Pyrimidine nucleotides impair phosphoribosylpyrophosphate (PRPP) synthetase subunit aggregation by sequestering magnesium. A mechanism for the decreased PRPP synthetase activity in hereditary erythrocyte pyrimidine 5'-nucleotidase deficiency

The activity of phosphoribosylpyrophosphate (PRPP) synthetase (ATP: D-ribose-5-phosphate pyrophosphotransferase, EC 2.7.6.1) is decreased in the erythrocyte in hereditary pyrimidine 5'-nucleotidase (P5N) deficiency. Given the increased pyrimidine nucleotide content of the P5N-deficient erythrocyte, we evaluated the effects of prototypic pyrimidine nucleotides on the activity of PRPP synthetase. In normal hemolysate a 1.0 mM combination of cytidine tri-, di- and monophosphate (CTP/CDP/CMP) inhibited PRPP synthetase activity and changed the ribose 5-phosphate (R5P) saturation curve from a hyperbola to a biphasic shape. Untreated crude hemolysate from P5N-deficient erythrocytes showed a biphasic R5P kinetic curve. Since the activity of PRPP synthetase is dependent on its state of subunit aggregation, we examined PRPP synthetase subunit aggregation using gel permeation chromatography. P5N-deficient erythrocytes had a decreased absolute amount of aggregated PRPP synthetase and almost a total loss of disaggregated PRPP synthetase. Using normal hemolysate, 1 mM CTP/CDP/CMP interfered with the ability of 1.0 mM ATP and 2.0 mM MgCl2 to promote PRPP synthetase subunit aggregation. Increasing the MgCl2 to 6.0 mM overcame the inhibitory effect of CTP/CDP/CMP. Thus, the decreased PRPP synthetase activity of the P5N-deficient erythrocyte is due, at least in part, to the ability of the accumulated pyrimidine nucleotides to sequester magnesium and to interfere with the subunit aggregation of PRPP synthetase.

In vitro synthesis of colominic acid by membrane-bound sialyltransferase of Escherichia coli K-235. Kinetic properties of this enzyme and inhibition by CMP and other cytidine nucleotides

The membrane-bound sialyltransferase obtained from Escherichia coli K-235 grown in a chemically defined medium (ideal for colominic acid production) was studied. The in vivo half-life calculated for this enzyme was 20 h. Kinetic tests revealed (at 33 degrees C and pH 8.3) hyperbolic behaviour with respect to CMP-Neu5Ac (Km250 microM) and a transition temperature at 31.3 degrees C. The enzyme was inhibited by NH4+, some divalent cations and by several agents that react with thiol groups. Detergents and fatty acids also inhibited the sialyltransferase activity. In vitro synthesis of colominic acid is strongly inhibited by CMP by blocking the incorporation of [14C]Neu5Ac into a protein-complex intermediate and therefore into free polymer. CDP and CTP also inhibited (91% and 84%) this enzyme activity whereas cytosine and cytidine had no effect. CMP inhibition corresponded to a competitive model the calculated Ki was 30 microM. Incubations of protein[14C]Neu5Ac with CMP, CDP and CTP led to de novo synthesis of CMP-[14C]Neu5Ac. The presence of colominic acid, which usually displaces the reaction equilibrium towards polymer synthesis, did not affect this de novo CMP-[14C]Neu5Ac formation. CMP also inhibited in vivo colominic acid biosynthesis.

[3H]arachidonic acid metabolism in rat brain minces: effects of nucleotide triphosphates, CDPcholine and CMP

Rat brain minces were used to investigate the effects of nucleotides on the metabolism of arachidonic acid in nerve tissue. Brain free fatty acids, neutral lipids and phospholipids, were radiolabeled in vivo following intracerebral injection of [3H]arachidonic acid. Minces were prepared from the radiolabeled cerebra and were incubated in a modified Krebs-Ringer buffer with and without various nucleotides. The incubation-induced accumulation of unesterified [3H]arachidonate was reduced in the presence of CDPcholine, ATP, CTP, GTP, and UTP. These nucleotides inhibited choline and inositol glycerophospholipid hydrolysis. They also reduced the amount of labeled diglycerides. However, CDPethanolamine had no effect on arachidonic acid metabolism in the mince preparation and CMP appeared to stimulate further hydrolysis of choline glycerophopholipids, resulting in increased accumulation of [3H]arachidonic acid and labeled diglycerides. We suggest that the production of unesterified [3H]arachidonate and labeled diglycerides is due to the involvement of more than one catabolic reaction, since the high energy nucleotides had similar effects on fatty acid accumulation, but different effects on phospholipid labeling.

