Pyrroline-5-carboxylate synthase activity in mammalian cells

Unraveling the evolutionary origin of the P5CS gene: a story of gene fusion and horizontal transfer

The accumulation of proline in response to the most diverse types of stress is a widespread defense mechanism. In prokaryotes, fungi, and certain unicellular eukaryotes (green algae), the first two reactions of proline biosynthesis occur through two distinct enzymes, γ-glutamyl kinase (GK E.C. 2.7.2.11) and γ-glutamyl phosphate reductase (GPR E.C. 1.2.1.41), encoded by two different genes, ProB and ProA, respectively. Plants, animals, and a few unicellular eukaryotes carry out these reactions through a single bifunctional enzyme, the Δ1-pyrroline-5-carboxylate synthase (P5CS), which has the GK and GPR domains fused. To better understand the origin and diversification of the P5CS gene, we use a robust phylogenetic approach with a broad sampling of the P5CS, ProB and ProA genes, including species from all three domains of life. Our results suggest that the collected P5CS genes have arisen from a single fusion event between the ProA and ProB gene paralogs. A peculiar fusion event occurred in an ancestral eukaryotic lineage and was spread to other lineages through horizontal gene transfer. As for the diversification of this gene family, the phylogeny of the P5CS gene in plants shows that there have been multiple independent processes of duplication and loss of this gene, with the duplications being related to old polyploidy events.

Single-Molecule Fluorescence Imaging Reveals Coassembly of CTPS and P5CS

The cellular compartmentation induced by self-assembly of natural proteins has recently attracted widespread attention due to its structural-functional significance. Among them, as a highly conserved metabolic enzyme and one of the potential targets for cancers and parasitic diseases in drug development, CTP synthase (CTPS) has also been reported to self-assemble into filamentous structures termed cytoophidia. To elucidate the dynamical mechanism of cytoophidium filamentation, we utilize single-molecule fluorescence imaging to observe the real-time self-assembly dynamics of CTPS and the coordinated assembly between CTPS and its interaction partner, Δ1-pyrroline-5-carboxylate synthase (P5CS). Significant differences exist in the direction of growth and extension when the two proteins self-assemble. The oligomer state distribution analysis of the CTPS minimum structural subunit under different conditions and the stoichiometry statistics of binding CTPS and P5CS by single-molecule fluorescence photobleach counting further confirm that the CTPS cytoophidia are mainly stacked with tetramers. CTPS can act as the nucleation core to induce the subsequent growth of the P5CS filaments. Our work not only provide evidence from the molecular level for the self-assembly and coordinated assembly (coassembly) of CTPS with its interaction partner P5CS in vitro but also offer new experimental perspectives for the dynamics research of coordinated regulation between other protein polymers.

Enhancement of l-Pipecolic Acid Production by Dynamic Control of Substrates and Multiple Copies of the pipA Gene in the Escherichia coli Genome

l-Pipecolic acid is an important rigid cyclic nonprotein amino acid, which is obtained through the conversion of l-lysine catalyzed by l-lysine cyclodeaminase (LCD). To directly produce l-pipecolic acid from glucose by microbial fermentation, in this study, a recombinant Escherichia coli strain with high efficiency of l-pipecolic acid production was constructed. This study involves the dynamic regulation of the substrate concentration and the expression level of the l-lysine cyclodeaminase-coding gene pipA. In terms of substrate concentration, we adopted the l-lysine riboswitch to dynamically regulate the expression of lysP and lysO genes. As a result, the l-pipecolic acid yield was increased about 1.8-fold as compared with the control. In addition, we used chemically inducible chromosomal evolution (CIChE) to realize the presence of multiple copies of the pipA gene on the genome. The resultant E. coli strain XQ-11-4 produced 61 ± 3.4 g/L l-pipecolic acid with a productivity of 1.02 ± 0.06 g/(L·h) and a glucose conversion efficiency (α) of 29.6% in fermentation. This is the first report that discovered multiple copies of pipA gene expression on the genome that improves the efficiency of l-pipecolic acid production in an l-lysine high-producing strain, and these results give us new insight for constructing the other valuable biochemicals derived from l-lysine.

Perspectives, past, present and future: the proline cycle/proline-collagen regulatory axis

In the 35 years since the introduction of the "proline cycle", its relevance to human tumors has been widely established. These connections are based on a variety of mechanisms discovered by many laboratories and have stimulated the search for small molecule inhibitors to treat cancer or metastases. In addition, the multi-layered connections of the proline cycle and the role of proline and hydroxyproline in collagen provide an important regulatory link between the extracellular matrix and metabolism.

