Serine Utilization Research Articles

Excessive serine from the bone marrow microenvironment impairs megakaryopoiesis and thrombopoiesis in Multiple Myeloma

Thrombocytopenia is a major complication in a subset of patients with multiple myeloma (MM). However, little is known about its development and significance during MM. Here, we show thrombocytopenia is linked to poor prognosis in MM. In addition, we identify serine, which is released from MM cells into the bone marrow microenvironment, as a key metabolic factor that suppresses megakaryopoiesis and thrombopoiesis. The impact of excessive serine on thrombocytopenia is mainly mediated through the suppression of megakaryocyte (MK) differentiation. Extrinsic serine is transported into MKs through SLC38A1 and downregulates SVIL via SAM-mediated tri-methylation of H3K9, ultimately leading to the impairment of megakaryopoiesis. Inhibition of serine utilization or treatment with TPO enhances megakaryopoiesis and thrombopoiesis and suppresses MM progression. Together, we identify serine as a key metabolic regulator of thrombocytopenia, unveil molecular mechanisms governing MM progression, and provide potential therapeutic strategies for treating MM patients by targeting thrombocytopenia.

FK506-binding protein, FKBP12, promotes serine utilization and negatively regulates threonine deaminase in fission yeast.

FK506-binding protein with a molecular weight of 12 kDa (FKBP12) is a receptor of the immunosuppressive drugs, FK506 and rapamycin. The physiological functions of FKBP12 remain ambiguous because of its nonessentiality and multifunctionality. Here, we show that FKBP12 promotes the utilization of serine as a nitrogen source and regulates the isoleucine biosynthetic pathway in fission yeast. In screening for small molecules that inhibit serine assimilation, we found that the growth of fission yeast cells in medium supplemented with serine as the sole nitrogen source, but not in glutamate-supplemented medium, was suppressed by FKBP12 inhibitors. Knockout of FKBP12 phenocopied the action of these compounds in serine-supplemented medium. Metabolome analyses and genetic screens identified the threonine deaminase, Tda1, to be regulated downstream of FKBP12. Genetic and biochemical analyses unveiled the negative regulation of Tda1 by FKBP12. Our findings reveal new roles of FKBP12 in amino acid biosynthesis and nitrogen metabolism homeostasis.

Effects of insufficient serine on health and selenoprotein expression in rats and their offspring.

ObjectiveTo observe the impact of insufficient exogenous and/or endogenous serine on selenoprotein expression and health of pregnant rats and their offspring.MethodExperiment 1 was conducted in male rats, in which the dose-dependent effects of serine on selenoprotein expression and thyroid hormones (T3, T4 and TSH) were investigated by feeding either a serine adequate diet (20C), serine-deprived diet (20CSD) or 20CSD with different serine levels (0.5, 1.0, and 2.0 times the amount of serine in 20C). In experiment 2, a PHGDH inhibitor was administrated to pregnant rats fed either 20C or 20CSD. Blood and organ tissues of pregnant rats and offspring were subjected to the analyses of thyroid hormone, serine and homocysteine and GPx3 and SELENOP in plasma and expression of GPx1 and DIO1, 2 in tissues respectively.ResultIn experiment 1, plasma SELENOP and GPx3 levels in adult male rats increased with the increasing dose of serine. Immunohistochemical results showed that GPx1 expression in liver and kidney of male rats also increased with increasing serine supplementation. Amongst all diet groups, only male rats fed 20CSD had significantly lower plasma TSH and T4 levels (P < 0.05). In experiment 2, GPx1 and DIO2 expression in the liver and kidney were suppressed in pregnant rats administered with a PHGDH compared to those who were not (P < 0.05). There were no significant differences in plasma T4 and T3 amongst all diet groups (P > 0.05). Also, offspring born to pregnant rats administered with a PHGDH inhibitor exhibited slower growth rates and hyperhomocysteinemia compared to offspring from mothers not administered with the inhibitor (P < 0.05). Conclusions: Insufficient exogenous serine through the diet decreased selenoprotein synthesis in adult male rats. However, this was not observed in pregnant rats, whereby exogenous or endogenous serine deficiency had no effect on the selenoprotein levels. A possible explanation is that dams may have an adaptive mechanism to limit maternal serine utilization and ensure adequate supply to the fetus.

