Comparative Studies on the Pathways for Serine Biosynthesis in Animal Tissues

Amino Acid Metabolic Vulnerabilities in Acute and Chronic Myeloid Leukemias

Amino acid (AA) metabolism plays an important role in many cellular processes including energy production, immune function, and purine and pyrimidine synthesis. Cancer cells therefore require increased AA uptake and undergo metabolic reprogramming to satisfy the energy demand associated with their rapid proliferation. Like many other cancers, myeloid leukemias are vulnerable to specific therapeutic strategies targeting metabolic dependencies. Herein, our review provides a comprehensive overview and TCGA data analysis of biosynthetic enzymes required for non-essential AA synthesis and their dysregulation in myeloid leukemias. Furthermore, we discuss the role of the general control nonderepressible 2 (GCN2) and-mammalian target of rapamycin (mTOR) pathways of AA sensing on metabolic vulnerability and drug resistance.

Amino Acids in Swine Nutrition and Production

Amino acids are the building blocks of proteins in animals, including swine. With the development of new analytical methods and biochemical research, there is a growing interest in fundamental and applied studies to reexamine the roles and usage of amino acids (AAs) in swine production. In animal nutrition, AAs have been traditionally classified as nutritionally essential (EAAs) or nutritionally nonessential (NEAAs). AAs that are not synthesized de novo must be provided in diets. However, NEAAs synthesized by cells of animals are more abundant than EAAs in the body, but are not synthesized de novo in sufficient amounts for the maximal productivity or optimal health (including resistance to infectious diseases) of swine. This underscores the conceptual limitations of NEAAs in swine protein nutrition. Notably, the National Research Council (NRC 2012) has recognized both arginine and glutamine as conditionally essential AAs for pigs to improve their growth, development, reproduction, and lactation. Results of recent work have also provided compelling evidence for the nutritional essentiality of glutamate, glycine, and proline for young pigs. The inclusion of so-called NEAAs in diets can help balance AAs in diets, reduce the dietary levels of EAAs, and protect the small intestine from oxidative stress, while enhancing the growth performance, feed efficiency, and health of pigs. Thus, both EAAs and NEAAs are needed in diets to meet the requirements of pigs. This notion represents a new paradigm shift in our understanding of swine protein nutrition and is transforming pork production worldwide.

3-Phosphoglycerate dehydrogenase: a potential target for cancer treatment

BACKGROUND

Metabolic changes have been recognized as an important hallmark of cancer cells. Cancer cells can promote their own growth and proliferation through metabolic reprogramming. Particularly, serine metabolism has frequently been reported to be dysregulated in tumor cells. 3-Phosphoglycerate dehydrogenase (PHGDH) catalyzes the first step in the serine biosynthesis pathway and acts as a rate-limiting enzyme involved in metabolic reprogramming. PHGDH upregulation has been observed in many tumor types, and inhibition of PHGDH expression has been reported to inhibit the proliferation of PHGDH-overexpressing tumor cells, indicating that it may be utilized as a target for cancer treatment. Recently identified inhibitors targeting PHGDH have already shown effectiveness. A further in-depth analysis and concomitant development of PHGDH inhibitors will be of great value for the treatment of cancer.

CONCLUSIONS

In this review we describe in detail the role of PHGDH in various cancers and inhibitors that have recently been identified to highlight progression in cancer treatment. We also discuss the development of new drugs and treatment modalities based on PHGDH targets. Overexpression of PHGDH has been observed in melanoma, breast cancer, nasopharyngeal carcinoma, parathyroid adenoma, glioma, cervical cancer and others. PHGDH may serve as a molecular biomarker for the diagnosis, prognosis and treatment of these cancers. The design and development of novel PHGDH inhibitors may have broad implications for cancer treatment. Therapeutic strategies of PHGDH inhibitors in combination with traditional chemotherapeutic drugs may provide new perspectives for precision medicine and effective personalized treatment for cancer patients.

Phosphoserine aminotransferase has conserved active site from microbes to higher eukaryotes with minor deviations

Serine is ubiquitously synthesized in all living organisms from the glycolysis intermediate 3-phosphoglycerate (PGA) by phosphoserine biosynthetic pathway, consisting of three different enzymes, namely: 3-phosphoglycerate dehydrogenase (PGDH), phosphoserine aminotransferase (PSAT), and phosphoserine phosphatase (PSP). Any functional defect or mutation in these enzymes may cause deliberating conditions, such as colon cancer progression and chemoresistance in humans. Phosphoserine aminotransferase (PSAT) is the second enzyme in this pathway that converts phosphohydroxypyruvate (PHP) to O-phospho-L-serine (OPLS). Humans encode two isoforms of this enzyme: PSAT1 and PSAT2. PSAT1 exists as a functional dimer, where each protomer has a large and a small domain; each large domain contains a Lys residue that covalently binds PLP. The PLP-binding site of human PSAT1 and most of its active site residues are highly conserved in all known PSAT structures except for Cys-80. Interestingly, Two PSAT structures from different organisms show halide binding near their active site. While the human PSAT1 shows a water molecule at this site with different interacting residues, suggesting the inability of halide binding in the human enzyme. Analysis of the human PSAT1 structure showed a big patch of positive charge around the active site, in contrast to the bacterial PSATs. Compared to human PSAT1, the PSAT2 isoform lacks 46 residues at its C-terminal tail. This tail region is present at the opening of the active site as observed in the other PSAT structures. Further structural work on human PSAT2 may reveal the functional importance of these 46 residues.

Molecular characterization and expression analysis of a phosphoserine aminotransferase involving l-serine synthesis from silkworm, Bombyx mori

In this study, we identified and characterized a phosphoserine aminotransferase (bmPSAT) from Bombyx mori (B. mori) that is responsible for l-serine biosynthesis. A complementary DNA that encodes bmPSAT was cloned by reverse transcriptase polymerase reaction and sequenced. The presumed amino acid sequence revealed 47-87% identity with known PSATs from insects, humans, plants, and bacteria. Through phylogenetic analysis, we found that bmPSAT is evolutionary related to insect PSATs. Recombinant bmPSAT was produced in Escherichia coli by using a cold-shock promotor and purified to homogeneity. This enzyme utilizes phosphohydroxypyruvate and glutamate for transamination. bmPSAT messenger RNA (mRNA) was expressed at higher levels in several tissues of standard strain silkworm including the silk gland, whereas a sericin-deficient silkworm strain exhibited a diminished expression of bmPSAT mRNA in the silk gland. These findings indicate that bmPSAT may play an important role in synthesizing and supplying l-serine in the larva of B. mori.

