The metabolism of phosphoric esters and of cytidine-diphosphate esters of choline and ethanolamine in the liver

The effect of oxiracetam treatment on alterations of lipid metabolism in brain areas from spontaneously hypertensive rats

It has been previously demonstrated that spontaneously hypertensive adult rats (SHR) develop severe hypertension and cerebrovascular lesions on drinking 1% NaCl from weaning and that the phospholipid metabolism in the whole brain is actively altered in these lesioned animals (SHR-NaCl) as compared to SHRs which drink only water and show only sporadic cerebrovascular lesions. We have now assayed the incorporation of labelled choline, ethanolamine, glycerol and arachidonic acid into the phospholipids from the cortex and hippocampus of SHR-water and SHR-NaCl at different time intervals from injection into the lateral ventricle of the brain. A noticeable decrease of both choline and arachidonate specific activity (SA) in the phospholipids was found in the cortex and hippocampus (where the effect is most evident) from SHR-NaCl. Based on the literature and the data obtained, we suggest that in SHR-NaCl brain areas a release of choline and fatty acid also occurs from choline glycerophospholipids as a consequence of the cerebrovascular lesions caused by NaCl treatment. Even if a relatively minor loss of the amount of the lipids studied is evident from our results as compared to their entire pool, this change may be quite important if it causes a modification of the lipidic bilayer in excitable membranes. In a parallel group of SHR-NaCl animals, treated with the nootropic drug oxiracetam, we observed that the metabolic utilization of the precursors was completely restored. These experimental data favour the hypothesis that oxiracetam is effective in stimulating the phospholipid metabolism rate at levels even higher than those of the SHR-water animals.

Factors affecting the reaggregation of rat brain microsomes solubilized with octyl glucoside and their relationship with the base-exchange activity of reaggregates

Rat brain microsomal membranes disaggregated by exposure to octyl glucoside were recovered by centrifugation after dialytic removal of the detergent. The composition of the dialysis medium (divalent cations, pH) was important to this effect; indeed, the reaggregation process which occurred during the dialytic step required the presence of either Ca2+ or Mg2+ and a slightly acidic pH. The lipid protein/ratio and choline and ethanolamine base-exchange of recovered particles depended on the conditions of dialysis although their lipid composition did not. The lipid composition of membranes was also varied by adding PE or PC to octyl glucoside-microsome suspensions. This treatment produced reaggregates possessing a low content of cholesterol and varying PC/PE ratios. Both choline and ethanolamine base-exchange activities were related to this parameter.

Cerebellar metabolism of phosphatidylethanolamine and its water-soluble precursors during bicuculline-induced convulsive seizures

The incorporation of labeled ethanolamine into phosphatidylethanolamine (PE) and its water-soluble precursors, phosphoethanolamine and CDP-ethanolamine, is measured in rat cerebella during the course of bicuculline-induced convulsive seizures. The labeling of CDP-ethanolamine and phosphoethanolamine diminishes 6 min after the administration of both bicuculline and radioactive ethanolamine whereas that of PE is unaffected in these conditions. Time is very important to this effect; indeed, no differences of the labeling of PE water-soluble precursors can be found in rat cerebellum of normal and convulsing animals 12 min after the administration. The cerebellar pool of CDP-ethanolamine doubles after 6 min of convulsions, which means that unlabeled CDP-ethanolamine forms from a non-radioactive source, such as lipid, may be through the reversal of the ethanolamine phosphotransferase reaction. This effect disappears 12 min after the injection of the convulsant.

Effect of cytidine on the modification of phospholipid metabolism induced by ischemia

[1-14C-]Arachidonic acid was injected into the lateral ventricle of the gerbils (meriones unguiculatus) two hours before producing brain ischemia by the bilateral ligation of the carotid arteries. Ten minutes before the carotid ligation a group of animals received an additional intraventricular injection of cold cytidine (2.5 mumol/brain). Control animals with and without cytidine, together with the ischemic group, were decapitated directly into liquid nitrogen ten minutes after carotid ligation or sham surgery. Cytidine is able to both stimulate arachidonic acid incorporation into lipids and noticeably correct the release of this acid from polar lipids induced by ischemia. Based on these findings, it is possible to assume that cytidine exerts an effect on the biosynthesis of phosphoglycerides as well as on their catabolic activities.