Serine synthesis through PHGDH coordinates nucleotide levels by maintaining central carbon metabolism

Phosphoglycerate dehydrogenase (PHGDH) catalyzes the committed step in de novo serine biosynthesis. Paradoxically, PHGDH and serine synthesis are required in the presence of abundant environmental serine even when serine uptake exceeds the requirements for nucleotide synthesis. Here, we establish a mechanism for how PHGDH maintains nucleotide metabolism. We show that inhibition of PHGDH induces alterations in nucleotide metabolism independent of serine utilization. These changes are not attributable to defects in serine-derived nucleotide synthesis and redox maintenance, another key aspect of serine metabolism, but result from disruption of mass balance within central carbon metabolism. Mechanistically, this leads to simultaneous alterations in both the pentose phosphate pathway and the tri-carboxylic acid cycle, as we demonstrate based on a quantitative model. These findings define a mechanism whereby disruption of one metabolic pathway induces toxicity by simultaneously affecting the activity of multiple related pathways.

P011 ADHERENT-INVASIVE ESCHERICHIA COLI UTILIZES SERINE CATABOLISM TO OUTCOMPETE ITS COMMENSAL COMPETITORS

Crohn’s disease (CD) patients are often colonized by adherent-invasive Escherichia coli (AIEC). Although the colonization by AIEC is known to contribute to the pathogenesis of CD, the mechanisms by which AIEC thrives within the inflamed intestine in CD patients remain elusive. To elucidate the mechanisms by which AIEC outcompetes commensal non-pathogenic E. coli (cEC) in the intestine, in vivo competition of AIEC (strain LF82) and cEC (strain MG1655) was performed using co-colonization of the two strains in germ-free mice. Dextran sodium sulfate (DSS) was given to the co-colonized mice to mimic inflammatory environments. To identify the unique metabolic pathways that are utilized by AIEC during inflammation, transcriptomic changes in E. coli strains in the inflamed gut were analyzed by RNAseq. Amino acid (AA) defined diets were applied to bacterial in vivo competition to assess the impact of dietary serine on AIEC bloom. AIEC gained the growth advantage over cEC during DSS colitis. Intestinal inflammation reprogramed bacterial metabolic pathways from carbohydrate metabolism toward amino acid metabolism, particularly serine utilization, in AIEC. An isogenic mutant AIEC strain that lacks serine utilization genes failed to outcompete cEC in the inflamed gut. The deprivation of dietary serine also prevented outgrowth of AIEC in the inflamed but not in the steady-state gut in vivo. AIEC utilizes serine catabolism pathways to gain a growth advantage over cEC in the inflamed gut. Targeting unique metabolic pathways exploited by AIEC may selectively eliminate the pathogenic strains of gut bacteria without influencing beneficial commensal microbiota.

An archaeal ADP-dependent serine kinase involved in cysteine biosynthesis and serine metabolism.

Routes for cysteine biosynthesis are still unknown in many archaea. Here we find that the hyperthermophilic archaeon Thermococcus kodakarensis generates cysteine from serine via O-phosphoserine, in addition to the classical route from 3-phosphoglycerate. The protein responsible for serine phosphorylation is encoded by TK0378, annotated as a chromosome partitioning protein ParB. The TK0378 protein utilizes ADP as the phosphate donor, but in contrast to previously reported ADP-dependent kinases, recognizes a non-sugar substrate. Activity is specific towards free serine, and not observed with threonine, homoserine and serine residues within a peptide. Genetic analyses suggest that TK0378 is involved in serine assimilation and clearly responsible for cysteine biosynthesis from serine. TK0378 homologs, present in Thermococcales and Desulfurococcales, are most likely not ParB proteins and constitute a group of kinases involved in serine utilization.

