Thyroid hormone and dehydroepiandrosterone permit gluconeogenic hormone responses in hepatocytes

The importance of the sn-glycerol- 3-phosphate (G-3-P) electron transfer shuttle in hormonal regulation of gluconeogenesis was examined in hepatocytes from rats with decreased mitochondrial G-3-P dehydrogenase activity (thyroidectomized) or increased G-3-P dehydrogenase activity [triiodothyronine (T(3)) or dehydroepiandrosterone (DHEA) treated]. Rates of glucose formation from 10 mM lactate, 10 mM pyruvate, or 2.5 mM dihydroxyacetone were somewhat less in hypothyroid cells than in cells from normal rats but gluconeogenic responses to calcium addition and to norepinephrine (NE), glucagon (G), or vasopressin (VP) were similar to the responses observed in cells from normal rats. However, with 2. 5 mM glycerol or 2.5 mM sorbitol, substrates that must be oxidized in the cytosol before conversion to glucose, basal gluconeogenesis was not appreciably altered by hypothyroidism but responses to calcium and to the calcium-mobilizing hormones were abolished. Injecting thyroidectomized rats with T(3) 2 days before preparing the hepatocytes greatly enhanced gluconeogenesis from glyc erol and restored the response to Ca(2+) and gluconeogenic hormones. Feeding dehydroepiandrosterone for 6 days depressed gluconeogenesis from lactate or pyruvate but substantially increased glucose production from glycerol in euthyroid cells and restored responses to Ca(2+) in hypothyroid cells metabolizing glycerol. Euthyroid cells metabolizing glycerol or sorbitol use the G-3-P and malate/aspartate shuttles to oxidize excess NADH generated in the cytosol. The transaminase inhibitor aminooxyacetate (AOA) decreased gluconeogenesis from glycerol 40%, but had little effect on responses to Ca(2+) and NE. However, in hypothyroid cells, with minimal G-3-P dehydrogenase, AOA decreased gluconeogenesis from glycerol more than 90%. Thus, the basal rate of gluconeogenesis from glycerol in the euthyroid cells is only partly dependent on electron transport from cytosol to mitochondria via the malate/aspartate shuttle and almost completely dependent in the hypothyroid state, and the hormone enhancement of the rate in euthyroid cells involves primarily the G-3-P cycle. These data are consistent with Ca(2+) being mobilized by gluconeogenic hormones and G-3-P dehydrogenase being activated by Ca(2+) so as to permit it to transfer reducing equivalents from the cytosol to the mitochondria.     

Involvement of a glycerol-3-phosphate dehydrogenase in modulating the NADH/NAD+ ratio provides evidence of a mitochondrial glycerol-3-phosphate shuttle in Arabidopsis

A mitochondrial glycerol-3-phosphate (G-3-P) shuttle that channels cytosolic reducing equivalent to mitochondria for respiration through oxidoreduction of G-3-P has been extensively studied in yeast and animal systems. Here, we report evidence for the operation of such a shuttle in Arabidopsis thaliana. We studied Arabidopsis mutants defective in a cytosolic G-3-P dehydrogenase, GPDHc1, which, based on models described for other systems, functions as the cytosolic component of a G-3-P shuttle. We found that the gpdhc1 T-DNA insertional mutants exhibited increased NADH/NAD+ ratios compared with wild-type plants under standard growth conditions, as well as impaired adjustment of NADH/NAD+ ratios under stress simulated by abscisic acid treatment. The altered redox state of the NAD(H) pool was correlated with shifts in the profiles of metabolites concerning intracellular redox exchange. The impairment in maintaining cellular redox homeostasis was manifest by a higher steady state level of reactive oxygen species under standard growth conditions and by a significantly augmented hydrogen peroxide production under stress. Loss of GPDHc1 affected mitochondrial respiration, particularly through a diminished capacity of the alternative oxidase respiration pathway. We propose a model that outlines potential involvements of a mitochondrial G-3-P shuttle in plant cells for redox homeostasis.     