D-3-Phosphoglycerate Dehydrogenase

l-Serine is the immediate precursor of d-serine, a major agonist of the N-methyl-d-aspartate (NMDA) receptor. l-Serine is a pivotal amino acid since it serves as a precursor to a large number of essential metabolites besides d-serine. In all non-photosynthetic organisms, including mammals, a major source of l-serine is the phosphorylated pathway of l-serine biosynthesis. The pathway consists of three enzymes, d-3-phosphoglycerate dehydrogenase (PGDH), phosphoserine amino transferase (PSAT), and l-phosphoserine phosphatase (PSP). PGDH catalyzes the first step in the pathway by converting d-3-phosphoglycerate (PGA), an intermediate in glycolysis, to phosphohydroxypyruvate (PHP) concomitant with the reduction of NAD⁺. In some, but not all organisms, the catalytic activity of PGDH can be regulated by feedback inhibition by l-serine. Three types of PGDH can be distinguished based on their domain structure. Type III PGDHs contain only a nucleotide binding and substrate binding domain. Type II PGDHs contain an additional regulatory domain (ACT domain), and Type I PGDHs contain a fourth domain, termed the ASB domain. There is no consistent pattern of domain content that correlates with organism type, and even when additional domains are present, they are not always functional. PGDH deficiency results in metabolic defects of the nervous system whose systems range from microcephaly at birth, seizures, and psychomotor retardation. Although deficiency of any of the pathway enzymes have similar outcomes, PGDH deficiency is predominant. Dietary or intravenous supplementation with l-serine is effective in controlling seizures but has little effect on psychomotor development. An increase in PGDH levels, due to overexpression, is also associated with a wide array of cancers. In culture, PGDH is required for tumor cell proliferation, but extracellular l-serine is not able to support cell proliferation. This has led to the hypothesis that the pathway is performing some function related to tumor growth other than supplying l-serine. The most well-studied PGDHs are bacterial, primarily from Escherichia coli and Mycobacterium tuberculosis, perhaps because they have been of most interest mechanistically. However, the relatively recent association of PGDH with neuronal defects and human cancers has provoked renewed interest in human PGDH.

Elucidation of a Self-Sustaining Cycle in Escherichia coli l-Serine Biosynthesis That Results in the Conservation of the Coenzyme, NAD<sup/>

The equilibrium of the reaction catalyzed by d-3-phosphoglycerate dehydrogenase (PGDH), the first enzyme in the l-serine biosynthetic pathway, is far in the direction away from serine synthesis. As such, the enzyme is usually assayed in this direction. To easily assay it in the direction of l-serine synthesis, it can be coupled to the next enzyme in the pathway, phosphoserine aminotransferase (PSAT), with the activity monitored by the conversion of NAD⁺ to NADH by PGDH. However, when PGDHs from several different species were coupled to PSAT, it was found that one of them, ecPGDH, conserves the coenzyme in the production of l-serine by utilizing an intrinsic cycle of NAD⁺/NADH interconversion coupled with the conversion of α-ketoglutarate (αKG) to α-hydroxyglutarate. Furthermore, the cycle can be maintained by production of αKG by the second enzyme in the pathway, PSAT, and does not require any additional enzymes. This is not the case for PGDH from another bacterial source, Mycobacterium tuberculosis, and a mammalian source, human liver, where net consumption of NAD⁺ occurs. Both NAD⁺ and NADH appear to remain tightly bound to ecPGDH during the cycle, effectively removing a requirement for the presence of an exogenous coenzyme pool to maintain the pathway and significantly reducing the energy requirement needed to maintain this major metabolic pathway.

Sulfolobus - A Potential Key Organism in Future Biotechnology

Extremophilic organisms represent a potentially valuable resource for the development of novel bioprocesses. They can act as a source for stable enzymes and unique biomaterials. Extremophiles are capable of carrying out microbial processes and biotransformations under extremely hostile conditions. Extreme thermoacidophilic members of the well-characterized genus Sulfolobus are outstanding in their ability to thrive at both high temperatures and low pH. This review gives an overview of the biological system Sulfolobus including its central carbon metabolism and the development of tools for its genetic manipulation. We highlight findings of commercial relevance and focus on potential industrial applications. Finally, the current state of bioreactor cultivations is summarized and we discuss the use of Sulfolobus species in biorefinery applications.

Regulatory Mechanism of Mycobacterium tuberculosis Phosphoserine Phosphatase SerB2

Almost all organisms contain the same biosynthetic pathway for the synthesis of l-serine from the glycolytic intermediate, d-3-phosphoglycerate. However, regulation of this pathway varies from organism to organism. Many organisms control the activity of the first enzyme in the pathway, d-3-phosphoglycerate dehydrogenase (PGDH), by feedback inhibition through the interaction of l-serine with the ACT domains within the enzyme. The last enzyme in the pathway, phosphoserine phosphatase (PSP), has also been reported to be inhibited by l-serine. The high degree of sequence homology between Mycobacterium tuberculosis PSP (mtPSP) and Mycobacterium avium PSP (maPSP), which has recently been shown to contain ACT domains, suggested that the mtPSP also contained ACT domains. This raised the question of whether the ACT domains in mtPSP played a functional role similar to that of the ACT domains in PGDH. This investigation reveals that l-serine allosterically inhibits mtPSP by a mechanism of partial competitive inhibition by binding to the ACT domains. Therefore, in mtPSP, l-serine is an allosteric feedback inhibitor that acts by decreasing the affinity of the substrate for the enzyme. mtPGDH is also feedback inhibited by l-serine, but only in the presence of millimolar concentrations of phosphate. Therefore, the inhibition of mtPSP by l-serine would act as a secondary control point for the regulation of the l-serine biosynthetic pathway under physiological conditions where the level of phosphate would be below that needed for l-serine feedback inhibition of mtPGDH.

'Trophic' and 'source' amino acids in trophic estimation: a likely metabolic explanation

Amino acid nitrogen isotopic analysis is a relatively new method for estimating trophic position. It uses the isotopic difference between an individual's 'trophic' and 'source' amino acids to determine its trophic position. So far, there is no accepted explanation for the mechanism by which the isotopic signals in 'trophic' and 'source' amino acids arise. Yet without a metabolic understanding, the utility of nitrogen isotopic analyses as a method for probing trophic relations, at either bulk tissue or amino acid level, is limited. I draw on isotopic tracer studies of protein metabolism, together with a consideration of amino acid metabolic pathways, to suggest that the 'trophic'/'source' groupings have a fundamental metabolic origin, to do with the cycling of amino-nitrogen between amino acids. 'Trophic' amino acids are those whose amino-nitrogens are interchangeable, part of a metabolic amino-nitrogen pool, and 'source' amino acids are those whose amino-nitrogens are not interchangeable with the metabolic pool. Nitrogen isotopic values of 'trophic' amino acids will reflect an averaged isotopic signal of all such dietary amino acids, offset by the integrated effect of isotopic fractionation from nitrogen cycling, and modulated by metabolic and physiological effects. Isotopic values of 'source' amino acids will be more closely linked to those of equivalent dietary amino acids, but also modulated by metabolism and physiology. The complexity of nitrogen cycling suggests that a single identifiable value for 'trophic discrimination factors' is unlikely to exist. Greater consideration of physiology and metabolism should help in better understanding observed patterns in nitrogen isotopic values.