Context-dependent utilization of serine in cancer

Serine and glycine have diverse biological functions but the general and context-dependent utilization of these nutrients in cancer is poorly understood. Our recent work integrates genomics data and isotope tracing using computational tools to study serine utilization across multiple cancer and normal human samples.

Multi-omics Approaches to Deciphering a Hypervirulent Strain of Campylobacter jejuni

Campylobacter jejuni clone SA recently emerged as the predominant cause of sheep abortion in the United States and is also associated with foodborne gastroenteritis in humans. A distinct phenotype of this clone is its ability to induce bacteremia and abortion. To facilitate understanding the pathogenesis of this hypervirulent clone, we analyzed a clinical isolate (IA3902) of clone SA using multi-omics approaches. The genome of IA3902 contains a circular chromosome of 1,635,045 bp and a circular plasmid of 37,174 bp. Comparative genomic analysis revealed that IA3902 is most closely related to C. jejuni NCTC11168, which is a reference strain and was previously shown to be non-abortifacient in pregnant animals. Despite the high genomic synteny and sequence homology, there are 12 variable regions (VRs) and 8,696 single-nucleotide polymorphisms and indels between the two genomes. Notably, the variable genes in the capsular polysaccharides biosynthesis and O-linked glycosylation loci of IA3902 are highly homogenous to their counterparts in C. jejuni subsp. doylei and C. jejuni G1, which are known to be frequently associated with bacteremia. Transcriptomic and proteomic profiles were conducted to compare IA3902 with NCTC11168, which revealed that the pathways of energy generation, motility, and serine utilization were significantly up-regulated in IA3902, whereas the pathways of iron uptake and proline, glutamate, aspartate, and lactate utilization were significantly down-regulated. These results suggest that C. jejuni clone SA has evolved distinct genomic content and gene expression patterns that modulate surface polysacharide structures, motilitiy, and metabolic pathways. These changes may have contributed to its hyper-virulence in abortion induction.

Contribution of Amino Acid Catabolism to the Tissue Specific Persistence of Campylobacter jejuni in a Murine Colonization Model

Campylobacter jejuni is a major cause of food-borne disease in industrialized countries. Carbohydrate utilization by C. jejuni is severely restricted, and knowledge about which substrates fuel C. jejuni infection and growth is limited. Some amino acids have been shown to serve as carbon sources both in vitro and in vivo. In the present study we investigated the contribution of serine and proline catabolism to the in vitro and in vivo growth of C. jejuni 81-176. We confirmed that the serine transporter SdaC and the serine ammonia-lyase SdaA are required for serine utilization, and demonstrated that a predicted proline permease PutP and a bifunctional proline/delta-1-pyrroline-5-carboxylate dehydrogenase PutA are required for proline utilization by C. jejuni 81-176. C. jejuni 81-176 mutants unable to utilize serine were shown to be severely defective for colonization of the intestine and systemic tissues in a mouse model of infection. In contrast, C. jejuni 81-176 mutants unable to utilize proline were only defective for intestinal colonization. These results further emphasize the importance of amino acid utilization in C. jejuni colonization of various tissues.

Genome-scale Reconstruction of Metabolic Network in Bacillus subtilis Based on High-throughput Phenotyping and Gene Essentiality Data

In this report, a genome-scale reconstruction of Bacillus subtilis metabolism and its iterative development based on the combination of genomic, biochemical, and physiological information and high-throughput phenotyping experiments is presented. The initial reconstruction was converted into an in silico model and expanded in a four-step iterative fashion. First, network gap analysis was used to identify 48 missing reactions that are needed for growth but were not found in the genome annotation. Second, the computed growth rates under aerobic conditions were compared with high-throughput phenotypic screen data, and the initial in silico model could predict the outcomes qualitatively in 140 of 271 cases considered. Detailed analysis of the incorrect predictions resulted in the addition of 75 reactions to the initial reconstruction, and 200 of 271 cases were correctly computed. Third, in silico computations of the growth phenotypes of knock-out strains were found to be consistent with experimental observations in 720 of 766 cases evaluated. Fourth, the integrated analysis of the large-scale substrate utilization and gene essentiality data with the genome-scale metabolic model revealed the requirement of 80 specific enzymes (transport, 53; intracellular reactions, 27) that were not in the genome annotation. Subsequent sequence analysis resulted in the identification of genes that could be putatively assigned to 13 intracellular enzymes. The final reconstruction accounted for 844 open reading frames and consisted of 1020 metabolic reactions and 988 metabolites. Hence, the in silico model can be used to obtain experimentally verifiable hypothesis on the metabolic functions of various genes.