[Immunomodulating activity of RNA mononucleotides]

The immunological action of RNA mononucleotides was studied in animal experiments. The most pronounced activation of macrophagal glycolysis urea cycle, oxidative phosphorylation, lysosomal hydrolases was induced by uridine 5'-monophosphate (5'-UMP) and guanosine 5'-monophosphate (5'-GMP); 5'-GMP also induced the maximum increase of the expression of FC gamma receptors. 5'-UMP ensured cell activation comparable with the total action of all mononucleotides. 5'-UMP and 5'-GMP, used in combination, produced the highest stimulating effect on macrophages, and the addition of low-active adenosine 5'-monophosphate (5'-AMP) to active 5'-UMP did not decrease the stimulating potency of the latter. The stimulating activity of sodium nucleinate exceeded that of all mononucleotides and their combinations. 5'-GMP and 5'-AMP induced the maximum activation of oxygen metabolism, evaluated by chemiluminescence, while 5'-UMP and cytidine 5'-monophosphate (5'CMP) proved to be inactive. The shift of the phosphate group to the third carbon atom or the production of the oligonucleotide 5'-UMP consisting of 5-15 nucleotides resulted in the appearance of the capacity for stimulating oxygen metabolism in macrophages. Their in vitro cultivation with 5'-GMP and 5'-AMP induced the maximum increase of cell spreading in comparison with other mononucleotides, while the maximum increase of phagocytosis was ensured only by 5'-UMP and 5'-GMP.2+ 5'-UMP and 5'-GMP enhanced nonospecific resistance to Salmonella typhi infection, and sodium nucleinate, to Pseudomonas pseudomallei and Pseudomonas mallei infections.

Ribonuclease structure and catalysis: effects of crystalline enzyme, alcohol cryosolvents, low temperatures, and product inhibition

In order to determine the necessary conditions to stabilize intermediates in ribonuclease A catalysis at subzero temperatures for structural studies, we have examined the suitability of alcohol-based cryosolvents. On the basis of thermal denaturation transition curves, the enzyme is in the native conformation in high concentrations of ethanol and methanol, provided the temperature is suitably low. The effects of methanol on the catalytic properties for the hydrolysis for mono- and dinucleotide substrates also are consistent with the absence of adverse effects of the cosolvent. Significant methanolysis occurs in the presence of methanol as cosolvent. The kinetics of 2',3'-CMP hydrolysis are complicated by severe competitive product inhibition, both in aqueous and in methanolic solvents, accounting for the previously observed effect of substrate concentration on the observed Km. Computer-aided analysis allowed the determination of the inhibition constant as a function of experimental parameters. The reaction of ribonuclease A with 2',3'-CMP was investigated at subzero temperatures. The turnover reaction could be made negligible at temperatures below -60 degrees C at pH 3-6 in 70% methanol and below -35 degrees C at pH 2.1. The rate of the catalytic reaction with crystalline enzyme was compared to that of enzyme in solution for both 2',3'-CMP and the dinucleotide CpC. The rates were 50- and 200-fold slower, respectively, in the crystal. These investigations allowed calculation of the necessary conditions for NMR and X-ray diffraction experiments on the trapped enzyme--substrate intermediate.

