Effects of base exchange reaction on the Na+, K+ ATPase in rat brain microsomes

Presence of phosphatidylserine synthesizing enzymes in triton insoluble floating fractions from cerebrocortical plasma membranes: do phosphatidylserine synthesizing enzymes in plasma membrane microdomains play a role in signal transduction?

Mammals synthesize phosphatidylserine (PS), a binding PKC molecule, by exchanging the nitrogen base of phosphatidylethanolamine or phosphatidylcholine with free serine. Serine base exchange enzyme (SBEE) was found in Triton insoluble floating fractions (TIFFs) from rat cerebellum which contained PKC. Consequently, SBEE might modulate PS levels in the PKC binding area (Buratta et al., J Neurochem 103:942-951, 2007). In the present study, we determined whether SBEE and PKC were localised in rat cerebral cortex TIFFs (cx-TIFFs) and in rat cerebrocortical plasma membrane-TIFFs (PM-TIFFs) which are more directly involved in signal transduction than intracellular membranes. Cx-and PM-TIFFs expressed SBEE activity and contained PKC. SBEE used ethanolamine as free exchanging base which may modulate PS level in the PKC binding area, transforming PS into PE and vice versa. The slight decrease in [(14)C]serine incorporation in the presence of choline indicated the existence of a SBEE isoform which may play a peculiar role in this brain area.

Metabolism and functions of phosphatidylserine in mammalian brain

Phosphatidylserine (PtdSer) is involved in cell signaling and apoptosis. The mechanisms regulating its synthesis and degradation are still not defined. Thus, its role in these processes cannot be clearly established at molecular level. In higher eukaryotes, PtdSer is synthesized from phosphatidylethanolamine or phosphatidylcholine through the exchange of the nitrogen base with free serine. PtdSer concentration in the nervous tissue membranes varies with age, brain areas, cells, and subcellular components. At least two serine base exchange enzymes isoforms are present in brain, and their biochemical properties and regulation are still largely unknown because their activities vary with cell type and/or subcellular fraction, developmental stage, and differentiation. These peculiarities may explain the apparent contrasting reports. PtdSer cellular levels also depend on its decarboxylation to phosphatidylethanolamine and conversion to lysoPtdSer by phospholipases. Several aspects of brain PtdSer metabolism and functions seem related to the high polyunsaturated fatty acids content, particularly docosahexaenoic acid (DHA).

Changes of nuclear membrane lipid composition affect RNA nucleocytoplasmic transport

We have previously shown that the nuclear membrane fluidity of rat liver, measured by fluorescence anisotropy of two probes, is higher in the hydrophobic core, with respect to the bilayer surface, in newborn rats compared to adult rats. The aim of the present research is to investigate whether the nuclear membrane fluidity influences RNA nucleocytoplasmic transport. To this end two experimental models were used: the fluidity of nuclear membrane isolated from adult rats was increased by a choline base exchange reaction, which is known to be accompanied by an increase of phosphatidylcholine unsaturated fatty acids, whereas that of nuclear membrane isolated from newborn rats was decreased by incubation with dimyristoylphosphatidylcholine-cholesterol liposomes. The RNA efflux, evaluated by using [3H]uridine, significantly increased in the adult nuclear membrane submitted to choline base exchange reaction, whereas a strong decrease in the newborn nuclear membrane enriched with cholesterol was found. The activity of nucleoside triphosphatase, a nuclear membrane-associated enzyme which is correlated with mRNA transport, showed parallel variations. Therefore, for the first time, we have provided evidence that the nuclear membrane fluidity plays a regulatory role in RNA nucleocytoplasmic transport, although the mechanism by which this effect takes place remains to be clarified.