Serine utilization in mouse liver: Influence of caloric restriction and aging

The influence of caloric restriction (CR) on the activities of hepatic serine metabolizing enzymes in young (3 months) and old (30 months) mice was studied. Serine dehydratase (SDH) activity increased markedly with age in both diet groups and in old mice was higher in the CR group. No effects of CR were observed in the young. Serine:pyruvate transaminase (SPT) and glycerate kinase activities were unaffected by age and diet. However, glycerate dehydrogenase activity was decreased in old CR mice but not in young CR. The results of this study show that long-term CR influenced serine utilization only in the pathway catalyzed by SDH. This suggests that in mouse liver this pathway is critical for serine utilization in gluconeogenesis, while the SPT pathway plays a minor role. The increase in SDH activity with long-term CR is consistent with sustained increase in gluconeogenesis.

Comportement métabolique de Lactococcus lactis en réponse à l'environnement

Metabolic behavlour of Lactococcus lactis in response to environmental conditions. Growth and metabolism of the lactic acid bacteria are closely dependent on the nutritional environment. Major environ mental constraints, when they are not directly related to the bacterial growth, are the nature and the availability of the nitrogen and carbon sources. ln this paper, the influence of the carbon substrate (sugar) on the metabolism of Lactococcus lactis NCDO 2118 was studied from a kinetic and enzymatic point of view. An original model of metabolism control has been developed and was successively tesied in other nutritional environments. This model differs from that generally assumed for lactic acid bacteria and can be applied to other lactococci. Fructose-J.o-bisphosphate plays only a minor role in controlling the further metabolism of pyruvate and NADH/NAD ratio exerts the major control over glycolytic flux. The metabolic consequences of co-substrate utilization were analyzed and the role of serine utilization on the metabolism of L. lactis NCDO 2118 was investigated in more detail. © Inra/Elsevier, Paris. Lactococcus lactis / growth / homolactic and mixed metabolism / co-substrate

Production and utilization of amino acids by ovine placenta in vivo.

Uterine and umbilical uptakes of plasma amino acids were measured simultaneously in eighteen singleton pregnant ewes at 130 +/- 1 days gestation for the purpose of establishing which amino acids are produced or used by the uteroplacenta under normal physiological conditions and at what rates. The branched-chain amino acids (BCAA) had uterine uptakes significantly greater than umbilical uptakes. Net uteroplacental BCAA utilization was 8.0 +/- 2.5 mumol.kg fetus-1.min-1 (P < 0.005) and represented 42% of the total BCAA utilization by fetus plus uteroplacenta. There was placental uptake of fetal glutamate (4.2 +/- 0.3 mumol.kg fetus-1.min-1, P < 0.001) and no uterine uptake of maternal glutamate. Umbilical uptake of glutamine was approximately 61% greater than uterine uptake, thus demonstrating net uteroplacental glutamine production of 2.2 +/- 0.9 mumol.kg fetus-1.min-1 (P < 0.021). In conjunction with other evidence, these data indicate rapid placental metabolism of glutamate, which is in part supplied by the fetus and in part produced locally via BCAA transamination. Most of the glutamate is oxidized, and some is used to synthesize glutamine, which is delivered to the fetus. There was net uteroplacental utilization of maternal serine and umbilical uptake of glycine produced by the placenta. Maternal serine utilization and glycine umbilical uptake were virtually equal (3.14 +/- 0.50 vs. 3.10 +/- 0.46 mumol.kg fetus-1.min-1). This evidence supports the conclusion that the ovine placenta converts large quantities of maternal serine into fetal glycine.