X-ray refinement study on the binding of cytidylic acid (2'-CMP) to ribonuclease A

The X-ray structure of the inhibitor complex of bovine ribonuclease A with cytidylic acid (2'-CMP) has been determined at 2.3 A (1 A = 0.1 nm) resolution and refined by restrained least-squares refinement to R = 0.132 for 5650 reflections. Incorporation of the inhibitor molecule has occurred with little disturbance of the protein main-chain atoms, although significant displacement of some side-chain atoms has occurred, particularly in the region of the active site. The binding of 2'-CMP to ribonuclease A is different from that of the related cytidine-N(3)-oxide 2'-phosphate, which has an extra oxygen on N(3) of the cytidine base. The PO4(2-) group is held by hydrogen bond interactions to the side-groups of His 12, Glu 11 and His119. Thr45 is involved in stabilizing the enzyme-ligand complex by forming hydrogen bond interactions between O(gamma) and the pyrimidine base N(3) atom and between the main-chain N(45) and O(2) of the base. Phe120 is much closer to the inhibitor than in the cytidine N(3)-oxide 2'-phosphate structure.

Inhibition of DNA polymerase-alpha by ara-CMP in the presence of a regulatory protein extracted from human promyelocytic leukemic cells (HL-60)

Cytotoxicity of arabinofuranosylcytosine (ara-C) has been related in vitro to the inhibition of the DNA polymerase activities by arabinosylcytosine triphosphate (ara-CTP) and the incorporation of ara-C into the DNA where, acting as a chain terminator, it slows the chain elongation. Induced in vitro cellular resistance to ara-C was shown to be secondary to altered deoxycytidine (dCyd) kinase activity, dCyd deaminase activity, or deoxynucleotides triphosphates (dNTP) pools. Recent studies reported no differences of ara-C metabolism in cells obtained from leukemic patients at diagnosis and at relapse after ara-C therapy, suggesting that unknown cellular biochemical determinants may be involved in acquisition of ara-C resistance. Using dialysed crude extracts of leukemic cells obtained from patients at diagnosis, we observed variable inhibition of their DNA polymerase activities by arabinosylcytosine monophosphate (ara-CMP) at 2 mmol/L (0% to 50% inhibition). In similar conditions, ara-CMP reduced the polymerase activities of human thymus extract by 35% and 55% in extract of HL-60 cells (cultured human promyelocytic cells). The ara-CMP factor responsible for inhibition of DNA polymerase activity was nondialysable, heat labile, proteinase K sensitive, and has an estimated molecular mass of 30 kilodalton by gel filtration. After partial purification, this protein had no DNA polymerase RNA polymerase activities. In presence of the regulator and ara-CMP at 2 mmol/L, we observed no inhibition of the HL-60 3'----5' and 5'----3' exonucleases activities, suggesting the regulator interaction being mainly with the DNA polymerases in presence of ara-CMP. The relevance of the presence or absence of this protein regarding the cell sensitivity to ara-C is under investigation.

CpA containing oligoribonucleotides specifically inhibit protein synthesis in rabbit reticulocytes

The diribonucleoside monophosphate CpA (and no others) inhibits polypeptide chain elongation in rabbit reticulocyte lysates at 10-50 microM. Furthermore, all the trinucleotides containing CpA, i.e., XpCpA and CpApX (X = U, C, A or G) block polypeptide chain elongation as well. At 10 microM the inhibition by XpCpA and not CpApX is transient because a 3'-exonucleolytic activity destroys the critical CpA moiety. The inhibitors do not appear to interfere with the aminoacylation of tRNAs or disrupt the interaction of amino-acyl-tRNAs with the protein synthetic machinery. High levels (200 microM) of CpA or the trinucleotides containing CpA have no effect on translation in a wheat germ cell-free system.

The control of CTP:choline-phosphate cytidylyltransferase activity in pea (Pisum sativum L.)

Several possible control mechanisms for CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15) activity in pea (Pisum sativum L.) stems were investigated. Indol-3-ylacetic acid (IAA) treatment of the pea stems decreased total cytidylyltransferase activity but did not affect its subcellular distribution. Oleate (2 mM) caused some stimulation of enzyme activity by release of activity from the microsomal fraction into the cytosol, but neither phosphatidylglycerol nor monoacyl phosphatidylethanolamine had an effect on activity or subcellular distribution. A decrease in soluble cytidylyltransferase protein concentrations was found in IAA-treated pea stems, but this was not sufficient to account for all of the decrease in cytidylyltransferase activity. A 50% inhibition of enzyme activity could be obtained with 0.2 mM-CMP, which indicated possible allosteric regulation. Similar inhibition was obtained with 1.5 mM-ATP, but other nucleotides had no effect. The cytidylyltransferase enzyme protein was not directly phosphorylated, and the inhibition with 1.5 mM-ATP occurred with the purified enzyme, thus excluding an obligatory mediation via a modulator protein. The results indicate that the cytosolic form of cytidylyltransferase is the most important in pea stem tissue and that the decrease in cytidylyltransferase activity in IAA-treated material appears to be brought about by several methods.