MSMEG_5684 down-regulation in Mycobacterium smegmatis affects its permeability, survival under stress and persistence

The Mycobacterium tuberculosis (Mtb) genome sequence and annotation details have been available for a long time; however physiological relevance of many ORFs remains poorly described. Mtb is a pathogenic strain; hence, surrogate strains such as Mycobacterium bovis BCG and Mycobacterium smegmatis (Msmeg) have also been studied to gain an understanding of mycobacterial physiology and metabolism. The Mycobacterium smegmatis mc² 155 ORF MSMEG_5684 is annotated as a part of serine biosynthetic pathway, however, its physiological significance remains to be established experimentally. To understand the relevance of SerC for Msmeg physiology we developed a recombinant Mycobacterium smegmatis with SerC knockdown (KD) and also complemented it with serC over-expressing construct (KDC). The KD showed reduced growth compared to wild-type (WT) and complemented strain on glycerol as carbon source. The growth of KD was restored after supplementation of serine. The survival studies with WT and KD under oxidative, nitrosative and detergent stresses showed increased susceptibility of KD. The KD also showed increased susceptibility to antimycobacterial agents and poor ability for in vitro persistence. Also, the serC transcript profiling showed increased expression under stress. The complementation studies with Msmeg serC showed growth restoration of E. coli-ΔserC in minimal medium.

Comparison of Type 1 D-3-phosphoglycerate dehydrogenases reveals unique regulation in pathogenic Mycobacteria

D-3-phosphoglycerate dehydrogenases (PGDH) from all organisms catalyze the conversion of D-3-phosphoglycerate to phosphohydroxypyruvate as the first step in the biosynthesis of l-serine. This investigation compares the properties of Type 1 PGDHs from seven different species and demonstrates that conserved residues in the ACT and ASB domains of some allow l-serine to act as a feedback inhibitor at low micromolar concentrations. In addition, the serine sensitivity is dependent on the presence of phosphate ions. These residues are most highly conserved among PGDHs from the actinomycetales family, but only certain pathogenic mycobacteria appear to have the full complement of residues required for high sensitivity to serine. These basic residues are also responsible for the presence of dual pH optima in the acidic region that is also phosphate dependent. Analytical ultracentrifugation analysis demonstrates that the dual pH optima do not require changes in oligomeric state. This study also demonstrates that substrate inhibition is a common feature of Type 1 PGDHs and that it is suppressed by phosphate, indicating that phosphate likely interacts at both the catalytic and regulatory sites. The unique features resulting from the complement of basic residues conserved in pathogenic mycobacteria may impart important metabolic advantages to these organisms.

Phosphoglycerate dehydrogenase: potential therapeutic target and putative metabolic oncogene

Exemplified by cancer cells' preference for glycolysis, for example, the Warburg effect, altered metabolism in tumorigenesis has emerged as an important aspect of cancer in the past 10–20 years. Whether due to changes in regulatory tumor suppressors/oncogenes or by acting as metabolic oncogenes themselves, enzymes involved in the complex network of metabolic pathways are being studied to understand their role and assess their utility as therapeutic targets. Conversion of glycolytic intermediate 3-phosphoglycerate into phosphohydroxypyruvate by the enzyme phosphoglycerate dehydrogenase (PHGDH)—a rate-limiting step in the conversion of 3-phosphoglycerate to serine—represents one such mechanism. Forgotten since classic animal studies in the 1980s, the role of PHGDH as a potential therapeutic target and putative metabolic oncogene has recently reemerged following publication of two prominent papers near-simultaneously in 2011. Since that time, numerous studies and a host of metabolic explanations have been put forward in an attempt to understand the results observed. In this paper, I review the historic progression of our understanding of the role of PHGDH in cancer from the early work by Snell through its reemergence and rise to prominence, culminating in an assessment of subsequent work and what it means for the future of PHGDH.

The use of 2D-DIGE to understand the regeneration of somatic embryos in avocado

Avocado embryogenic cell cultures can be classified into two groups based on their morphology when cultured on a medium containing auxin: somatic embryo (SE) and proembryonic masses (PEM) type cultures. The calli of SE-type cell lines are able to go through the maturation process, whereas the calli of PEM cell lines rarely mature. We have investigated four independent avocado cell cultures (two SE and two PEM). The aim of this study was to link the differential regeneration capacity of the four cell cultures to a proteomic pattern and to gain insight into the regeneration capacity. A 2D-DIGE analysis followed by a blind multivariate analysis was able to separate the two SE lines from the PEM lines indicating that the protein profiles of SE and PEM calli are different. Based on the variable importance, that is, the differential protein pattern, we hypothesize that the regeneration capacity in avocado is correlated to the ability to overcome the physicochemical stress stimuli associated with the in vitro culture. Our identical culture conditions do not seem to trigger an appropriate response in PEM lines.

Regulation of Mycobacterium tuberculosis D-3-phosphoglycerate dehydrogenase by phosphate-modulated quaternary structure dynamics and a potential role for polyphosphate in enzyme regulation

D-3-phosphoglycerate dehydrogenase (PGDH) catalyzes the first reaction in the "phosphorylated" pathway of l-serine biosynthesis. In Mycobacterium tuberculosis, it is a type 1 enzyme (mtPGDH) in that it contains both an ACT domain and an ASB domain in addition to a catalytic domain. The published crystal structures (Protein Data Bank entries 1YGY and 3DC2) show a tartrate molecule interacting with cationic residues at the ASB-ACT domain interfaces and a serine molecule bound at the ACT domain interface. These sites have previously been shown to be involved in the mechanism of serine and substrate inhibition of catalytic activity. This investigation has revealed a mechanism of allosteric quaternary structure dynamics in mtPGDH that is modulated by physiologically relevant molecules, phosphate and polyphosphate. In the absence of phosphate and polyphosphate, the enzyme exists in equilibrium between an inactive dimer and an active tetramer that is insensitive to inhibition of catalytic activity by L-serine. Phosphate induces a conversion to an active tetramer and octamer that are sensitive to inhibition of catalytic activity by L-serine. Small polyphosphates (pyrophosphate and triphosphate) induce a conversion to an active dimer that is insensitive to L-serine inhibition. The difference in the tendency of each respective dimer to form a tetramer as well as slightly altered elution positions on size exclusion chromatography indicates that there is likely a conformational difference between the serine sensitive and insensitive states. This appears to constitute a unique mechanism in type 1 PGDHs that may be unique in pathogenic Mycobacterium species and may provide the organisms with a unique metabolic advantage.