Synthesis of ethanolamine phosphoglycerides in rat cerebral cortex subjected in vitro to experimental hypoxia with and without hypocapnia

Slices and homogenates from rat cerebral cortex were used to study the effect of hypoxia, with or without hypocapnia, on phosphatidylethanolamine synthesis. The incorporation of [1-3H]ethanolamine into the corresponding phospholipid was greatest in slices treated with pure nitrogen, intermediate when the nitrogen contained 5% CO2, and least in slices treated with 95% O2-5% CO2. The role of hypocapnia in reinforcing the effect due to hypoxia did not require the integrity of the cell because similar results were obtained by treating homogenates with pure nitrogen or nitrogen plus 5% CO2. In both cases the synthesis of phosphatidylethanolamine was abolished by the addition of EGTA and the degradation of newly synthesized phospholipid by phospholipases was similar to that obtained in controls. When the homogenate was not buffered, changes in the pH due to experimental treatment influenced the response to Ca2+ and to hypoxia plus hypocapnia. Intracellular calcium ions are thought to play a role in the response of cerebrocortical slices to N2-treatment. In fact, although the incorporation was greater in complete medium that contains 2 mM Ca2+ than in the same medium prepared without the addition of this ion, the relative increase of incorporation due to N2-treatment was greater in the medium lacking added Ca2+.

Effect of lipid composition on rat liver nuclear membrane fluidity

Nuclear membrane fluidity is measured in rat liver by use of the fluorescence anisotropy of two probes: diphenylhexatriene and its cationic derivative trimethylammonium-diphenylhexatriene. It has been shown that, in 2-month-old rat liver cells, the bilayer surface is less fluid than the hydrophobic core. The fluidity was higher in 6-day-old rat liver nuclei, in which both the amount of cholesterol and the cholesterol/phospholipid ratio decreased. The influence of the single phospholipids, and in particular of phosphatidylcholine, has been studied by increasing the phosphatidylcholine with a choline base exchange reaction in isolated nuclear membranes. After this reaction, the fluorescence anisotropy of the bilayer surface increased, whereas at the hydrophobic core it decreased. Analysis of fatty acid composition shows an increase of phosphatidylcholine unsaturated fatty acids. The results show that the fluidity of nuclear membranes changes in relation to the lipid content and to the fatty acid composition. The role of nuclear membrane fluidity in cell function is discussed.

Membrane integrity and phospholipid movement influence the base exchange reaction in rat liver microsomes

Properties of Ca(2+)-stimulated incorporation of amincalcohols, serine and ethanolamine, into phospholipids, and factors regulating the reaction were studied in endoplasmic reticulum membranes isolated from rat liver. In contrast to apparent K(m) values for either aminoalcohol, maximal velocities of the reaction were significantly affected by Ca2+ concentration. No competition between these two soluble substrates used at equimolar concentrations close to their K(m) values was observed, suggesting the existence of two distinct phospholipid base exchange activities. The enzyme utilizing the electrically neutral serine was not sensitive to changes of membrane potential evoked by valinomycin in the presence of KCl. On the other hand, when positively charged ethanolamine served as a substrate, the enzyme activity was inhibited by 140 mM KCl and this effect was reversed by valinomycin. The rates of inhibition of phospholipid base exchange reactions by various thiol group modifying reagents were also found to differ. Cd2+ and lipophylic p-chloromercuribenzoic acid at micromolar concentrations were most effective. It can be suggested that -SH groups located within the hydrophobic core of the enzymes molecules are essential for the recognition of membrane substrates. However, the influence of the -SH group modifying reagents on the protein-facilitated phospholipid motion across endoplasmic reticulum membranes can not be excluded, since an integral protein-mediated transverse movement of phospholipids within the membrane bilayer and Ca(2+)-mediated changes in configuration of the phospholipid polar head groups seem to be a regulatory step of the reaction. Indeed, when the membrane integrity was disordered by detergents or an organic solvent, the reaction was inhibited, although not due to the transport of its water-soluble substrates is affected, but due to modulation of physical state of the membrane bilayer and, in consequence, the accessibility of phospholipid molecules.

Phospholipid base exchange enzyme activity in sarcolemmal membranes from the heart of cardiomyopathic hamsters

The activity of phospholipid base exchange enzymes has been evaluated in cardiac sarcolemmal membranes from Syrian Golden hamsters and from a hamster strain (UM-X7.1) characterized by a genetic form of hypertrophic cardiomyopathy. No choline base exchange activity and only a little serine base exchange activity were detected, whereas the ethanolamine base exchange enzyme was found highly active in membranes from both strains. For this reason, the present study is focussed on the ethanolamine base exchange enzyme. The apparent Km for ethanolamine of ethanolamine base exchange enzyme from Syrian Golden membranes and from UM-X7.1 strain membranes are 18 and 32 microM, respectively. The specific activity of the sarcolemmal ethanolamine base exchange enzyme is lower in the UM-X7.1 strain than in Syrian Golden hamsters. The calcium-dependence of the enzyme appears different when the membranes from the two strains are compared. Indeed, after removal of the membrane-bound divalent cations, comparable activities are found in both membrane preparations, whereas, upon addition of Ca2+ to the incubation mixtures, the activity of the enzyme is enhanced in the membranes from Syrian Golden strain more than in those from UM-X7.1 strain. The cholesterol content of sarcolemmal membranes is higher in the cardiomyopathic strain than in the Syrian Golden hamsters. A possible relation between changes of the membrane lipid composition and of the ethanolamine base exchange activity is discussed.