Reversibility of cholinephosphotransferase in lung microsomes

The effect of cytidine 5'-monophosphate (CMP) on the incorporation of cytidine 5'-diphosphate (CDP) [methyl-14C]choline or [1-14C]dipalmitoylglycerol into phosphatidylcholine (PC) catalyzed by rabbit lung microsomal CDPcholine:1,2-diacyl-sn-glycerol cholinephosphotransferase (EC 2.7.8.2) was studied. In the presence of 0.85 mM CMP and nonsaturating diacylglycerol concentration, the incorporation of CDP[14C]choline into PC was markedly stimulated, but the incorporation of [14C]dipalmitoylglycerol into PC was inhibited. This was due to the increase of endogenous diacylglycerol generated from microsomal PC by the cholinephosphotransferase reverse reaction. However, the newly synthesized PC was not readily hydrolyzed in the presence of CMP. The results of this study suggest that the endogenous membranous diacylglycerol is utilized more preferentially for PC synthesis than the exogenous diacylglycerol and that the newly synthesized PC could rapidly equilibrate with the endogenous membrane PC pool.

Inhibition of DNA replication in Saccharomyces cerevisiae by araCMP

Cytosine arabinoside (araC), a potent inhibitor of DNA replication in mammalian cells, was found to be completely ineffective in Saccharomyces cerevisiae. The 5' monophosphate derivative, araCMP, is toxic and effectively inhibits both nuclear and mitochondrial DNA synthesis in this organism. Although wild-type strains can be inhibited by araCMP, dTMP permeable (tup-) strains were found to be much more sensitive to the analogue. In vivo labelling experiments indicate that araC enters yeast cells; however, it is extensively catabolized by deamination and breakage of the glycosidic bond. In addition, the analogue is not efficiently phosphorylated in S. cerevisiae owing to an apparent lack of deoxynucleoside kinase activity. These results provide further evidence that deoxyribonucleotides can be synthesized only through de novo pathways in this organism. Finally, araCMP was found to be recombinagenic in S. cerevisiae which suggests, together with other previous studies, that, in general, inhibition of DNA synthesis in yeast promotes mitotic recombination events.

Phosphatidylinositol:myo-inositol exchange activity in intact nerve endings: substrate and cofactor dependence, nucleotide specificity, and effect on synaptosomal handling of myo-inositol

Micromolar concentrations of CMP produced a large increase in Mn2+-dependent phosphatidylinositol:myo-inositol exchange activity in isolated nerve endings or synaptosomes. The apparent Km for CMP was 2 microM, and that for myo-inositol was 38 microM. Only cytidine nucleotides were capable of enhancing activity, and this effect is probably specific for CMP, because the synaptosomal preparation rapidly converted CTP or CDP to CMP. Manganese did not affect the uptake of myo-inositol into the synaptosomal cytosolic fraction or myo-inositol levels. Determinations of myo-inositol specific activity showed that the Mn2+-enhanced labeling of phosphatidylinositol was not accompanied by a decrease of label content in free myo-inositol. This lack of an effect on intrasynaptosomal myo-inositol and the dependence of exchange on cytidine nucleotides whereas cytidine itself was previously found to be without effect show that for the bulk of Mn2+-dependent exchange activity, it is the myo-inositol in the incubation medium that is being directly incorporated into membrane-bound phosphatidyl-inositol. Because CMP dependence is the hallmark of exchange catalyzed by CDP-diacylglycerol:inositol phosphatidyl transferase, this enzyme is likely to be responsible for most of the exchange activity in synaptosomes. The strong affinity of this exchange system for CMP suggests that endogenous levels of this nucleotide might support Mn2+-dependent exchange in the absence of added nucleotide.