Cerebellar metabolism of phosphatidylcholine and its hydrosoluble precursors during bicuculline-induced convulsive seizures

The cerebellar incorporation of labeled choline into phosphatidylcholine (PC) and its hydrosoluble choline-containing precursors has been examined during the course of bicuculline-induced convulsive seizures. The labeling of phosphocholine and of PC diminished in these conditions whereas that of cytidine-5'-diphosphate choline (CDP-choline) was practically unaffected. Moreover, the cerebellar pools of phosphocholine and CDP-choline increased by 75-100% after 6 min of convulsions; these compounds were formed from lipid through the action of phospholipases or through the reverse action of choline phosphotransferase. From the data reported in this paper it should also be inferred that the cytidylyltransferase reaction was activated. It is therefore concluded that the cerebellar metabolism of PC and its precursors was affected in various ways by the bicuculline-induced convulsive seizures.

The reaggregation of rat brain microsomal membranes after the treatment with octyl-beta-D-glucopyranoside. A study on ethanolamine base-exchange

The ethanolamine base-exchange activity of rat brain microsomes has been studied after treating the membranes with the non-ionic detergent n-octyl-beta-D-glucopyranoside. The detergent could solubilize membrane lipid and protein. The concentrations of the detergent and of membrane protein were both important for this effect. The presence of disaggregating concentrations of octylglucopyranoside in the base-exchange incubation mixture strongly inhibited the incorporation of radioactive ethanolamine into lipid; however, the removal of the detergent through dialytic procedures before assaying the base-exchange reaction restored the enzymic activity almost completely. As shown by exposing the membranes to trinitrobenzenesulfonic acid (TNBS), the phosphatidylethanolamine (PE) which was newly synthesized by base-exchange was also compartmented in the microsomal membrane. The treatment with the detergent after the base-exchange reaction abolished the compartmentation of the newly synthesized lipid. However, if microsomes were solubilized and the detergent was removed by dialysis before the assay of base-exchange, the reassembly of membranes occurred with a recovery of the compartmentation of the newly synthesized PE. The presence of Ca2+ in the dialytic medium was important for the preservation of base-exchange activity, probably affecting the reassembly of membrane components.

Turnover of palmitic and arachidonic acids in the phospholipids from different brain areas of adult and aged rats

[3H]Palmitic acid and [14C]arachidonic acid were injected together into the cerebral ventricle of 4-month and 24-month-old rats. At different time intervals from the injection, the distribution of these fatty acids in the lipids from different brain areas was examined. The fatty acids were rapidly incorporated into the lipids through different mechanisms. The time-specific activity relationship indicate that the utilization of the fatty acid differs according to the different areas and aging decreases the utilization of both the fatty acids. The decline of arachidonic acid incorporation into phospholipids is particularly evident, indicating that aging affects mainly the utilization of polyunsaturated fatty acids.

Ethanolamine base-exchange reaction in rat brain microsomal subfractions

Crude microsomal fractions have been subfractionated by differential ultracentrifugation into subfractions A, B, and C, corresponding to light smooth, heavy smooth, and rough microsomal membranes, respectively. The purity and the vesiculation of the membranes were checked biochemically. Subfraction C showed the highest ethanolamine base-exchange activity, both on phospholipid and protein bases. The other two subfractions had roughly similar activities. The kinetic behavior of the enzyme activity, although anomalous, was similar in the three subfractions. Treatment of the vesicles with Pronase or with mercury-dextran produced inactivation of the ethanolamine base-exchange reaction in the three subfractions. These findings suggest that the active site of base-exchange activity would be localized on the external leaflet of the vesicles. Treatment of the membranes with trinitrobenzenesulfonic acid (TNBS) has shown that the newly synthesized phosphatidylethanolamine (PE) belongs to a pool easily reacting with the probe, independent of the subfraction investigated. On the other hand, the distribution of the bulk membrane PE reacting with TNBS differs in the three subfractions examined. It is concluded that the newly synthesized PE and probably the active site of the enzyme are on the external leaflet of the membrane in all subfractions and that the ethanolamine base-exchange reaction has similar properties in all subfractions.