Discovery of novel allosteric effectors based on the predicted allosteric sites for Escherichia coli D-3-phosphoglycerate dehydrogenase

D-3-phosphoglycerate dehydrogenase (PGDH) from Escherichia coli catalyzes the first critical step in serine biosynthesis, and can be allosterically inhibited by serine. In a previous study, we developed a computational method for allosteric site prediction using a coarse-grained two-state Gō Model and perturbation. Two potential allosteric sites were predicted for E. coli PGDH, one close to the active site and the nucleotide binding site (Site I) and the other near the regulatory domain (Site II). In the present study, we discovered allosteric inhibitors and activators based on site I, using a high-throughput virtual screen, and followed by using surface plasmon resonance (SPR) to eliminate false positives. Compounds 1 and 2 demonstrated a low-concentration activation and high-concentration inhibition phenomenon, with IC₅₀ values of 34.8 and 58.0 µM in enzymatic bioassays, respectively, comparable to that of the endogenous allosteric effector, L-serine. For its activation activity, compound 2 exhibited an AC₅₀ value of 34.7 nM. The novel allosteric site discovered in PGDH was L-serine- and substrate-independent. Enzyme kinetics studies showed that these compounds influenced K_m, k_cat, and k_cat/K_m. We have also performed structure-activity relationship studies to discover high potency allosteric effectors. Compound 2-2, an analog of compound 2, showed the best in vitro activity with an IC₅₀ of 22.3 µM. Compounds targeting this site can be used as new chemical probes to study metabolic regulation in E. coli. Our study not only identified a novel allosteric site and effectors for PGDH, but also provided a general strategy for designing new regulators for metabolic enzymes.

Glycine metabolism in animals and humans: implications for nutrition and health

Glycine is a major amino acid in mammals and other animals. It is synthesized from serine, threonine, choline, and hydroxyproline via inter-organ metabolism involving primarily the liver and kidneys. Under normal feeding conditions, glycine is not adequately synthesized in birds or in other animals, particularly in a diseased state. Glycine degradation occurs through three pathways: the glycine cleavage system (GCS), serine hydroxymethyltransferase, and conversion to glyoxylate by peroxisomal D-amino acid oxidase. Among these pathways, GCS is the major enzyme to initiate glycine degradation to form ammonia and CO2 in animals. In addition, glycine is utilized for the biosynthesis of glutathione, heme, creatine, nucleic acids, and uric acid. Furthermore, glycine is a significant component of bile acids secreted into the lumen of the small intestine that is necessary for the digestion of dietary fat and the absorption of long-chain fatty acids. Glycine plays an important role in metabolic regulation, anti-oxidative reactions, and neurological function. Thus, this nutrient has been used to: (1) prevent tissue injury; (2) enhance anti-oxidative capacity; (3) promote protein synthesis and wound healing; (4) improve immunity; and (5) treat metabolic disorders in obesity, diabetes, cardiovascular disease, ischemia-reperfusion injuries, cancers, and various inflammatory diseases. These multiple beneficial effects of glycine, coupled with its insufficient de novo synthesis, support the notion that it is a conditionally essential and also a functional amino acid for mammals (including pigs and humans).

Contrasting catalytic and allosteric mechanisms for phosphoglycerate dehydrogenases

D-3-Phosphoglycerate dehydrogenases (PGDH) exist with at least three different structural motifs and the enzymes from different species display distinctly different mechanisms. In many species, particularly bacteria, the catalytic activity is regulated allosterically through binding of l-serine to a distinct structural domain, termed the ACT domain. Some species, such as Mycobacterium tuberculosis, contain an additional domain, called the "allosteric substrate binding" or ASB domain, that functions as a co-domain in the regulation of catalytic activity. That is, both substrate and effector function synergistically in the regulation of activity to give the enzyme some interesting properties that may have physiological relevance for the persistent state of tuberculosis. Both enzymes function through a V-type regulatory mechanism and, in the Escherichia coli enzyme, it has been demonstrated that this results from a dead-end complex that decreases the concentration of active species rather than a decrease in the velocity of the active species. This review compares and contrasts what we know about these enzymes and provides additional insight into their mechanism of allosteric regulation.

Transient kinetic analysis of L-serine interaction with Escherichia coli D-3-phosphoglycerate dehydrogenase containing amino acid mutations in the hinge regions

In Escherichia colid-3-phosphoglycerate dehydrogenase, the amino acid sequences G294-G295 and G336-G337 are found between structural domains and appear to function as hinge regions. Mutagenesis studies of these sequences showed that bulky side chains had significant effects on the kinetic properties of the enzyme. Placement of a tryptophanyl residue near the serine binding site (W139F/E360W) allows serine binding to be monitored by fluorescence quenching analysis. Pre-steady-state analysis has demonstrated that serine binds to two forms of the free enzyme, E and E*. Conversion of Gly-336 to valine has its main effect on the Kd of serine binding to one form of the free enzyme (E) while maintaining the cooperativity of binding observed in the native enzyme. Conversion of Gly-294 to valine eliminates a rate limiting conformational change that follows serine binding to E. The conformational change between the two forms of free enzyme is maintained, but the Hill coefficient for cooperativity is significantly lowered. The data indicate that the cooperative transmission induced by serine binding is transmitted through the Gly294-Gly295 hinge region to the opposite serine binding interface and that this is most likely propagated by way of the substrate binding domain-regulatory domain interface. In the G294 mutant enzyme, both serine bound species, E·Ser and E*·Ser, are present in significant amounts indicating that cooperativity of serine binding does not result from the binding to two different forms. The data also suggest that the E* form may be inactive even when serine is not bound.

Transient kinetic analysis of the interaction of L-serine with Escherichia coli D-3-phosphoglycerate dehydrogenase reveals the mechanism of V-type regulation and the order of effector binding

Pre-steady state stopped-flow analysis of Escherichia coli d-3-phosphoglycerate dehydrogenase (PGDH) reveals that the physiological inhibitor, l-serine, exerts its effect on at least two steps in the kinetic mechanism, but to very different degrees. First, there is a small but significant effect on the dissociation constant of NADH, the first substrate to bind in the ordered mechanism. The effect of serine is mainly on the binding off rate, increasing the K(d) to 5 and 23 muM from 0.6 and 9 muM, respectively, for the two sets of sites in the enzyme. A more profound effect is seen after the second substrate is added. Serine reduces the amplitude of the signal without a significant effect on the observed rate constants for binding. The serine concentration that reduces the amplitude by 50% is equal to the K(0.5) for serine inhibition. The data are consistent with the conclusion that serine binding eliminates a conformational change subsequent to substrate binding by formation of a dead-end quaternary complex consisting of enzyme, coenzyme, substrate, and effector. Thus, the mechanistic basis for V-type regulation in this enzyme is a reduction in the population of active species rather than a differential decrease in the velocity of active species. Pre-steady state analysis of binding of serine to a mutant PGDH (W139F/E360W) demonstrates that each serine binding interface produces an integrated fluorescent signal. The observed rate data are complex but conform to a model in which serine can bind to two forms of the enzyme with different affinities. The integrated signal from each interface allows the amplitude data to clearly define the order of binding to each site, and modeling the amplitude data with species distribution equations clearly demonstrates an alternate interface binding mechanism and the direction of binding cooperativity.