Cytidine monophosphate-dependent synthesis of phosphatidylglycerol in permeabilized type II pneumonocytes

Results of previous investigations support the proposition that, in type II pneumonocytes, CMP is involved in integration of the synthesis of phosphatidylcholine and phosphatidylglycerol for lung surfactant. In the present investigation, the amount of CMP in rat type II pneumonocytes was altered directly and resultant changes in the synthesis of phosphatidylglycerol were examined. Type II pneumonocytes were made permeable to CMP by treatment with Ca2+-free medium, and phosphatidylglycerol synthesis was then assessed by measurement of the incorporation of a radiolabelled precursor, [14C]glycerol 3-phosphate, that was not effectively utilized by cells that resisted permeabilization. Incorporation of [14C]glycerol 3-phosphate into phosphatidylglycerol (but not into other lipids) was stimulated greatly by CMP (half-maximal stimulation at approx. 0.1 mM). CMP stimulated the incorporation of [14C]glycerol 3-phosphate into both the phosphatidyl moiety and the head group of phosphatidylglycerol. Incorporation of [14C]palmitate into phosphatidylglycerol was also stimulated by CMP. myo-Inositol, at concentrations found in foetal-rat serum (0.2-2.0 mM), inhibited CMP-dependent incorporation of [14C]glycerol 3-phosphate into phosphatidylglycerol and promoted, instead, CMP-dependent incorporation into phosphatidylinositol. These data, when extrapolated to foetal type II pneumonocytes, are consistent with the view that the developmental increase in the synthesis of phosphatidylglycerol for surfactant by foetal lungs is promoted by the increase in intracellular CMP and the declining availability of myo-inositol that were found previously to be associated with this period of development.

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.

Phosphatidylcholine as a source of diacylglycerols in neuronal nuclei incubated in the presence of EGTA and CMP

A neuronal nuclear fraction (N1), isolated from immature rabbit cerebral cortex, was preincubated with 1-[14C]palmitoyl-sn-glycero-3-phosphorylcholine and oleoyl-CoA. Most of the radioactivity was recovered in N1 phosphatidylcholine, and subsequent incubations in the presence of EGTA and CMP indicated an increase in radioactivity in N1 diacylglycerol and triacylglycerol which was matched by a decline in the labelling of N1 phosphatidylcholine. N1 phosphatidylcholine was also prelabelled using [14C]oleate and 1-acyl-sn-glycero-3-phosphorylcholine in vitro, or by intrathecal injection of [3H]oleate prior to N1 isolation. In the following incubations with EGTA and CMP there was a good correspondence between the radioactive decline in N1 phosphatidylcholine and the increase in radioactivity in N1 diacylglycerol. In all these experiments the generation of radioactive diacylglycerol depended upon the presence of EGTA and CMP in the incubations and could be largely inhibited by the addition of CDP-choline. During the prelabelling procedures noted above, other complex lipids had less of the total radioactivity than phosphatidylcholine and showed little or no decline in radioactivity in the presence of EGTA and CMP. In N1 preincubations with [14C]oleate and lysophosphatidylethanolamine, phosphatidylethanolamine could be more highly labelled than phosphatidylcholine, but in subsequent incubations with EGTA and CMP no decline was seen in phosphatidylethanolamine radioactivity. It is concluded that the back reaction of cholinephosphotransferase in N1 represents an active route for the production of diacylglycerols bearing palmitate and/or oleate.

Effect of nucleotides on translocation of sugar nucleotides and adenosine 3'-phosphate 5'-phosphosulfate into Golgi apparatus vesicles