Identification of a mitochondrial glycerol-3-phosphate dehydrogenase from Arabidopsis thaliana: evidence for a mitochondrial glycerol-3-phosphate shuttle in plants

We report molecular characterization of an Arabidopsis gene encoding a mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase (FAD-GPDH) that oxidizes glycerol-3-phosphate (G-3-P) to dihydroxyacetone phosphate. We demonstrate through in vitro targeting assays that the encoded gene product can be imported into mitochondrial membrane systems. Enzyme activity of the protein was confirmed through heterologous expression in Escherichia coli. The Arabidopsis gene is expressed throughout plant development, but at the highest level during seed germination. We also show that expression of the Arabidopsis FAD-GPDH gene is coupled to oxygen consumption and affected by ABA and stress conditions. Together with an NAD(+)-dependent GPDH, this enzyme could form a G-3-P shuttle, as previously established in other eukaryotic organisms, and links cytosolic G-3-P metabolism to carbon source utilization and energy metabolism in plants.     

Glyceraldehyde-3-phosphate, a glycolytic intermediate, prevents cells from apoptosis by lowering S-nitrosylation of glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate (G-3-P), the substrate of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is a key intermediate in several metabolic pathways. Recently, we reported that G-3-P directly inhibits caspase-3 activity in a reversible noncompetitive mode, suggesting the intracellular G-3-P level as a cell fate decision factor. It has been known that apoptotic stimuli induce the generation of NO, and NO S-nitrosylates GAPDH at the catalytic cysteine residue, which confers GAPDH the ability to bind to Siah-1, an E3 ubiquitin ligase. The GAPDH-Siah-1 complex is translocated into the nucleus and subsequently triggers the apoptotic process. Here, we clearly showed that intracellular G-3-P protects GAPDH from S-nitrosylation at above a certain level, and consequently maintains the cell survival. In case G-3-P drops below a certain level as a result of exposure to specific stimuli, G-3-P cannot inhibit S-nitrosylation of GAPDH anymore, and consequently GAPDH translocates with Siah-1 into the nucleus. Based on these results, we suggest that G-3-P functions as a molecule switch between cell survival and apoptosis by regulating S-nitrosylation of GAPDH.     

Comparative changes in the lipogenic enzyme activities and in the in vivo fatty acid synthesis in liver and adipose tissues during the post-weaning growth of male rats

Changes in the specific activities of acetyl-CoA-carboxylase (ACX), malic enzyme (ME) and glucose-6-phosphate dehydrogenase (G-6-PD) were compared to changes in de novo lipogenesis measured by in vivo incorporation of [3H] of tritiated water into fatty acids of liver and of perirenal and dorsal subcutaneous adipose tissues. In the adipose tissues, the specific activities of the three enzymes rather closely followed fluctuations in the rate of fatty acid synthesis. In the liver, ACX and especially ME activities were satisfactory indicators of de novo lipogenesis; G-6-PD activity did not depend on de novo lipogenesis.     

Glycolytic enzymes in the premolt field crab Paratelphusa hydrodromus (Milne-Edwards) (Crustacea)

The quantitative assay of hexokinase (HK), phosphorylase, phosphofructokinase (PFK), glucose 6-phosphate dehydrogenase (G-6-PDH), glycerol 3-phosphate dehydrogenase (G-3 PDH) and lactate dehydrogenase (LDH) revealed that coxal muscles compared to hepatopancreas contained higher activities of all the enzymes investigated. It appears that the coxal muscles of the premolt field crab has carbohydrate-based fuel economy. The hepatopancreas is a rich source of lipid and very poor source of glycogen. The activity of G-6-PDH is moderately high in the hepatopancreas. It seems that in this lipogenic tissue conversion of G-6-P to triose phosphate occurs predominately via pentose-phosphate pathway thus generating NADPH for lipogenesis. The relative G-3PDH ad LDH activities in hepatopancreas and coxal muscles led us to believe that the reconversion of NAD from NADH in hepatopancreas nd muscle flexor is effected by glycerol 3-phosphate shuttle, whereas in muscle extensor it is achieved by both G-3PDH and LDH activities.     

Differences between glycogen biogenesis in fast- and slow-twitch rabbit muscle

Skeletal muscle glycogen is an essential energy substrate for muscular activity. The biochemical properties of the enzymes involved in de novo synthesis of glycogen were analysed in two types of rabbit skeletal muscle fiber (fast- and slow-twitch). Glycogen concentration was higher in fast-twitch muscle than in slow-twitch muscle, but the latter contained many more small intermediate-acceptor molecules that could act as glycogen synthase substrates. The enzymes involved in de novo synthesis of glycogen in fast-twitch muscle were strongly stimulated by Glc-6-P, but those in slow-twitch muscle were not.     

