alkaline methanolysis; and (3), its hydrolysis by the CDPdiacylglycerol hydrolase of Escherichia coli to yield tritiated 5'-CMP. At most, only about 5% of yeast liponucleotide could be dCDPdiacylglycerol, in contrast to the presence of nearly equimolar amounts of CDP-and dCDPdiacylglycerols in E. coli. Although no CDPceramide could be detected in the liponucleotide fraction of this organism, the possibility still exists that it may be an intermediate in the biosynthesis of sphingolipids in systems yet to be examined.

Studies on nucleotide diphosphate diacylglycerol specificity of acidic phospholipid biosynthesis in rat liver subcellular fractions

(1)Cytidine diphosphate diacylglycerol, uridine diphosphate diacylglycerol, adenosine diphosphate diacylglycerol and guanosine diphosphate diacylglycerol were synthesized chemically and their purity assessed. The acticity of these compounds in acidic phospholipid synthesis was examined in rat liver mitochondria and microsomes. (2) Phosphatidylglycerol synthesis in rat liver mitochondria exhibited considerable activity with CDP diacylglycerol (v 7.0 nmol mg-1 h-1). UDPdiacylglycerol (v 5.4) and ADP diacylglyc erol (v 4.2). GDP diacylglycerol activity was detectable but very low. (3) Diphophatidylglycerol formation in mitochondria and phosphatidylinositolsythesis in microsomes exhibited considerable more specificity for CDP diacylglycerol. However, at high concentrations, measurable diphosphaticylglycerol and phophatidylinositol synthesis was observed with the other liponucleotides. (4) Although considerable phosphatidylglycerol formation was observed with UDPdiacaylglycerol and ADP diacylglycerol, it is unlikelky that these compounds are of physiologic importance, at least in rat liver, since CTP:phophatidic acid cytidyltransferase in microsomes and mitochondria was shown to be specific for cytidine triphosphate. The lack of specificity of phosphatiidylglycerol synthesis for CDP diacylglycerol is currently unexplained but may be of some importance in other tissues or in other organisms.

Cytidine diphosphate diglyceride of bovine brain. Positional distribution of fatty acids and analysis of major molecular species

A method is described for the isolation of CDP-diglyceride from bovine brain. Yields of the product ranged from 9.2-15.5 mumol per kilogram of tissue, which corresponds to about 1% of the level of phosphatidic acid. Mild alkaline hydrolysis of the product gave three water-soluble phosphate esters which had the same electrophoretic mobilities as CMP, CDP-glycerol and glycerol 3-phosphate. The liponucleotide was quantitatively hydrolysed by CDP-diglyceride hydrolase from Escherichia coli to phosphatidic acid and CMP. No dCMP was recovered in enzymatic or alkaline hydrolysates and it is concluded there can be little or no dCDP-diglyceride in bovine brain. Brain CDP-diglyceride was similar to phosphatidylinositol in that in both lipids stearate was the major saturated fatty acid and arachidonate the most abundant unsaturated fatty acid. This differed significantly from the fatty acid patterns of other metabolically related phospholipids, phosphatidic acid and cardiolipin. Brain CDP-diglyceride was hydrolysed with phospholipase C from Clostridium welchii with the liberation of the diglyceride moiety in high yield. Treatment of the diglyceride with pancreatic lipase showed CDP-diglyceride with the asymmetric distribution of fatty acids characteristic of most mammalian phospholipids, saturated fatty acids being found mostly at position 1 and polyunsaturated fatty acids at position 2. The derived diglyceride acetates were separated into different molecular species by argentation thin-layer chromatography. These analyses showed that 1-stearoyl, 2-arachidonoyl was the major species of brain CDP-diglyceride.

Biosynthesis of molecular species of CDP-diglyceride from endogenously-labeled phosphatidate in rat liver microsomes

The biosynthesis of [14C] CDP-diglyceride was studied using rat liver microsomes which were endogenously labeled with [14C] phosphatidic acid by preincubation of unlabeled microsomes with sn-[14C] glycerol-3-phosphate and appropriate cofactors. The formation of CDP-diglyceride from radioactive phosphatidate showed an absolute requirement for CTP and MgCl2. The newly formed [14C] CDP-diglyceride was characterized by thin layer chromatography (TLC), isotopic labeling from radioactive CTP, and its ability to serve as substrate for the microsomal enzyme, CDP-diglyceride: inositol phosphatidyltransferase. The distributions of radioactive glycerol-3-phosphate among the various chemical classes of microsomal [14C] phosphatidate and [14C] CDP-diglyceride were determined following argentation TLC of their 1,2-diglyceride acetate derivatives. Most of the radioactivity among the phosphatidic acids was present in the monoenoic (36%) and dienoic (33%) molecular species, whereas 10, 8, 4, and 8% were associated with the saturates, trienes, tetraenes, and polyenes, respectively. Similar distributions of radioactivity were found among the corresponding classes of newly formed CDP-diglyceride. Only a slight enrichment of radioactivity in the tetraenoic CDP-diglyceride was found relative to the corresponding phosphatidates. Therefore, under the conditions of study, the microsomal CTP: phosphatidate cytidylyltransferase produces mainly monoenoic and dienoic species of CDP-diglyceride and shows little specificity towards different molecular species of phosphatidic acids. The present results suggest also that the arachidonoyl phosphatidate derived from the microsomal acylation of sn-glycerol-3-phosphate is not likely the major source of arachidonic acid in liver phosphatidylinositol.