Recent studies from this laboratory have suggested that rat-liver Golgi apparatus derived membranes contain different proteins which can translocate in vitro CMP-N-acetylneuraminic acid, GDP-fucose and adenosine 3'-phosphate 5'-phosphosulfate from an external compartment into a lumenal one. The aim of this study was to define the role of the nucleotide, sugar and sulfate moieties of sugar nucleotides and adenosine 3'-phosphate 5'-phosphosulfate in translocation of these latter compounds across Golgi vesicle membranes. Indirect evidence was obtained suggesting that the nucleotide (but not sugar or sulfate) is a necessary recognition feature for binding to the Golgi membrane (measured as inhibition of translocation) but is not sufficient for overall translocation; this latter event also depends on the type of sugar. Important recognition features for inhibition of translocation of the above sugar nucleotides and adenosine 3'-phosphate 5'-phosphosulfate were found to be the type of nucleotide base (purine or pyrimidine) and the position of the phosphate group in the ribose. Thus, UMP and CMP were found to be competitive inhibitors of translocation of CMP-N-acetylneuraminic acid, while AMP did not inhibit. Structural features of the nucleotides which were less important in inhibition of translocation (and thus presumably in binding) of the above sugar nucleotides and adenosine 3'-phosphate 5'-phosphosulfate were the number of phosphate groups in the nucleotide (CDP and CMP inhibited to a similar extent), the presence of ribose or deoxyribose in the nucleotide, a replacement of hydrogen in positions 5 of pyrimidines or 8 in purines by halogens or an azido group. The sugar or sulfate did not inhibit translocation of the above sugar nucleotides and adenosine 3'-phosphate 5'-phosphosulfate into Golgi vesicles and therefore appear not to be involved in their binding to the Golgi membrane.

An increased incorporation of fatty acid into triacylglycerols of neuronal nuclei in vitro in the presence of CMP and EGTA

Neuronal nuclei (N1) were isolated from cerebral cortices of 15-day-old rabbits. With the addition of both EGTA and CMP the incorporation of radioactive oleate into N1 triacylglycerols in vitro (in the presence of ATP, CoA, and MgCl2) was increased threefold. The same large increase could not be achieved using citrate or EDTA in the presence of CMP or using AMP, UMP, or TMP in the presence of EGTA. The increased labelling of N1 triacylglycerols could be greatly reduced when CDP-choline was added to incubations containing EGTA and CMP. Levels of endogenous N1 diacylglycerols increased threefold following a 10-min incubation in the presence of buffer (pH 7.4) and MgCl2, when CMP and EGTA were also added. Of the major N1 phospholipids, phosphatidylcholine was most similar in fatty acid composition to the enlarged endogenous diacylglycerol pool. The rate of formation of oleoyl-CoA in fraction N1 was not significantly changed by the presence of EGTA and CMP. Rates of triacylglycerol labelling could only be modestly increased when EGTA and CMP were added to incubations containing N1 samples with artificially enlarged endogenous diacylglycerol pools (produced by phospholipase C preincubations). It is suggested that EGTA, as a Ca2+ chelator, and CMP, as a substrate, may allow an enhanced diacylglycerol production mediated by the back reaction of cholinephosphotransferase in N1. The endogenous N1 diacylglycerol produced in the absence of EGTA and CMP may come from another metabolic route.

Inhibition of endoribonuclease VI from Artemia larvae by cytidine 2'-phosphate

The endoribonuclease VI from Artemia larvae is non-competitively inhibited by cytidine 2'-phosphate with a Ki ca 1 microM. Neither of the cytidine monophosphates isomers with the phosphate group in the 3' or 5' position nor the cyclic 2':3' phosphate are inhibitors at concentrations up to 100 microM. Adenosine, guanosine and uridine 2' or 3' phosphates are also ineffective in this range of concentrations. Certain polyribonucleotides are potent competitive inhibitors of the ribonuclease activity.

Effect of nucleotides on the incorporation of myo-inositol into phosphatidylinositol in rat liver microsomes

In rat liver microsomes the incorporation of inositol in the presence of Mn2+ was stimulated by cytidine nucleotides, whereas it was inhibited by other nucleotides. At low concentrations of CMP, AMP and other nucleotides stimulated inositol incorporation. No such effect was observed when the concentration of CMP was 2 mM or higher. It was found that an appreciable hydrolysis of CMP to cytidine and inorganic phosphate occurred during incubation with microsomes in the presence of Mg2+ or Mn2+. AMP was hydrolysed at a comparable rate. The activatory effect of AMP and other nucleotides on the CMP-dependent incorporation of inositol could be ascribed to protecting CMP against hydrolysis.

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.