Effects of glucocorticoid and thyroid hormones on regulatory enzymes of fatty acid synthesis and glycogen metabolism in developing fetal rat lung

Although glucocorticoid and thyroid hormones are known to act synergistically to stimulate surfactant production, they have opposite effects on other parameters of fetal lung maturation. We recently reported that the developmental increases in de novo fatty acid synthesis and glycogen accumulation in fetal rat lung were accelerated by dexamethasone but prevented by triiodothyronine and that the dexamethasone-induced increases were diminished when the two hormones were administered together. We have now examined the effects of maternal administration of these hormones on activities of enzymes of lung fatty acid synthesis and glycogen metabolism in the rat. There was a developmental increase in fatty-acid synthase activity between 19 and 21 days gestation. This activity was increased by dexamethasone but decreased by triiodothyronine. When the two hormones were administered together the stimulatory effect of dexamethasone was decreased from 56% to 29%. The stimulatory effect on fatty-acid synthase was also observed in fetal lung explants cultured in the presence of dexamethasone. This shows that the effect of the hormone was directly on the fetal lung. Dexamethasone had no effect on liver fatty-acid synthase. There was a developmental decrease in acetyl-CoA carboxylase activity but it was not affected by the hormones. These data show that the developmental and hormone-induced changes in fetal lung de novo fatty acid synthesis are mediated by fatty-acid synthase. Although there were developmental changes in fetal lung 6-phosphofructokinase, glycogen synthase and glycogen phosphorylase activities, these enzymes were not affected by the hormones.     

The effect of ribose 5-phosphate and 5-phosphoribosyl-1-pyrophosphate availability on de novo synthesis of purine nucleotides in rat liver slices.

The effect of increasing cellular ribose 5-phosphate (ribose-5-P) availability by methylene blue-induced acceleration of the oxidative pentose phosphate pathway on the rate of 5-phosphoribosyl-1-pyrophosphate (P-ribose-PP) generation, was studied in slices of rat liver at varying Pi concentration. It was found that at Pi concentration prevailing in the tissue of extracellular physiological Pi concentration, ribose-5-P availability is saturating for P-ribose-PP generation, as gauged by the rate of adenine incorporation into tissue nucleotides. The effect of altering P-ribose-PP availability on the rate of de novo purine production gauged by the rate of formate incorporation into purines, was also studied. It was found that the physiological P-ribose-PP concentration in rat liver tissue is limiting for purine synthesis de novo. Depletion of cellular P-ribose-PP, achieved by increase of P-ribose-PP consumption, decelerated purine synthesis, while increase of P-ribose-PP availability, achieved by activation of P-ribose-PP synthetase occurring at elevated Pi concentration, resulted in acceleration of purine synthesis.     

Time-course of hormonal induction of mitochondrial glycerophosphate dehydrogenase biogenesis in rat liver

Thyroid hormones are important regulators of mitochondrial metabolism. Due to their complex mechanism of action, the timescale of different responses varies from minutes to days. In this work, we studied selective T3 induction of the inner mitochondrial membrane enzyme-glycerophosphate dehydrogenase (mGPDH) in liver of euthyroid rats. We correlated the kinetics of the T3 level in blood, the mRNA level in liver, the activity and amount of mGPDH in liver mitochondria after a single dose of T3. The T3 level reached maximum after 1 h (80 nmol/l) and subsequently rapidly decreased. mGPDH mRNA increased also relatively fast, reaching a maximum after 12 h and fell to the control level after 72 h. An increase of mGPDH activity could be already found after 6 h and reached a maximum after 24 h in accordance with the increase in mGPDH content (2.4-fold vs. 2.7-fold induction). After 72 h, the mGPDH activity showed a significant 30% decrease. When the rats received three subsequent doses of T3, the increase of mGPDH activity was 2-fold higher than after a single T3 dose. The results demonstrate that mGPDH displays rapid induction as well as decay upon disappearance of a hormonal stimulus, indicating a rather short half-life of this inner mitochondrial membrane enzyme.     