A stopped flow transient kinetic analysis of substrate binding and catalysis in Escherichia coli D-3-phosphoglycerate dehydrogenase

Pre-steady state, stopped flow analysis of Escherichia coli D-3-phosphoglycerate dehydrogenase was performed by following the fluorescence of protein tryptophan and the fluorescence resonance energy transfer from protein tryptophan to bound NADH. The results indicate that binding of substrates is ordered, with coenzyme, NADH, binding first. Furthermore, the analysis indicated that there are two sets of sites on the tetrameric enzyme that can be differentiated by their kinetic behavior. NADH binding was consistent with an initial binding event followed by a slow conformational change for each site. The slow conformational change is responsible for the apparent tight binding of NADH to the apoenzyme but is too slow to participate in the catalytic cycle when the enzyme is rapidly turning over. Subsequent binding of the substrate, alpha-ketoglutarate, was characterized by a rapid equilibrium binding event followed by a conformational change for each site. Catalysis in the direction of NAD(+) reduction showed a distinct burst of activity followed by a slow rate of turnover, indicating that the rate-limiting step is after hydride transfer. Catalysis in the direction of NADH oxidation did not display burst kinetics, indicating that the rate-limiting step is at or before the hydride transfer step. The burst data indicated that the rate of NAD(+) reduction (3.8 s(-1)) is similar to the k(cat) of the enzyme (2-3 s(-1)) in that direction. However, analysis of the reaction with deuterated NADH failed to show an effect on the velocity of the reaction with a V(H)/V(D)=1.07+/-0.06. None of the other rates determined by stopped flow analysis could account for the k(cat) of the enzyme in either direction (forward k(cat)=0.01 s(-1), reverse k(cat)=2-3 s(-1)), suggesting that the rate-limiting step in both directions is a conformational change in the enzyme that is not detected optically.

Effect of pH on the structure and stability of Bacillus circulans ssp. alkalophilus phosphoserine aminotransferase: Thermodynamic and crystallographic studies

pH is one of the key parameters that affect the stability and function of proteins. We have studied the effect of pH on the pyridoxal-5'-phosphate-dependent enzyme phosphoserine aminotransferase produced by the facultative alkaliphile Bacillus circulans ssp. alkalophilus using thermodynamic and crystallographic analysis. Enzymatic activity assay showed that the enzyme has maximum activity at pH 9.0 and relative activity less than 10% at pH 7.0. Differential scanning calorimetry and circular dichroism experiments revealed variations in the stability and denaturation profiles of the enzyme at different pHs. Most importantly, release of pyridoxal-5'-phosphate and protein thermal denaturation were found to occur simultaneously at pH 6.0 in contrast to pH 8.5 where denaturation preceded cofactor's release by approximately 3 degrees C. To correlate the observed differences in thermal denaturation with structural features, the crystal structure of phosphoserine aminotransferase was determined at 1.2 and 1.5 A resolution at two different pHs (8.5 and 4.6, respectively). Analysis of the two structures revealed changes in the vicinity of the active site and in surface residues. A conformational change in a loop involved in substrate binding at the entrance of the active site has been identified upon pH change. Moreover, the number of intramolecular ion pairs was found reduced in the pH 4.6 structure. Taken together, the presented kinetics, thermal denaturation, and crystallographic data demonstrate a potential role of the active site in unfolding and suggest that subtle but structurally significant conformational rearrangements are involved in the stability and integrity of phosphoserine aminotransferase in response to pH changes.

Enzyme adaptation to alkaline pH: atomic resolution (1.08 A) structure of phosphoserine aminotransferase from Bacillus alcalophilus

The crystal structure of the vitamin B(6)-dependent enzyme phosphoserine aminotransferase from the obligatory alkaliphile Bacillus alcalophilus has been determined at 1.08 A resolution. The model was refined to an R-factor of 11.7% (R(free) = 13.9%). The enzyme displays a narrow pH optimum of enzymatic activity at pH 9.0. The final structure was compared to the previously reported structure of the mesophilic phosphoserine aminotransferase from Escherichia coli and to that of phosphoserine aminotransferase from a facultative alkaliphile, Bacillus circulans subsp. alkalophilus. All three enzymes are homodimers with each monomer comprising a two-domain architecture. Despite the high structural similarity, the alkaliphilic representatives possess a set of distinctive structural features. Two residues directly interacting with pyridoxal-5'-phosphate are replaced, and an additional hydrogen bond to the O3' atom of the cofactor is present in alkaliphilic phosphoserine aminotransferases. The number of hydrogen bonds and hydrophobic interactions at the dimer interface is increased. Hydrophobic interactions between the two domains in the monomers are enhanced. Moreover, the number of negatively charged amino acid residues increases on the solvent-accessible molecular surface and fewer hydrophobic residues are exposed to the solvent. Further, the total amount of ion pairs and ion networks is significantly reduced in the Bacillus enzymes, while the total number of hydrogen bonds is increased. The mesophilic enzyme from Escherichia coli contains two additional beta-strands in a surface loop with a third beta-strand being shorter in the structure. The identified structural features are proposed to be possible factors implicated in the alkaline adaptation of phosphoserine aminotransferase.

Crystal Structure of Mycobacterium tuberculosis D-3-Phosphoglycerate Dehydrogenase

Phosphoglycerate dehydrogenases exist in at least three different structural motifs. The first D-3-phosphoglycerate dehydrogenase structure to be determined was from Escherichia coli and is a tetramer composed of identical subunits that contain three discernable structural domains. The crystal structure of D-3-phosphoglycerate dehydrogenase from Mycobacterium tuberculosis has been determined at 2.3 A. This enzyme represents a second structural motif of the D-3-phosphoglycerate dehydrogenase family, one that contains an extended C-terminal region. This structure is also a tetramer of identical subunits, and the extended motif of 135 amino acids exists as a fourth structural domain. This intervening domain exerts quite a surprising characteristic to the structure by introducing significant asymmetry in the tetramer. The asymmetric unit is composed of two identical subunits that exist in two different conformations characterized by rotation of approximately 180 degrees around a hinge connecting two of the four domains. This asymmetric arrangement results in the formation of two different and distinct domain interfaces between identical domains in the asymmetric unit. As a result, the surface of the intervening domain that is exposed to solvent in one subunit is turned inward in the other subunit toward the center of the structure where it makes contact with other structural elements. Significant asymmetry is also seen at the subunit level where different conformations exist at the NAD-binding site and the putative serine-binding site in the two unique subunits.