Folding of homologous proteins. The refolding of different ribonucleases is independent of sequence variations, proline content and glycosylation

The refolding kinetics of four different pancreatic ribonucleases have been compared. Bovine and ovine RNAase contain 4 proline residues, red deer RNAase has 5 prolines, the enzyme from roe deer 6 prolines. Despite the variation in the amount of prolines, all four proteins show a constant value of 20% fast refolding species UF. The extra proline residues of the deer enzymes do not increase the amount of slow refolding species US. Consequently these residues may be non-essential for folding. Despite many differences in the amino acid sequence, the rates if the fast and slow refolding reactions are very similar for all investigated ribonucleases. This indicates that the pathway of refolding has been conserved during evolution, i.e. the positions where amino acid substitutions occur are not critically important for the rate-determining steps of the folding process. A carbohydrate chain attached to ribonuclease does not alter the folding properties of the protein: RNAase A and RNAase B from roe deer show identical refolding kinetics.

The effect of CMP on the release of free fatty acids of rat brain in vitro

With CMP, phosphatidylcholine can be converted to diacylglycerols and CDPcholine by reversal of the cholinephosphotransferase that is normally used for synthesis. Incubation of homogenates of rat brains at pH 8 with 20 mM MgCl2 increased the free fatty acid (FFA) levels 30 to 117%. The FFA levels increased 62 to 212% when 4 mM CMP was included. Diacylglycerols were also produced. Hydrolysis of the diacylglycerols to FFA was markedly inhibited by inclusion of 3 mM diisopropylphosphofluoridate in the incubation mixture. The composition of the fatty acids released by CMP resembles that of phosphatidylcholine except for some polyunsaturated fatty acids. These may have been released from the ethanolamine glycerophospholipids. Most of the CMP-stimulated release of FFA was blocked by inclusion of 1 mM CDPcholine in the incubation mixture. Rat brains were labeled by intracerebral injection of [3H]oleic acid. The labeled oleic acid was released primarily from phosphatidylcholine. Thus, measurements of both mass and radioactivity confirm that the reversal of cholinephosphotransferase followed by diacylglycerol lipase can be an important pathway for the liberation of FFA from phosphatidylcholine.

CMP-dependent phosphatidylinositol:myo-inositol exchange activity in isolated nerve-endings

Greatly enhanced manganese-dependent phosphatidylinositol:myo-inositol exchange activity was observed when isolated, intact nerve-endings were incubated with the nucleotide, CMP, suggesting that the enzyme, CDP-diglyceride:inositol phosphatidyl transferase, catalyzes this exchange. CMP, at 10 microM, produced as much myo-[2-3H] inositol incorporation into phosphatidylinositol as did 1 mM. This CMP-stimulated exchange activity may reside on the plasma membrane.

Biosynthesis in vitro of sialyl(alpha 2-3)neolactotetraosylceramide by a sialyltransferase from embryonic chicken brain

A sialyltransferase activity present in 7- to 12-day-old embryonic chicken brain catalyzes the transfer of sialic acid from CMP-sialic acid to the terminal galactose residue of [3H]nLcOse4Cer ([3H]Gal(beta 1-4).GlcNAc(beta 1-3)Gal(beta 1-4)Glc-Cer) to form NeuAc(alpha 2-3)-[3H]nLcOse4Cer (LM1 ganglioside). The product is sialidase-labile (96%), and the NeuAc group is linked to O-3 of the terminal galactose residue. The (alpha 2-3) linkage between sialic acid and the terminal galactose was determined on the basis of identification of 2,4,6-tri-O-methyl[3H]galactose obtained after hydrolysis of the permethylated enzymatic product. The CMP-sialic acid:nLcOse4Cer (alpha 2-3)sialyltransferase activity sediments (90%) at the junction of 1.2 M and 1.5 M on a discontinuous sucrose density gradient when still membrane bound (insoluble in 0.2% Triton X-100). The enzyme preparation also catalyzes the transfer of sialic acid from CMP-sialic acid to O-3 of GgOse4Cer (Gal(beta 1-3)GalNAc(beta 1-4)Gal(beta 1-4)Glc-Cer) to form NeuAc (alpha 2-3)GgOse4Cer (GM1b). Substrate inhibition studies indicate that these two reactions are probably catalyzed by the same enzyme.