Effect of adrenaline, hydrocortisone, insulin and dibutyryl-cAMP on glycolysis and glycogenolysis in white rat liver slices]

Epinephrine, hydrocortisone, and dibutyril cAMP inhibited glycolysis and glucogenolysis. The inhibitory effect was also found when glucose-6-phosphate (G-6-P) was used as a glycolysis substrate, but not for fructose-1,6-diphosphate. This is the evidence of hexokinase activity inhibition by hormones and dibutyril cAMP, and presumably of phospholylase and phosphofructokinase as well. In the simulated cell-free system the hormones produced no effect, dibutyril cAMP inhibiting hexokinase alone. For the realization of hormones effect their interaction with the cell membrane is required. Inhibition of glycogen and G-6-P decomposition to lactic acid in the rat liver slices was not associated with the hormone action on phosphorylase and phosphofructokinase through cAMP and proteinkinase directly. The results obtained indicated the existence of a supplementary mechanism that modified cAMP effect on the activity of the said enzymes. Insulin was effective in any of the cases.     

Control of glycogen content in retina: allosteric regulation of glycogen synthase

Retinal tissue is exceptional because it shows a high level of energy metabolism. Glycogen content represents the only energy reserve in retina, but its levels are limited. Therefore, elucidation of the mechanisms controlling glycogen content in retina will allow us to understand retina response under local energy demands that can occur under normal and pathological conditions. Thus, we studied retina glycogen levels under different experimental conditions and correlated them with glucose-6-phosphate (G-6-P) content and glycogen synthase (GS) activity. Glycogen and G-6-P content were studied in ex vivo retinas from normal, fasted, streptozotocin-treated, and insulin-induced hypoglycemic rats. Expression levels of GS and its phosphorylated form were also analyzed. Ex vivo retina from normal rats showed low G-6-P (14±2 pmol/mg protein) and glycogen levels (43±3 nmol glycosyl residues/mg protein), which were increased 6 and 3 times, respectively, in streptozotocin diabetic rats. While no changes in phosphorylated GS levels were observed in any condition tested, a positive correlation was found between G-6-P levels with GS activity and glycogen content. The results indicated that in vivo, retina glycogen may act as an immediately accessible energy reserve and that its content was controlled primarily by G-6-P allosteric activation of GS. Therefore, under hypoglycemic situations retina energy supply is strongly compromised and could lead to the alterations observed in type 1 diabetes.     

Effect of hibernation, thyroid hormones and dexamethasone on cytosolic and mitochondrial glycerol-3-phosphate dehydrogenase from jerboa (Jaculus orientalis)

Tissue distribution of the cytosolic and mitochondrial glycerol-3-phosphate dehydrogenase (cGPDH and mGPDH) activities in jerboa (Jaculus orientalis), a hibernator, shows the highest level of enzyme activity in skeletal muscle and brown adipose tissue, respectively. The effect of hibernation on cGPDH indicates an increase of activity in all tissues examined. In contrast, hibernation decreases mGPDH activity in all tissues, except skeletal muscle. The effect of thyroid hormones on GPDH activity was tissue specific: in kidneys, cGPDH activity doubled in euthermic jerboas treated with T4. In contrast, 6-n-propyl-2-thiouracil treatment provokes an increase of enzyme activity in brown adipose tissue, liver and brain. T4 treatment leads to a 2.7-fold increase in liver mGPDH activity. 6-n-propyl-2-thiouracil treatment decreases mGPDH activity in the skeletal muscle whereas the opposite effect was observed in brain. Dexamethasone stimulates cGPDH in all tissues examined, except skeletal muscle and kidneys. In the case of mGPDH activity, this increase was observed only for brown adipose tissue and brain. Our results suggest that hibernation, thyroid hormones and dexamethasone probably play a role in the regulation of cGPDH and mGPDH activities in jerboa. Our findings confirm that these enzymes are involved in metabolic adaptation to thermal stress in Jaculus orientalis.     

Starch and fatty acid synthesis in plastids from developing embryos of oilseed rape (Brassica napus L.)