D-3-Phosphoglycerate dehydrogenase from Mycobacterium tuberculosis is a link between the Escherichia coli and mammalian enzymes

D-3-Phosphoglycerate dehydrogenase (PGDH) from Mycobacterium tuberculosis has been isolated to homogeneity and displays an unusual relationship to the Escherichia coli and mammalian enzymes. In almost all aspects investigated, the M. tuberculosis enzyme shares the characteristics of the mammalian PGDHs. These include an extended C-terminal motif, substrate inhibition kinetics, dependence of activity levels and stability on ionic strength, and the inability to utilize alpha-ketoglutarate as a substrate. The unique property that the M. tuberculosis enzyme shares with E. coli PGDH that it is very sensitive to inhibition by L-serine, with an I(0.5) = 30 microm. The mammalian enzymes are not inhibited by L-serine. In addition, the cooperativity of serine inhibition appears to be modulated by chloride ion, becoming positively cooperative in its presence. This is modulated by the gain of cooperativity in serine binding for the first two effector sites. The basis for the chloride modulation of cooperativity is not known, but the sensitivity to serine inhibition can be explained in terms of certain amino acid residues in critical areas of the structures. The differential sensitivity to serine inhibition by M. tuberculosis and human PGDH may open up interesting possibilities in the treatment of multidrug-resistant tuberculosis.

Mouse 3-phosphoglycerate dehydrogenase gene: genomic organization, chromosomal localization, and promoter analysis

d-3-Phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95) is the first committed enzyme of l-serine biosynthesis in the phosphorylated pathway. We have recently demonstrated that, in developing and mature brain, expression of Phgdh is highly regulated in a cell lineage-specific manner, mainly in neuroepithelial stem cells, radial glia, and astrocytes (J. Neurosci. 21 (2001) 7691; Arch. Histol. Cytol. 66 (2003) 109). To gain insight into the regulatory mechanism of Phgdh expression, we have isolated a mouse genomic clone that contains the entire mouse Phgdh gene. Structural analysis demonstrated that the Phgdh gene spans approximately 27 kilobases (kb) in length and comprises 12 exons with 11 intervening introns. Using fluorescent in situ hybridization (FISH), we mapped the gene to mouse chromosome 3, region F2-F3. Analysis of a 1.8 kb fragment of the 5'-flanking region showed that the classical TATA-box motif near transcription initiation sites was absent. Instead, a GC-rich proximal region containing a potential Sp1 recognition sequence was present; this region is conserved in mouse, rat, and human counterparts. Transient transfection analysis revealed that the cis-acting elements necessary for basal transcription of Phgdh are contained within the -196/+4 proximal sequence of the promoter, in which the conserved Sp1 recognition sites play an important role for basal promoter activity.

Targeted disruption of the mouse 3-phosphoglycerate dehydrogenase gene causes severe neurodevelopmental defects and results in embryonic lethality

D-3-Phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95) is the first committed enzyme of L-serine biosynthesis in the phosphorylated pathway. To determine the physiological importance of Phgdh-dependent L-serine biosynthesis in vivo, we generated Phgdh-deficient mice using targeted gene disruption in embryonic stem cells. The absence of Phgdh led to a drastic reduction of L-serine metabolites such as phosphatidyl-L-serine and sphingolipids. Phgdh null embryos have small bodies with abnormalities in selected tissues and died after days post-coitum 13.5. Striking abnormalities were evident in the central nervous system in which the Phgdh null mutation culminated in hypoplasia of the telencephalon, diencephalon, and mesencephalon; in particular, the olfactory bulbs, ganglionic eminence, and cerebellum appeared as indistinct structures. These observations demonstrate that the Phgdh-dependent phosphorylated pathway is essential for normal embryonic development, especially for brain morphogenesis.

Nucleotide sequence and differential expression of the human 3-phosphoglycerate dehydrogenase gene

The nucleotide sequence of Hs 3-PGDH gene, encoding human 3-phosphoglycerate dehydrogenase that catalyzes the initiating step in the phosphorylated pathway of serine biosynthesis, has been determined. The 3-PGDH gene has a predicted 533 amino acid open reading frame, encoding a 56.8kDa protein that shares 94.0% similarity with rat-liver 3-PGDH. Two different transcripts corresponding to 3-PGDH mRNA were detected in human normal tissues. A dominant 2.1kb transcript was expressed at high levels in prostate, testis, ovary, brain, liver, kidney, and pancreas, and weakly expressed in thymus, colon, and heart. A 710bp transcript also appeared as a weaker band where the 2.1kb mRNA was expressed, and it was more significant than the 2.1kb mRNA in heart and skeletal muscle. The TPA-induced monocytic differentiation of U937, which also resulted in growth arrest, abruptly downregulated the expression of 3-PGDH. Removal of TPA restored cell growth through the retrodifferentiation process and subsequent expression of 3-PGDH. The 3-PGDH mRNA was markedly expressed in human leukemias, lymphoma Sup-T1, colon adenocarcinoma COLO 320DM, epitheloid carcinoma HeLa S3, and murine lymphoma BW5147.G.1.4, but not in human leukemia K562. This report demonstrates that the human 3-PGDH gene is regulated at the transcriptional level depending on tissue specificty and cellular proliferative status, and its transcriptional regulation mechanism may be a useful target for diagnosis and therapy of cancer.

Crystal structure of phosphoserine aminotransferase from Escherichia coli at 2.3 A resolution: comparison of the unligated enzyme and a complex with alpha-methyl-l-glutamate

Phosphoserine aminotransferase (PSAT; EC 2.6.1.52), a member of subgroup IV of the aminotransferases, catalyses the conversion of 3-phosphohydroxypyruvate to l-phosphoserine. The crystal structure of PSAT from Escherichia coli has been solved in space group P212121 using MIRAS phases in combination with density modification and was refined to an R-factor of 17.5% (Rfree=20.1 %) at 2.3 A resolution. In addition, the structure of PSAT in complex with alpha-methyl-l-glutamate (AMG) has been refined to an R-factor of 18.5% (Rfree=25.1%) at 2.8 A resolution. Each subunit (361 residues) of the PSAT homodimer is composed of a large pyridoxal-5'-phosphate binding domain (residues 16-268), consisting of a seven-stranded mainly parallel beta-sheet, two additional beta-strands and seven alpha-helices, and a small C-terminal domain, which incorporates a five-stranded beta-sheet and two alpha-helices. A three-dimensional structural comparison to four other vitamin B6-dependent enzymes reveals that three alpha-helices of the large domain, as well as an N-terminal domain (subgroup II) or subdomain (subgroup I) are absent in PSAT. Its only 15 N-terminal residues form a single beta-strand, which participates in the beta-sheet of the C-terminal domain. The cofactor is bound through an aldimine linkage to Lys198 in the active site. In the PSAT-AMG complex Ser9 and Arg335 bind the AMG alpha-carboxylate group while His41, Arg42 and His328 are involved in binding the AMG side-chain. Arg77 binds the AMG side-chain indirectly through a solvent molecule and is expected to position itself during catalysis between the PLP phosphate group and the substrate side-chain. Comparison of the active sites of PSAT and aspartate aminotransferase suggests a similar catalytic mechanism, except for the transaldimination step, since in PSAT the Schiff base is protonated. Correlation of the PSAT crystal structure to a published profile sequence analysis of all subgroup IV members allows active site modelling of nifs and the proposal of a likely molecular reaction mechanism.