Effect of modification of membrane phospholipid composition on phospholipid methylation in aggregating cell culture

The effect of the presence of nitrogenous bases in the growth medium of fetal rat brain aggregating cell cultures was investigated. The presence of either N-methylethanolamine (MME) or N,N-dimethylethanolamine (DME) in the growth medium resulted in significant increase of the corresponding phospholipid, phosphatidyl-N-monomethylethanolamine (PMME) or phosphatidyl-N,N-dimethylethanolamine (PDME). They represented 28% and 32% of the total phospholipids, respectively. The presence of the new phospholipids was accompanied by a significant decrease of phosphatidylethanolamine (PE) and phosphatidylcholine (PC). Cells grown in the presence of ethanolamine or choline had only barely detectable amounts of PMME and PDME. Intact cells previously grown with the bases were incubated with [methyl-3H]methionine. Incubation of cells previously grown in presence of the bases MME and DME resulted in a marked increase of radioactivity in the corresponding phospholipids possessing one additional methyl group, PDME and PC respectively. The incorporation of S-adenosyl[methyl-3H]methionine (AdoMet) was examined in cell homogenates incubated in presence or absence of either PMME or PDME acceptors. The addition of these exogenous phospholipids caused a three-or fourfold stimulation of radioactivity incorporated into the total phospholipids of cells grown in the absence of nitrogen bases. The cells grown in presence of either MME or DME in the culture medium did not show an increased incorporation of methyl groups from AdoMet into the total phospholipids after addition of exogenous acceptors. This work suggests that MME and DME incorporated into the corresponding phospholipids function as effective substrates for phospholipid-N-methylation.

The base-exchange enzyme activities of sarcolemma and sarcoplasmic reticulum from rat heart

The Ca2+ dependent incorporation of [14C]ethanolamine, L-[14C]serine and [14C]choline into phosphatidylethanolamine, phosphatidylserine and phosphatidylcholine, respectively, were investigated in membrane preparations from rat heart. The ethanolamine and serine base-exchange enzyme-catalyzed reactions were associated with the sarcolemma and sarcoplasmic reticulum. There was a 17.2-fold and 6.8-fold enrichment, respectively, of the serine and the ethanolamine base-exchange enzyme activities in the sarcolemma compared to the starting whole homogenate. The sarcoplasmic reticulum was enriched in the ethanolamine and serine base-exchange enzyme activities. The choline base-exchange enzyme activity of all membranes fractions was negligible compared to the ethanolamine or serine base-exchange enzyme activities. The apparent Km for the ethanolamine and serine base-exchange enzyme in sarcolemma was 14 microM and 25 microM, respectively. The pH optimum for these base-exchange activities was 7.5-8.0. There was a dependence upon Ca2+ for these reactions with a 1 or 4 mM concentration required for maximal activity. The properties of the sarcoplasmic reticulum base-exchange enzymes were similar to the sarcolemmal base-exchange enzymes.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of rat brain microsomal Na+,K+-ATPase by S-adenosylmethionine

Rat brain microsomes were preincubated with S-adenosylmethionine (SAM), MgCl2, and CaCl2, then reisolated, and the activity of Na+,K+-ATPase determined. SAM inhibited the Na+,K+-ATPase activity compared with microsomes subjected to similar treatment in the absence of SAM. A biphasic inhibitory effect was observed with a 50% decrease at a SAM concentration range of 0.4 microM-3.2 microM and a 70% reduction at a concentration range above 100 microM. Inclusion of either S-adenosylhomocysteine or 3-deazaadenosine in the preincubations prevented the SAM inhibition of Na+,K+-ATPase activity. The inhibition by SAM appeared to be Mg2+- or Ca2+-dependent.