P5C as an Interface of Proline Interconvertible Amino Acids and Its Role in Regulation of Cell Survival and Apoptosis

Studies of cancer metabolism have focused on the production of energy and the interconversion of carbons between cell cycles. More recently, amino acid metabolism, especially non-essential amino acids (NEAAs), has been investigated, underlining their regulatory role. One of the important mediators in energy production and interconversion of carbons in the cell is Δ¹-pyrroline-5-carboxylate (P5C)—the physiological intracellular intermediate of the interconversion of proline, ornithine, and glutamate. As a central component of these conversions, it links the tricarboxylic acid cycle (TCA), urea cycle (UC), and proline cycle (PC). P5C has a cyclic structure containing a tertiary nitrogen atom (N) and is in tautomeric equilibrium with the open-chain form of L-glutamate-γ-semialdehyde (GSAL). P5C is produced by P5C synthase (P5CS) from glutamate, and ornithine via ornithine δ-amino acid transferase (δOAT). It can also be converted to glutamate by P5C dehydrogenase (P5CDH). P5C is both a direct precursor of proline and a product of its degradation. The conversion of P5C to proline is catalyzed by P5C reductase (PYCR), while proline to P5C by proline dehydrogenase/oxidase (PRODH/POX). P5C-proline-P5C interconversion forms a functional redox couple. Their transformations are accompanied by the transfer of a reducing-oxidizing potential, that affect the NADP+/NADPH ratio and a wide variety of processes, e.g., the synthesis of phosphoribosyl pyrophosphate (PRPP), and purine ribonucleotides, which are crucial for DNA synthesis. This review focuses on the metabolism of P5C in the cell as an interconversion mediator of proline, glutamate, and ornithine and its role in the regulation of survival and death with particular emphasis on the metabolic context.

Activation of proline biosynthesis is critical to maintain glutamate homeostasis during acute methamphetamine exposure

Methamphetamine (METH) is a highly addictive psychostimulant that causes long-lasting effects in the brain and increases the risk of developing neurodegenerative diseases. The cellular and molecular effects of METH in the brain are functionally linked to alterations in glutamate levels. Despite the well-documented effects of METH on glutamate neurotransmission, the underlying mechanism by which METH alters glutamate levels is not clearly understood. In this study, we report an essential role of proline biosynthesis in maintaining METH-induced glutamate homeostasis. We observed that acute METH exposure resulted in the induction of proline biosynthetic enzymes in both undifferentiated and differentiated neuronal cells. Proline level was also increased in these cells after METH exposure. Surprisingly, METH treatment did not increase glutamate levels nor caused neuronal excitotoxicity. However, METH exposure resulted in a significant upregulation of pyrroline-5-carboxylate synthase (P5CS), the key enzyme that catalyzes synthesis of proline from glutamate. Interestingly, depletion of P5CS by CRISPR/Cas9 resulted in a significant increase in glutamate levels upon METH exposure. METH exposure also increased glutamate levels in P5CS-deficient proline-auxotropic cells. Conversely, restoration of P5CS expression in P5CS-deficient cells abrogated the effect of METH on glutamate levels. Consistent with these findings, P5CS expression was significantly enhanced in the cortical brain region of mice administered with METH and in the slices of cortical brain tissues treated with METH. Collectively, these results uncover a key role of P5CS for the molecular effects of METH and highlight that excess glutamate can be sequestered for proline biosynthesis as a protective mechanism to maintain glutamate homeostasis during drug exposure.

The proline synthesis enzyme P5CS forms cytoophidia in Drosophila

Compartmentation of enzymes via filamentation has arisen as a mechanism for the regulation of metabolism. In 2010, three groups independently reported that CTP synthase (CTPS) can assemble into a filamentous structure termed the cytoophidium. In searching for CTPS-interacting proteins, here we perform a yeast two-hybrid screening of Drosophila proteins and identify a putative CTPS-interacting protein, △¹-pyrroline-5-carboxylate synthase (P5CS). Using the Drosophila follicle cell as the in vivo model, we confirm that P5CS forms cytoophidia, which are associated with CTPS cytoophidia. Overexpression of P5CS increases the length of CTPS cytoophidia. Conversely, filamentation of CTPS affects the morphology of P5CS cytoophidia. Finally, in vitro analyses confirm the filament-forming property of P5CS. Our work links CTPS with P5CS, two enzymes involved in the rate-limiting steps in pyrimidine and proline biosynthesis, respectively.