Regulation of serine biosynthesis in Arabidopsis. Crucial role of plastidic 3-phosphoglycerate dehydrogenase in non-photosynthetic tissues

In plants, Ser is synthesized through a couple of pathways. 3-Phosphoglycerate dehydrogenase (PGDH), the first enzyme that is involved in the phosphorylated pathway of Ser biosynthesis, is responsible for the oxidation of 3-phosphoglycerate to phosphohydroxypyruvate. Here we report the first molecular cloning and characterization of PGDH from Arabidopsis thaliana. Sequence analysis of cDNA and a genomic clone revealed that the PGDH gene is composed of three exons, encoding a 623-amino acid polypeptide (66, 453 Da). The deduced protein, containing three of the most conserved regions in the NAD-dependent 2-hydroxyacid dehydrogenase family, has 38-39% identity to its animal and bacterial counterparts. The presence of an N-terminal signal sequence for translocation into plastids was confirmed by particle-gun bombardment experiments using green fluorescence protein as a reporter protein for subcellular localization. Southern hybridization analysis and restriction fragment length polymorphism mapping indicated that PGDH is a single-copy gene that is mapped to the upper arm of chromosome 1. Northern hybridization analysis indicated preferential expression of PGDH mRNA in root tissues of light-grown plants, suggesting that the phosphorylated pathway of Ser biosynthesis plays an important role in supplying Ser to non-photosynthetic tissues. The recombinant enzyme overproduced in Escherichia coli displayed hyperbolic kinetics with respect to 3-phosphoglycerate and NAD+.

Pathway of gluconeogenesis from D- and L-glycerate in rat hepatocytes

Synthesis of glucose from 10 mM D-glycerate by hepatocytes isolated from rats fasted 24 h was inhibited by 3-aminopicolinate (3AP), 3-mercaptopicolinate (3MP), and aminoxyacetate (AOA). The inhibition by AOA can be exerted over that caused by either of the picolinates. The effects of 3AP and 3MP were not additive, and cannot be ascribed totally to inhibition of reducing equivalent transfer by their inhibitory effect on phosphoenolpyruvate carboxykinase. The time course and dependence on substrate concentration of gluconeogenesis from L-glycerate was similar to those from D-glycerate, even though the cellular content of D-glycerate was markedly less when L-glycerate was the substrate. Gluconeogenesis from L-glycerate was not significantly affected by 3AP or 3MP, but was inhibited by AOA. The degree of inhibition by AOA of glucose synthesis from L-glycerate was greater than that from D-glycerate. Studies employing [14C]paraformaldehyde revealed that hydroxypyruvate may be converted to dihydroxyacetone in the hepatocyte. A hitherto unknown inhibition site of 3AP and 3MP is inferred and a dual pathway of D- and L-glycerate gluconeogenesis consistent with these results is proposed.

Effect of acute ethanol on serine biosynthesis in liver

The effect of an acute intraperitoneal dose of ethanol (1 g/kg), glucose (7.2 g/kg), or the combination of the two on the metabolite pattern of the biosynthetic pathway of L-serine has been determined in rabbit liver in vivo as has the effect of 10 mM ethanol on the glucose-, fructose-, or pyruvate-stimulated accumulation of L-serine in rabbit hepatocytes in vitro. In vivo, the 50% increase in L-serine and 80% increase in L-phosphoserine content of liver following glucose injection was completely prevented by ethanol. In fact, the L-phosphoserine content fell to only 6% of the control value. In spite of these and other significant changes in the metabolite pattern of the pathway of L-serine biosynthesis (D-3-phosphoglycerate dehydrogenase, L-phosphoserine aminotransferase (PSAT), and L-phosphoserine phosphatase), the mass action ratio of the combined reactions of the first two steps remained close to their equilibrium position. As a consequence it is estimated that the tissue content of phosphohydroxypyruvate fell to less than 2% of the control value, to approximately 0.3% of its Km for the PSAT reaction. The conclusion that acute ethanol blocks L-serine biosynthesis (presumably by redox effects) was supported by the prevention or inhibition of L-serine accumulation in hepatocytes metabolizing glucose, fructose, or pyruvate. Because L-serine is an important source of one-carbon fragments, the inhibition of its biosynthesis may be another mechanism by which ethanol interferes with folate and one-carbon metabolism.

Purification and properties of phosphoserine aminotransferase from bovine liver

L-Phosphoserine aminotransferase was purified from bovine liver to apparent homogeneity as judged by nondenaturing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, analytical ultracentrifugation, and immunochemical analysis. The purification procedure described involves the specific elution of the enzyme from Cibacron blue-agarose by micromolar concentrations of its substrate, phosphohydroxypyruvate. The purified enzyme had a specific activity of approximately 13 mumol of phosphohydroxypyruvate formed min-1 mg-1 of protein at 38 degrees C. Determinations of the native molecular weight and the subunit molecular weight indicated that the phosphoserine aminotransferase from bovine liver was a dimer composed of two subunits with identical molecular weights of 43,000.

Enzymes of serine metabolism in normal and neoplastic rat tissues

Enzymes involved in the pathway of de novo serine biosynthesis (L-phosphoserine aminotransferase) and in alternative pathways of serine utilization (L-serine hydroxymethyltransferase, L-serine dehydratase and L-serine aminotransferase) were assayed in normal adult and fetal rat tissues and in a range of transplantable rat tumors. Serine dehydratase and serine aminotransferase activities were essentially confined to normal adult liver and kidney, whereas phosphoserine aminotransferase and serine hydroxymethyltransferase activities showed a more ubiquitous tissue distribution. In particular, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were appreciable in neoplastic tissues, in the absence of the other enzymes of serine utilization. The pattern of enzyme distribution suggests that the synthesis of serine de novo is metabolically coupled to its utilization for nucleotide biosynthesis in tumors of differing tissue origins.

Enzymes of serine metabolism in normal, developing and neoplastic rat tissues

The cellular pattern of serine metabolism was conceptualized into four main areas of metabolic sequences: the biosynthesis of serine from intermediates of the glycolytic pathway (the so-called "phosphorylated pathway"); and alternative pathways of serine utilization initiated by serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. The known regulatory and adaptive properties of the enzymes involved in these pathways were reviewed in detail and key enzymes associated with each pathway (phosphoserine aminotransferase, serine dehydratase, serine aminotransferase, and serine hydroxymethyltransferase, respectively) were selected for further investigation. Tissue distribution studies in the rat revealed that whereas serine dehydratase and serine aminotransferase activities were largely confined to the liver, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were more broadly distributed. In particular in tissues with a high rate of cell turnover, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were coordinately increased. An increase in serine hydroxymethyltransferase activity coincided temporally with the incorporation of [3-14C]serine and thymidine into DNA in normal human lymphocytes during proliferation after mitogenic stimulation by phytohemagglutinin. The evidence suggested a primarily gluconeogenic role for serine dehydratase and serine aminotransferase. Serine hydroxymethyltransferase has a role in providing glycine and one-carbon folate co-factors as precursors for nucleotide biosynthesis and in some situations serves to metabolically couple the pathway of serine biosynthesis to utilization for de novo purine and pyrimidine synthesis. Multiple enzymic forms were distinguished for serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. For serine dehydratase the two cytosolic multiple forms had no apparent functional significance; the multiple forms were catalytically unmodified by conditions promoting phosphorylation-dephosphorylation in vitro. The mitochondrial form of serine aminotransferase showed adaptive responses in gluconeogenic situations, and the hypothesis was proposed that the mitochondrial isoenzyme of serine hydroxymethyltransferase is associated together with serine aminotransferase in a pathway for gluconeogenesis from protein-derived amino acids such as glycine and hydroxyproline. The adaptive behaviour of the enzymes during the neonatal development of rat liver revealed that serine aminotransferase reached a peak in the mid-suckling period at a time when gluconeogenesis is known to be increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydroxypyruvate-mediated regulation of oxalate synthesis by lactate dehydrogenase and its relevance to primary hyperoxaluria type II