Evidence for a different metabolic behaviour of cytidine diphosphate choline after oral and intravenous administration to rats

Radioactivity plasma decay was studied in rats after intravenous and oral administration of cytidine diphosphate [methyl-14C]choline at doses of 25 and 300 mg/kg. The kinetics fitted well with a two compartment open model and showed a long lasting elimination phase with a half-life ranging from 2.0 to 2.6 days for the two doses and the two administration routes. Absorption of cytidine diphosphate choline radioactivity was complete after oral treatment with the low dose and accounted for 94.5% of the dose when 300 mg/kg of cytidine diphosphate [methyl-14C]choline were administered. However the distribution of radioactivity in tissues, urine and expired air suggest metabolic differences, at least from a quantitative point of view, between the oral and intravenous treatments. In particular, the higher excretion of radioactivity associated with trimethylamine in urine found when cytidine diphosphate [methyl-14C]choline was given orally, suggest that the compound may be metabolized, at least in part, previous to its gastrointestinal absorption.

Effect of various drugs producing convulsive seizures on rat brain glycerolipid metabolism

Convulsive seizures were elicited in the rat by the injection of several different drugs (pyridoxal phosphate, bicuculline, penicillin and ouabain). Glycerolipid metabolism was studied after the intraventricular injection of [2-3H]glycerol, which was incorporated into rat brain glycerides. The percentage of total lipid label found in each lipid class (phosphatidylethanolamine, PE; phosphatidylcholine, PC; phosphatidylserine, PS; phosphatidic acid, PA; phosphatidylinositol, PI; diacylglycerol (+ monoacylglycerol), DG and triacylglycerol, TG) depended on the time elapsed from the injection of the labeled precursor. The percent of total lipid radioactivity as PE and PC increased with time (3-60 min), whereas the opposite was true for the radioactivity of DG and PA. The radioactivity of other lipid classes did not appreciably vary between 3 and 60 min from the injection of the labeled glycerol. The intraventricular administration of pyridoxal phosphate together with labeled glycerol decreased the percent of lipid radioactivity as PE and increased that as DG. This 'lipid effect' was detected also after the administration of other convulsants, such as ouabain and penicillin. The intraperitoneal administration of bicuculline affected lipid metabolism in cerebellum.

Turnover of ethanolamine phosphoglycerides in different brain areas of adult and aged rats

The turnover of ethanolamine glycerophospholipids (EGP) has been determined in six different cerebral areas of 4-month and 22-month-old rats, by injecting [3H]glycerol together with [14C]ethanolamine into the lateral ventricle of the brain. The areas examined behave quite differently in respect to their utilization of the most simple precursors of phosphoglyceride biosynthesis. The incorporation of both glycerol and ethanolamine is already complete as early as 2-4 hours and then reutilization begins, at least for the so called fast pools of phosphoglycerides. The different slopes of the specific activity of the two precursors in EGP suggest a high degree of variance among catabolic rates in the different brain regions. In aged rats the utilization of the water-soluble precursors of EGP synthesis decreases in all brain areas and these data suggest that aging may have a different effect on the catabolic activities as well as phospholipid biosynthesis.

The effect of pyridoxal phosphate-induced convulsive seizures on rat brain phospholipid metabolism

The intraventricular injection of pyridoxal phosphate (PLP; 1 mumole/brain) to rats causes convulsive seizures beginning 3 min after injection and lasting for about 20 min. The incorporation of [2-3H] glycerol into rat brain glycerides has been studied to ascertain whether treatment with PLP affects the incorporation of label into various lipid classes. The labeling pattern of glycerides is changed by the administration of PLP. The observed alterations begin a few min after injection, together with the convulsive seizures. 1 h after the injection the pattern of labeling of brain glycerides returns to normal. Different glycerides are differently affected by PLP. This work demonstrates that the labeling of diglyceride increases whereas that of phosphatidylethanolamine decreases following PLP administration.