Upregulation of the Glutaminase II Pathway Contributes to Glutamate Production upon Glutaminase 1 Inhibition in Pancreatic Cancer

The targeting of glutamine metabolism specifically via pharmacological inhibition of glutaminase 1 (GLS1) has been translated into clinical trials as a novel therapy for several cancers. The results, though encouraging, show room for improvement in terms of tumor reduction. In this study, the glutaminase II pathway is found to be upregulated for glutamate production upon GLS1 inhibition in pancreatic tumors. Moreover, genetic suppression of glutamine transaminase K (GTK), a key enzyme of the glutaminase II pathway, leads to the complete inhibition of pancreatic tumorigenesis in vivo unveiling GTK as a new metabolic target for cancer therapy. These results suggest that current trials using GLS1 inhibition as a therapeutic approach targeting glutamine metabolism in cancer should take into account the upregulation of other metabolic pathways that can lead to glutamate production; one such pathway is the glutaminase II pathway via GTK.

The Proline Cycle As a Potential Cancer Therapy Target

Interest in how proline contributes to cancer biology is expanding because of the emerging role of a novel proline metabolic cycle in cancer cell survival, proliferation, and metastasis. Proline biosynthesis and degradation involve the shared intermediate Δ1-pyrroline-5-carboxylate (P5C), which forms l-glutamate-γ-semialdehyde (GSAL) in a reversible non-enzymatic reaction. Proline is synthesized from glutamate or ornithine through GSAL/P5C, which is reduced to proline by P5C reductase (PYCR) in a NAD(P)H-dependent reaction. The degradation of proline occurs in the mitochondrion and involves two oxidative steps catalyzed by proline dehydrogenase (PRODH) and GSAL dehydrogenase (GSALDH). PRODH is a flavin-dependent enzyme that couples proline oxidation with reduction of membrane-bound quinone, while GSALDH catalyzes the NAD+-dependent oxidation of GSAL to glutamate. PRODH and PYCR form a metabolic relationship known as the proline-P5C cycle, a novel pathway that impacts cellular growth and death pathways. The proline-P5C cycle has been implicated in supporting ATP production, protein and nucleotide synthesis, anaplerosis, and redox homeostasis in cancer cells. This Perspective details the structures and reaction mechanisms of PRODH and PYCR and the role of the proline-P5C cycle in cancer metabolism. A major challenge in the field is to discover inhibitors that specifically target PRODH and PYCR isoforms for use as tools for studying proline metabolism and the functions of the proline-P5C cycle in cancer. These molecular probes could also serve as lead compounds in cancer drug discovery targeting the proline-P5C cycle.

Alpha-Ketoglutarate as a Molecule with Pleiotropic Activity: Well-Known and Novel Possibilities of Therapeutic Use

Alpha-ketoglutarate (AKG), an endogenous intermediary metabolite in the Krebs cycle, is a molecule involved in multiple metabolic and cellular pathways. It functions as an energy donor, a precursor in the amino acid biosynthesis, a signalling molecule, as well as a regulator of epigenetic processes and cellular signalling via protein binding. AKG is an obligatory co-substrate for 2-oxoglutarate-dependent dioxygenases, which catalyse hydroxylation reactions on various types of substrates. It regulates the activity of prolyl-4 hydroxylase, which controls the biosynthesis of collagen, a component of bone tissue. AKG also affects the functioning of prolyl hydroxylases, which, in turn, influences the function of the hypoxia-inducible factor, an important transcription factor in cancer development and progression. Additionally, it affects the functioning of enzymes that influence epigenetic modifications of chromatin: ten-eleven translocation hydroxylases involved in DNA demethylation and the Jumonji C domain containing lysine demethylases, which are the major histone demethylases. Thus, it regulates gene expression. The metabolic and extrametabolic function of AKG in cells and the organism open many different fields for therapeutic interventions for treatment of diseases. This review presents the results of studies conducted with the use of AKG in states of protein deficiency and oxidative stress conditions. It also discusses current knowledge about AKG as an immunomodulatory agent and a bone anabolic factor. Additionally, the regulatory role of AKG and its structural analogues in carcinogenesis as well as the results of studies of AKG as an anticancer agent are discussed.

Evolution of plant δ(1)-pyrroline-5-carboxylate reductases from phylogenetic and structural perspectives

Proline plays a crucial role in cell growth and stress responses, and its accumulation is essential for the tolerance of adverse environmental conditions in plants. Two routes are used to biosynthesize proline in plants. The main route uses glutamate as a precursor, while in the other route proline is derived from ornithine. The terminal step of both pathways, the conversion of δ(1)-pyrroline-5-carboxylate (P5C) to L-proline, is catalyzed by P5C reductase (P5CR) using NADH or NADPH as a cofactor. Since P5CRs are important housekeeping enzymes, they are conserved across all domains of life and appear to be relatively unaffected throughout evolution. However, global analysis of these enzymes unveiled significant functional diversity in the preference for cofactors (NADPH vs. NADH), variation in metal dependence and the differences in the oligomeric state. In our study we investigated evolutionary patterns through phylogenetic and structural analysis of P5CR representatives from all kingdoms of life, with emphasis on the plant species. We also attempted to correlate local sequence/structure variation among the functionally and structurally characterized members of the family.