Hydroxypyruvate (HP) brought about the decarboxylation of [1-14C] glyoxylate nonenzymically at all pH values considered. The rate of decomposition of glyoxylate increased with each increase in the concentrations of the reactants, the pH, and temperature and on the addition of the cations Fe2+, Mn2+, Mg2+, Zn2+, Co2+, and Cu2+. The addition of HP to a purified preparation of lactate dehydrogenase (LDH) catalyzing the oxidation of [1-14C]glyoxylate to [14C]oxalate in the presence of either NAD or NADH inhibited the production of oxalate. These observations have their implications in L-glyceric aciduria (primary hyperoxaluria type II), a syndrome characterized by the accumulation of HP and recurrent oxalosis. They suggest that the accumulating HP may reduce the contribution of intracellular glyoxylate to the formation of oxalate by competitively inhibiting the liver LDH. The involvement of liver LDH in oxalate synthesis and its postulated induction by HP and NAD in vivo are, therefore, reexamined.

Adaptation of urate synthesis in chicken liver

1. Urate synthesis was measured in hepatocytes from chickens after starvation or high-protein feeding. Adaptation occurred only on the high-protein diet. 2. The theoretical balances of reactions from alanine (5 alanine + 3 O2 = urate + 1.5 glucose + glycine) and asparagine (3 asparagine + 2 O2 = urate + ammonia + 0.5 glucose + glycine) agree reasonably well with the experimental results. 3. Enzymes directly involved in urate synthesis from these amino acids increase up to 12-fold on the high-protein diet; only amidophosphoribosyltransferase activity appears to be rate-limiting for urate synthesis. 4. The processes of nitrogen disposal in chicken and rat are compared and discussed.

Nitrogen economy and the metabolism of serine and glycine in reticulocytes of rabbits

In this work it is demonstrated the NH3 potentially available from the oxidation of amino acids by the reticulocyte is utilized for the new-synthesis of serine via transamination reactions with hydroxypyruvate and phosphohydroxypyruvate. These compounds are derived from glucose, which furnishes in this manner the carbon skeleton of serine. It was shown that serine is mainly degraded via glycine, while glycine is in the main formed from serine. Furthermore it was found that serine synthesis is localized in the cytosol, while the reversible transformation of serine to glycine takes place in the mitochondria. A transfer of methylenetetrahydrofolate occurs in both directions across the mitochondrial membrane. A concentrative uptake of serine by the mitochondria was found, while glycine is transported slowly in both directions. On the basis of the data an overall balance is given of the quantitative relations between protein breakdown and hemoglobin synthesis as well as for the new-formation and utilization of serine and glycine. The new-formation of serine may amount to about one tenth of the glucose utilized by the reticulocyte and furnishes about one-half of the serine and glycine moieties required for the synthesis of heme and globin. The remainder is provided by the energy-dependent protein breakdown.

The formation of oxalate from hydroxypyruvate, serine, glycolate and glyoxylate in the rat

[14C]Oxalate was found in the urine of fasted male Wistar rats fed L-[14C3]-serine, DL-[14C1]serine, [14C1]ethanolamine, [14C1]hydroxypyruvate, [14C3]-hydroxypyruvate, [14C1]glycolate or [14C2]glyoxylate. [14C1]Hydroxypyruvate and [14C1]ethanolamine were the least effective precursors of [14C]oxalate. 20% of the [14C1]serine and [14C3]serine administered was recovered as 14CO2, while less than 4% was metabolized by pathways known to contribute to oxalate synthesis. Oxalate synthesis from serine involved both the transamination to hydroxypyruvate and the conversion to glycine, but not decarboxylation to ethanolamine. The oxidation of [14C2]glyoxylate to [14C]oxalate in the rat was inhibited by hydroxypyruvate, but the oxidation of [14C1]glycolate to [14C]-oxalate was not significantly altered. [14C]Glycolaldehyde, [14C]glycolate, [14C]glyoxylate and [14C]oxalate were recovered in the urine of rats administered [14C3]hydroxypyruvate. This is consistent with the oxidation of hydroxypyruvate via glycolaldehyde leads to glycolate leads to glyoxylate leads to oxalate and is identical to the metabolic pathway for the oxidation of ethylene glycol to oxalate. However, the major metabolic intermediate recovered from [14C3]hydroxypyruvate was [14C]glyoxylate rather than [14C]glycolate, suggesting that an alternate pathway is contributing to the oxidation of hydroxypyruvate to oxalate in the rat.

The synthesis of oxylate from hydroxypyruvate by isolated perfused rat liver. The mechanism of hyperoxaluria in L-glyceric aciduria

Hydroxypyruvate and glycolate inhibited the oxidation of [U-14C]glyoxylate to [14C]oxalate in isolated perfused rat liver, but stimulated total oxalate and glycolate synthesis. [14C]Oxalate synthesis from [14C]glycine was similarly inhibited by hydroxypyruvate, but conversion of [14C1]glycolate to [14C]oxalate was increased three-fold. Pyruvate had no effect on the synthesis of [14C]-oxalate or total oxalate. The inhibition studies suggest that hydroxypyruvate is a precursor of glycolate and oxalate and that the conversion of glycolate to oxalate does not involve free glyoxylate as an intermediate. [14C35Hydroxypyruvate, but not [14C1]hydroxypyruvate, was oxidized to [14C]oxalate in isolated perfused rat liver. Isotope dilution studies indicate the major pathway involves the decarboxylation of hydroxypyruvate forming glycolaldehyde which is subsequently oxidized to oxalate via glycolate. The oxidation of serine which is subsequently oxidized to oxalate via glycolate. The oxidation of serine to oxalate appears to proceed predominantly via hydroxypyruvate rather than glycine or ethanolamine. The hyperoxaluria of L-glyceric aciduria, primary hyperoxaluria type II, is induced by the oxidation of the hydroxypyruvate, which accumulates because of the deficiency of D-glyceric dehydrogenase, to oxalate.