The effect of cytidine-diphosphate choline (CDP-choline) on brain lipid changes during aging

Lipid synthesis has been tested in vivo in different brain areas of 12-month-old male rats. Cortex, striatum, brainstem, and subcortex of brain have been examined. The cerebellum was discarded. Mixtures of (2-3H)glycerol and (Me-14C)choline were injected into the lateral ventricle of the brain as lipid precursors, and their incorporation into total lipid, water-soluble intermediates and choline-containing phospholipids was examined 1 hr after isotope injection. In another series of experiments cytidine-5'-diphosphate choline (CDP-choline) was injected intraventricularly to the aged rats 10 min before sacrifice with a simultaneous injection, and radioactivity assays were performed as above. Distribution of radioactivity content of CDP-choline among brain areas 10 min after its administration showed a noticeable enrichment of the nucleotide and water-soluble-related compounds in the examined areas, but to a lesser degree in the cerebral cortex. The incorporation of labelled glycerol, which is severely depressed in aged rats in all four areas [Gaiti et al, 1982, 1983], was increased only in the cortex, and apparently decreased in the other areas. This last result is probably due to a dilution effect brought about by the administered cold CDP-choline upon the (14C)-containing water-soluble metabolites. As a consequence, the (3H)/(14C) ratio in total lipid and in isolated phosphatidylcholine and choline plasmalogen increased after CDP-choline treatment.

Compartmentation of membrane phosphatidylethanolamine formed by base-exchange reaction in rat brain microsomes

The compartmentation of membrane phosphatidylethanolamine (PE) formed by base-exchange reaction in rat brain microsomal vesicles has been investigated. After labelling membrane PE by base-exchange in vitro, microsomal vesicles were treated with trinitrobenzenesulfonic acid (TNBS). The amount of membrane PE reacting with TNBS depends on the duration and the temperature of the reaction as well as on the TNBS concentration. It was found that almost all of the labelled PE molecules, but only about 24% of membrane PE, were accessible to TNBS, under very mild reaction conditions. It is concluded that PE labelled by base-exchange is completely localized in the cytoplasmic leaflet of microsomal vesicles.

Cholinephosphotransferase activity in human platelets

Disrupted human platelets possess a cholinephosphotransferase activity (EC 2.7.8.2) whose properties have been studied in this work. The labeling of choline glycerophospholipid (CGP) from radioactive cytidine-5'-diphosphate choline (CDP-choline) in vitro shows a maximum at pH 8.0 (using Hepes [4-(2-hydroxyethyl)-piperazine-1-ethane-2-sulfonic acid] as a buffer) and is stimulated by Mn2+, Mg2+ and diacylglycerol. The enzymic activity is inhibited by Ca2+. The dependence of human platelet cholinephosphotransferase upon CDP-choline concentration does not follow the Michaelis-Menten equation. CMP strongly inhibits the reaction. The functional implications of this newly discovered platelet activity are briefly considered.

The transport of cytidine into rat brain in vivo, and its conversion into cytidine metabolites

Double-labeled cytidine, with a 3H/14C isotope ratio of 20.00, has been intraventricularly injected into the brain of young rats, and its fate followed up to 90 min from administration together time-course of labeling. The injected nucleoside enters the brain as an intact molecule and is immediately utilized without prior degradation. Cytidine is actively converted into uridine and CMP, the latter being then transformed by a stepwise mechanism into CDP and CTP, and finally into CDP-choline and CDP-ethanolamine. The results indicate that administered cytidine represents a compound likely to enter metabolic events, which lead to CDP-choline and CDP-ethanolamine synthesis, and presumably to phospholipid production.

The synthesis in vivo of choline and ethanolamine phosphoglycerides in different brain areas during aging

The biosynthesis of choline and ethanolamine phosphoglycerides was tested in vivo in different brain areas of the rat during aging. Mixtures of [2-3H] glycerol and [Me-14C] choline or [2-3H] glycerol and [2-14C] ethanolamine were injected into lateral ventricle of the brain as lipid precursors and their incorporation into corresponding phospholipid was examined. A significant decrease of synthesis of both phosphoglycerides takes place in cerebral cortex and in the striatum, and is already apparent at 9 months of age with no further decrease or change thereafter. No significant change takes place in the cerebellum. The unchanged absorption of injected water-soluble precursors, together with the lack of any significant change of phospholipid/protein ratio in all examined brain areas, suggests that the incorporation of both glycerol and nitrogen bases are affected by aging.