The aim of this work was to investigate the capacity for synthesis of starch and fatty acids from exogenous metabolites by plastids from developing embryos of oilseed rape (Brassica napus L.). A method was developed for the rapid isolation from developing embryos of intact plastids with low contamination by cytosolic enzymes. The plastids contain a complete glycolytic pathway, NADP-glucose-6-phosphate dehydrogenase, NADP-6-phosphogluconate dehydrogenase, fructose-1,6-bisphosphatase, NADP-malic enzyme, the pyruvate dehydrogenase complex (PDC), and acetyl-CoA carboxylase. Organelle fractionation studies showed that 67% of the total cellular PDC activity was in the plastids. The isolated plastids were fed with ¹⁴C-labelled carbon precursors and the incorporation of ¹⁴C into starch and fatty acids was determined. ¹⁴C from glucose-6-phosphate (G-6-P), fructose, glucose, fructose-6-phosphate and dihydroxyacetone phosphate (DHAP) was incorporated into starch in an intactness- and ATP-dependent manner. The rate of starch synthesis was highest from G-6-P, although fructose gave rates which were 70% of those from G-6-P. Glucose-1-phosphate was not utilized by intact plastids for starch synthesis. The plastids utilized pyruvate, G-6-P, DHAP, malate and acetate as substrates for fatty acid synthesis. Of these substrates, pyruvate and G-6-P supported the highest rates of synthesis. These studies show that several cytosolic metabolites may contribute to starch and/or fatty acid synthesis in the developing embryos of oilseed rape.     

Effects of NaCl salinity in vivo and in vitro on ribonuclease activity of Vigna unguiculata cotyledons during germination

Pitiuba bean [ Vigna unguiculata (L.) Walp.] seeds were sown in water or. in 0.1 M NaCl. Seedling growth and cotyledon nucleic acid mobilization were delayed by NaCl salinity. The differences in cotyledonary RNase activity between seeds sown in water and in NaCl solutions suggest that salinity delays the activation and/or de novo synthesis of the enzyme. Cotyledon extracts were subjected to gel filtration through Sephadex G-100, and RNase activity measured. Only one cotyledonary RNase appeared during germination, and salinity did not induce any change in molecular weight of the enzyme. Salinity inhibited 45% of the specific activity of the RNase on the 5th day of the experimental period. The same salt concentration (0.1 M NaCl) added in vitro inhibited only 8 % of the specific activity of the enzyme. This difference may indicate that NaCl in vivo affects mainly the de novo synthesis of the RNase.     

Substrate-Induced Stability of Glyceraldehyde 3-Phosphate Dehydrogenase from Mung Beans (Vigna radiata L.)

Time-dependent thermal inactivation of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) present in the extract of mung beans at different periods of germination showed biphasic kinetics in the 12-h germinated seeds but single exponential decay at 24 h of germination. The glyceraldehyde 3-phosphate (G-3-P) concentration in the deproteinated extracts was found to increase with period of germination up to 36 h, parallel to that of GAPDH activity. G-3-P was found to offer protection of the enzyme against thermal inactivation and trypsin digestion. It is suggested that accumulation of G-3-P in germinating mung beans may be of physiological significance and it might offer protection to the enzyme in vivo against thermal inactivation and proteolysis.     

The initial step of the glycerolipid pathway: identification of glycerol 3-phosphate/dihydroxyacetone phosphate dual substrate acyltransferases in Saccharomyces cerevisiae.

The initial step of phospholipid biosynthesis in yeast is carried out through the acylation of glycerol 3-phosphate (G-3-P) and dihydroxyacetone phosphate by stereospecific sn-1 acyltransferases. Here we report the identification of two key fatty acyltransferases of the glycerolipid biosynthesis pathway in Saccharomyces cerevisiae. Disruption of the open reading frame YBL011w, corresponding to a gene previously identified as a choline transporter suppressor (SCT1), resulted in a substantial decrease of total cellular G-3-P acyltransferase activity. A yeast strain disrupted at the open reading frame YKR067w, which encodes a protein closely related to Sct1p, also exhibited a dramatic reduction in G-3-P acyltransferase activity. Molecular characterizations of the genes revealed that a missense mutation in YKR067w accounted for a defect in the activities of the G-3-P acyltransferase in the yeast mutant strain TTA1. Heterologous expression of YKR067w in Escherichia coli further confirmed its enzyme activity. These results indicate that YKR067w and YBL011w, designated herein as GAT1 and GAT2(SCT1), respectively, are yeast G-3-P acyltransferase genes. Furthermore, biochemical results are presented to show that both Gat1p and Gat2p(Sct1p) are G-3-P/dihydroxyacetone phosphate dual substrate-specific sn-1 acyltransferases. The fatty acyl specificity of Gat1p is similar to that of the mammalian microsomal G-3-P acyltransferase, as it can effectively utilize a broad range of fatty acids as acyl donors. In contrast, Gat2p(Sct1p) displayed preference toward 16-carbon fatty acids. The most notable of the altered phospholipid compositions of the gat1Delta and gat2(sct1)Delta strains are a decreased phosphatidic acid pool and an increased phosphatidylserine/phosphatidylinositol ratio. This did not appear to affect the mutants as no growth defect was found. However, null mutations of both GAT1 and GAT2(SCT1) are synthetically lethal to yeast.     