Serine and Glycine Metabolism in Hepatocytes from Mid Gestation Fetal Lambs

Using stable isotopes of serine, glycine, and glutamine, the metabolism of serine and glycine was investigated in primary hepatocytes from six mid-gestation fetal lambs (mean gestational age = 81 +/- 6 d, normal gestation = 145 d). Serine production was 6.84 +/- 1.22 mumol/24 h/mg of DNA and exceeded serine utilization (3.76 +/- 1.44 mumol/24 h/mg of DNA) with a resultant net increase in medium serine of 2.58 +/- 1.70 mumol/24 h/mg of DNA. In contrast, glycine production (6.84 +/- 1.16 mumol/24 h/mg of DNA) was less than glycine utilization (12.10 +/- 1.78 mumol/24 h/mg of DNA) with a net decline in medium glycine of -5.44 +/- 2.03 mumol/24 h/mg of DNA. Of the serine produced, 50.4 +/- 4.3% was derived from glycine via the action of serine hydroxymethyltransferase (SHMT) and the glycine cleavage enzyme complex (GCS). Increasing the medium serine concentration resulted in an increase in serine utilization and sparing of the utilization of other amino acids. Biosynthesis of glycine from serine accounts for only 18.1 +/- 5.6% of glycine production, and this percentage is not affected by changes in medium serine concentration. Using 2.5-[15N2]glutamine as the tracer, an estimated 18 +/- 7% of serine production was derived from transamination reactions. The specific activity of both cytosolic and mitochondrial SHMT was constant for the duration of the cultures. We conclude that, in mid-gestation fetal ovine hepatocytes, there is net production of serine (with glycine as the primary metabolic source of this serine biosynthesis) and net glycine utilization. These data suggest that flux through SHMT and GCS accounts for 50% of serine biosynthesis in mid-gestation fetal ovine hepatocytes. The sparing of the utilization of other amino acids by serine suggests that serine a conditionally essential amino acid for the mid-gestation fetal liver.

Evidence for intracellular partitioning of serine and glycine metabolism in Chinese hamster ovary cells.

Serine hydroxymethyltransferase (SHMT) is the primary enzyme in the interconversion of serine and glycine. The roles of mitochondrial and cytosolic SHMT in the interconversion of serine and glycine were determined in two Chinese hamster ovary (CHO) cell lines that both contain cytosolic SHMT but either have (CHOm+) or lacK (CHOm-) mitochondrial SHMT. Mitochondrial SHMT activity was significantly reduced in CHOm- (0.24 +/- 0.11 nmol/min per mg of mitochondrial protein) compared with CHOm+ (3.21 +/- 0.66 nmol/min per mg of mitochondrial protein; P = 0.02) cells, whereas cytosolic SHMT activity was similar in CHOm- and CHOm+ cells (1.09 +/- 0.31 and 1.53 +/- 0.12 nmol/min per mg of cytosolic protein respectively; P = 0.57). In CHOm+ and CHOm- cells, the relative flux of glycine to serine measured with either [1-13C]- or [2-13C]-glycine was similar (CHOm-: 538 +/- 82 nmol/24 per mg of DNA; CHOm+: 616 +/- 88 nmol/24 h per mg of DNA; P = 0.42). In contrast, the relative flux of serine to glycine measured with [1-13C]serine was low in CHOm- cells (80 +/- 28 nmol/24 h per mg of DNA) compared with CHOm+ cells (3080 +/- 320 nmol/24 h per mg of DNA; P = 0.0001). The rate of glycine production determined by [1-(13)C]glycine dilution was lower in CHOm- (1200 +/- 200 nmol/24 h per mg of DNA) than CHOm+ (10200 +/- 1800 nmol/24 h per mg of DNA; P = 0.03) cells, whereas glycine utilization was similar in the two cell lines. Serine production was similar in the two cell lines but serine utilization was lower in CHOm- (3800 +/- 1200 mu mol/24 h per mg of DNA) than CHOm+ (6600 +/- 1000 nmol/24 h per mg of DNA; P = 0.0002) cells. Increasing the serine concentration in the medium resulted in an increase in glycine production in CHOm+ but not in CHOm- cells. Intracellular studies with [1-13C]serine confirm the findings of decreased glycine production from serine. In CHO cells there is partitioning of intracellular serine and glycine metabolism. Our data support the hypothesis that mitochondrial SHMT is the primary pathway for serine into glycine interconversion.