The reverse reaction of cholinephosphotransferase in rat brain microsomes. A new pathway for degradation of phosphatidylcholine

The synthesis of phosphatidylcholine is catalyzed by cholinephosphotransferase (EC 2.7.8.2) which is known to be reversible in liver. The reversibility of cholinephosphotransferase in rat brain in demonstrated in this paper. Labeled microsomes were prepared from young rats which had been given an intracerebral injection of labeled choline or oleate 2 h before killing. During incubation of choline-labeled microsomes with CMP, label was lost from ;choline glycerophospholipids and labeled CDPcholine was produced. The Km for CMP was 0.35 mM and V was 3.3 nmol/min per mg protein. Neither AMP nor UMP could substitute for CMP. Oleate-labeled microsomes were pretreated with e mM diisopropylfluorophosphate (lipase inhibitor). During incubation with CMP, label was lost from choline, and ethanolamine glycerophospholipid and labeled diacylglycerols were produced. When the lipase was not inhibited, labeled oleate was produced. We propose that a principal pathway for degradation of phosphatidylcholine, particularly during brain ischemia, is by reversal of cholinephosphotransferase, followed by hydrolysis of diacylglycerols by the lipase.

Phospholipid metabolism in neuronal and glial cells during aging

The incorporation of cytidine-containing precursors (CDP-Cho and CDP-Etn) into the main phospholipid classes of cellular fractions enriched in neurons and glial cells from whole rat brains of different ages was examined. The rate of synthesis of choline phosphoglycerides in neuronal homogenates significantly decreased with age up to 18 months; after this time no additional decrease was found. The decrease of CDP-Etn incorporation in neurons was found to be less significantly affected by age up to 18 months, but the enzymic activity decreased after 18 months of age. No changes were found in the corresponding glial activity at any age. Biochemical phenomena that occur in 18-month-old rat brain (aged animals) were compared with phenomena occurring in 2-month-old rat brain (adult animals). No significant variations of lipid composition were found in neurons from either 18-month-old or 2-month-old rat brain. These results, together with some kinetic parameters, suggest that ethanolamine and choline phosphotransferases are affected differently by aging.

Synthesis of phosphatidylcholine and phosphatidylethanolamine at different ages in the rat brain in vitro

The de novo synthesis of choline and ethanolamine phosphoglycerides in brain microsomes from 18 month-old male rats was investigated in vitro by using labeled cytidine-5'-diphosphate choline and cytidine-5'-diphosphate ethanolamine as lipid precursors. The rate of synthesis of the two phospholipid classes was found to be noticeably decreased, as compared to that of adult animals. The addition of exogenous diacyl glycerols to microsomes from ageing rat brain brings the rate of synthesis nearly to the adult levels. The synthesis of choline and ethanolamine phosphoglycerides is not affected in the liver microsomes of ageing rats. The molar distribution of fatty acids in brain microsomal diacyl glycerols of ageing rats is noticeably different from that of adult animals. The content of monoenoic and dienoic species is increased, whereas that of the tetraenoic species is decreased. Base exchange reaction for choline and ethanolamine incorporation into respective phospholipids is not affected in the brain microsomes of the aged rats.

Biosynthesis of rat brain phosphatidylethanolamines from intracerebrally injected ethanolamine

[2-3H]Ethanolamine was injected intracerebrally into male rats and the brains of the animals immediately removed by particular procedures at regular intervals over the first 1200 sec. The incorporation of radioactivity into brain phosphorylethanolamine, cytidine-5'-diphosphate (CDP) ethanolamine and phosphatidylethanolamines was examined and quantitated. The nature of phosphatidylethanolamine molecular subspecies, which became labelled, was also investigated after isotope administration. Phosphorylethanolamine, CDP-ethanolamine and phosphatidylethanolamines were all labelled already 5 sec after the administration of labelled ethanolamine. The specific radioactivities of different phosphatidylethanolamine molecular subspecies varied according to the time elapsed from the injection to the sacrifice of the animals. This last result, together with the data on time course of labelling of ethanolamine phosphoglycerides and their precursors, provides indications that this base may be incorporated into lipids not only by net synthesis pathway, but also by base-exchange reaction.