Feedback inhibition of amidophosphoribosyltransferase regulates the rate of cell growth via purine nucleotide, DNA, and protein syntheses

To clarify the contributions of amidophosphoribosyltransferase (ATase) and its feedback regulation to the rates of purine de novo synthesis, DNA synthesis, protein synthesis, and cell growth, mutated human ATase (mhATase) resistant to feedback inhibition by purine ribonucleotides was engineered by site-directed mutagenesis and expressed in CHO ade (-)A cells (an ATase-deficient cell line of Chinese hamster ovary fibroblasts) and in transgenic mice (mhATase-Tg mice). In Chinese hamster ovary transfectants with mhATase, the following parameters were examined: ATase activity and its subunit structure, the metabolic rates of de novo and salvage pathways, DNA and protein synthesis rates, and the rate of cell growth. In mhATase-Tg mice, ATase activity in the liver and spleen, the metabolic rate of the de novo pathway in the liver, serum uric acid concentration, urinary excretion of purine derivatives, and T lymphocyte proliferation by phytohemagglutinin were examined. We concluded the following. 1) ATase and its feedback inhibition regulate not only the rate of purine de novo synthesis but also DNA and protein synthesis rates and the rate of cell growth in cultured fibroblasts. 2) Suppression of the de novo pathway by the salvage pathway is mainly due to the feedback inhibition of ATase by purine ribonucleotides produced via the salvage pathway, whereas the suppression of the salvage pathway by the de novo pathway is due to consumption of 5-phosphoribosyl 1-pyrophosphate by the de novo pathway. 3) The feedback inhibition of ATase is more important for the regulation of the de novo pathway than that of 5-phosphoribosyl 1-pyrophosphate synthetase. 4) ATase superactivity leads to hyperuricemia and an increased bromodeoxyuridine incorporation in T lymphocytes stimulated by phytohemagglutinin.     

The Role of Phosphatidylserine Decarboxylase in Brain Phospholipid Metabolism

In brain, phosphatidylethanolamine can be synthesized from free ethanolamine either by a pathway involving the formation of CDP-ethanolamine and its transfer to diglyceride, or by base-exchange of ethanolamine with existing phospholipids. Although de novo synthesis from serine has also been demonstrated, the metabolic pathway involved is not known. The enzyme phosphatidylserine decarboxylase appears to be involved in the synthesis of much of the phosphatidylethanolamine in liver, but the significance of this route in brain has been challenged. Our in vitro studies demonstrate the existence of phosphatidylserine decarboxylase activity in rat brain and characterize some of its properties. This enzyme is localized in the mitochondrial fraction, whereas the enzymes involved in base-exchange and the cytidine pathway are localized to microsomal membranes. Parallel in vivo studies showed that after the intracranial injection of L-[G-3H]serine, the specific activity of phosphatidylserine was greater in the microsomal fractions than in the mitochondrial fraction, whereas the opposite was true for phosphatidylethanolamine. When L-[U-14C]serine and [1-3H]ethanolamine were simultaneously injected, the 14C/3H ratio in mitochondrial phosphatidylethanolamine was 10 times that in microsomal phosphatidylethanolamine. The results demonstrate that serine is incorporated into the base moiety of phosphatidylethanolamine primarily through the decarboxylation of phosphatidylserine in brain mitochondria. A minimal value of 7% for the contribution of phosphatidylserine decarboxylase to whole-brain phosphatidylethanolamine synthesis can be estimated from the in vivo data.