Comparison of leucine, serine and glycine transport across the ovine placenta

To estimate the transport rate of maternal glycine across the placenta [1- 13C]glycine and L-[1- 13]serine were infused intravenously in pregnant sheep using both continuous and bolus infusions. Each tracer was infused together with L-[1- 13C]leucine, to enable a comparison with the placental transport of an essential amino acid. At steady state, fetal plasma leucine enrichment was 40 per cent of maternal enrichment, indicating that approximately 60 per cent of the entry rate of leucine into fetal plasma is derived from protein breakdown in the placenta and fetus. Fetal plasma glycine enrichment was 11 per cent of maternal and there was no detectable fetal serine enrichment. The direct flux of maternal leucine into the fetal circulation was approximately 3.0 (bolus experiments) to 3.6 (continuous infusion experiments) pmol/min (kg fetus) and greater than the estimated 1.4 μmol/min (kg fetus) direct flux of maternal glycine, despite the fact that the net umbilical uptake of glycine exceeds that of leucine. This supports the conclusion that placental glycine production is a quantitatively important contribution to fetal glycine uptake via the umbilical circulation. The fetal glycine supply from the placenta is provided by a relatively small direct maternal glycine transplacental flux and a larger contribution derived from serine utilization within the placenta for glycine production.

Serine Utilization as a Potential Competition Mechanism Between Salmonella and a Chicken Cecal Bacterium

Our objective in this study was to use batch culture to estimate growth kinetic parameters for a growth-limiting amino acid, serine, in Salmonella typhimurium and a in chicken cecal bacterium, Escherichia fergusonii, and to test these predictions for competitiveness in a mixed culture of the two organisms. Under anaerobic growth conditions, the two bacteria grew only when serine was provided as the nitrogen source, When serine was used as a carbon source in aerobic media, the maximum growth rates of the two organisms were considerably lower and the affinity constants were higher than when serine was used as a nitrogen source. The maximum growth rates of the two organisms were lower in anaerobic media than in aerobic media, but serine anaerobic affinity constants were lower than those from aerobic media. In aerobic mixed culture, S. typhimurium outgrew E. fergusonii in N-limited minimal media containing 0.1mM serine but in N-nonlimited minimal media containing 10mM serine, E. fergusonii outgrew S. typhimurium. In anaerobic mixed culture, E. fergusonii outgrew S. typhimurium in media containing both 0.1mM and 10mM serine concentrations. It appears that the cause of inhibition of Salmonella with serine as the variable nutrient differs depending on the oxidation-reduction status.

Regulatory elements for the tissue-specific expression of the rat serine dehydratase-encoding gene

l-Serine dehydratase (SDH; EC 4.2.1.13), the key enzyme for serine utilization in the rat, is synthesized primarily in the liver. Cis-acting DNA elements required for liver-specific expression of the SDH gene were identified by two approaches: (7) transient expression assays in primary cultured rat hepatocytes, and in rat fibrosarcoma and normal rat kidney epithelial (NRK-52E) cell lines; and (2) in vitro transcription assays with nuclear extracts prepared from rat liver and spleen. Deletion analyses of the 5' flanking sequences of the gene have defined two functionally different regions: (a) a cell-type-specific promoter located between positions -62 and + 10, which is sufficient for liver-specific expression; and (b) a distal promoter region between bp −133 and −63 containing positive cu-acting elements that regulate the promoter activity in a non-tissue-specific fashion. No other cis-acting elements essential for liver-specific expression were found in the region of −134 to 2.1 kb upstream relative to the cap site of SDH.

Serine utilization as a precursor of phosphatidylserine and alkenyl-(plasmenyl)-, alkyl-, and acylethanolamine phosphoglycerides in cultured glioma cells

In several tissues and cell lines, serine utilized for phosphatidylserine (PS) synthesis is an eventual precursor of the base moiety of ethanolamine phosphoglycerides (PE). We investigated the biosynthesis and decarboxylation of PS in cultured C6 glioma cells, with particular attention to 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine (plasmenylethanolamine) biosynthesis. Incorporation of [3H]serine into PS reached a maximum within 4-8 h, and label in nonplasmenylethanolamine phosphoglyceride (NP-PE) and plasmenylethanolamine was maximal by 12-24 h and 48 h, respectively. After 8 h, label in PS decreased even though 40-60% of initial label remained in the culture medium. Serial additions of fresh [3H]serine restored PS synthesis to higher levels of labeled PS accumulation followed by a subsequent decrease in 4-8 h. High performance liquid chromatographic analyses confirmed that medium serine was depleted by 8 h, and thereafter metabolites, including acetate and formate, accounted for radioactivity in the medium. The rapid but transient appearance of labeled glycine and ATP inside the cells indicated conversion of serine by hydroxymethyltransferase. 78-85% of label from serine was in headgroup of PS or of PE formed by decarboxylation. A precursor-product relationship was suggested for label from [3H]serine appearing in the headgroup of diacyl, alkylacyl, and alkenylacyl subclasses of PE. By 48 h, a constant specific activity, ratio of approximately 1:1 was reached between plasmenylethanolamine and NP-PE, similar to the molar distribution of these lipids. In contrast, equilibrium was not achieved in cells incubated with [1,2-14C]ethanolamine; plasmenylethanolamine had 2-fold greater specific activity than labeled NP-PE by 72-96 h. These observations indicate that in cultured glioma cells 1) serine serves as a precursor of the head group of PS and of both plasmenyl and non-plasmenyl species of PE; 2) exchange of headgroup between NP-PE and plasmenylethanolamine may involve different donor pools of PE depending on whether the headgroup originates with exogenous serine or ethanolamine; 3) serine is rapidly converted to other metabolites, which limits exogenous serine as a direct phospholipid precursor.

Metabolic control analysis of mammalian serine metabolism

1. (1) Mammalian serine metabolism is discussed in relation to its synthesis and utilization in proliferating cells, particularly during the nonmalignant proliferation of lymphocytes. 2. (2) An analysis of the control of serine biosynthesis de novo under conditions of high pathway flux has been carried out using metabolic control theory. 3. (3) The important and novel conclusions are that control of pathway flux is localized exclusively at the final step of this biosynthetic pathway, phosphoserine phosphatase. This conclusion challenges the frequently stated maxim that control of biosynthetic pathways is always directed at the first pathway enzyme in a sequence. In the case of phosphoserine phosphatase, the enzyme is inhibited uncompetitively by its product serine, and this feedback control mechanism has the most significant controlling influence on overall pathway flux. Thus, the serine biosynthesis pathway, under these conditions, is controlled by product demand (serine utilization) and not by substrate supply (glycolytic provision of 3-phosphoglycerate), despite the high rate of glycolysis associated with cell proliferation. 4. (4) The control structure of the pathway is not immutable. As has been observed with other pathways analyzed by metabolic control theory, the key points of control in the pathway can shift according to physiological circumstances. At low pathway flux, the control of serine biosynthesis is shared between all the component enzymes of the pathway, and the responsiveness of flux shifts from product demand to substrate supply. 5. (5) Serine utilization has been studied in mitogenically-stimulated human peripheral lymphocytes. Cell proliferation and serine utilization for nucleic acid synthesis have been shown to be responsive to serine concentrations in the normal plasma range. 6. (6) It is concluded that the maintenance of normal plasma serine concentrations is an important factor in the rate of lymphocyte proliferation and hence the effectiveness with which the body can mount an immune response to an antigenic challenge, such as in infection.