The structure of Medicago truncatula δ(1)-pyrroline-5-carboxylate reductase provides new insights into regulation of proline biosynthesis in plants

The two pathways for proline biosynthesis in higher plants share the last step, the conversion of δ(1)-pyrroline-5-carboxylate (P5C) to L-proline, which is catalyzed by P5C reductase (P5CR, EC 1.5.1.2) with the use of NAD(P)H as a coenzyme. There is increasing amount of evidence to suggest a complex regulation of P5CR activity at the post-translational level, yet the molecular basis of these mechanisms is unknown. Here we report the three-dimensional structure of the P5CR enzyme from the model legume Medicago truncatula (Mt). The crystal structures of unliganded MtP5CR decamer, and its complexes with the products NAD(+), NADP(+), and L-proline were refined using x-ray diffraction data (at 1.7, 1.85, 1.95, and 2.1 Å resolution, respectively). Based on the presented structural data, the coenzyme preference for NADPH over NADH was explained, and NADPH is suggested to be the only coenzyme used by MtP5CR in vivo. Furthermore, the insensitivity of MtP5CR to feed-back inhibition by proline, revealed by enzymatic analysis, was correlated with structural features. Additionally, a mechanism for the modulation of enzyme activity by chloride anions is discussed, as well as the rationale for the possible development of effective enzyme inhibitors.

Apicidin F: characterization and genetic manipulation of a new secondary metabolite gene cluster in the rice pathogen Fusarium fujikuroi

The fungus F. fujikuroi is well known for its production of gibberellins causing the ‘bakanae’ disease of rice. Besides these plant hormones, it is able to produce other secondary metabolites (SMs), such as pigments and mycotoxins. Genome sequencing revealed altogether 45 potential SM gene clusters, most of which are cryptic and silent. In this study we characterize a new non-ribosomal peptide synthetase (NRPS) gene cluster that is responsible for the production of the cyclic tetrapeptide apicidin F (APF). This new SM has structural similarities to the known histone deacetylase inhibitor apicidin. To gain insight into the biosynthetic pathway, most of the 11 cluster genes were deleted, and the mutants were analyzed by HPLC-DAD and HPLC-HRMS for their ability to produce APF or new derivatives. Structure elucidation was carried out be HPLC-HRMS and NMR analysis. We identified two new derivatives of APF named apicidin J and K. Furthermore, we studied the regulation of APF biosynthesis and showed that the cluster genes are expressed under conditions of high nitrogen and acidic pH in a manner dependent on the nitrogen regulator AreB, and the pH regulator PacC. In addition, over-expression of the atypical pathway-specific transcription factor (TF)-encoding gene APF2 led to elevated expression of the cluster genes under inducing and even repressing conditions and to significantly increased product yields. Bioinformatic analyses allowed the identification of a putative Apf2 DNA-binding (“Api-box”) motif in the promoters of the APF genes. Point mutations in this sequence motif caused a drastic decrease of APF production indicating that this motif is essential for activating the cluster genes. Finally, we provide a model of the APF biosynthetic pathway based on chemical identification of derivatives in the cultures of deletion mutants.

A Molecular Dynamics (MD) and Quantum Mechanics/Molecular Mechanics (QM/MM) study on Ornithine Cyclodeaminase (OCD): a tale of two iminiums

Ornithine cyclodeaminase (OCD) is an NAD+-dependent deaminase that is found in bacterial species such as Pseudomonas putida. Importantly, it catalyzes the direct conversion of the amino acid L-ornithine to L-proline. Using molecular dynamics (MD) and a hybrid quantum mechanics/molecular mechanics (QM/MM) method in the ONIOM formalism, the catalytic mechanism of OCD has been examined. The rate limiting step is calculated to be the initial step in the overall mechanism: hydride transfer from the L-ornithine's C(α)-H group to the NAD+ cofactor with concomitant formation of a C(α)=NH(2)+ Schiff base with a barrier of 90.6 kJ mol-1. Importantly, no water is observed within the active site during the MD simulations suitably positioned to hydrolyze the C(α)=NH(2)+ intermediate to form the corresponding carbonyl. Instead, the reaction proceeds via a non-hydrolytic mechanism involving direct nucleophilic attack of the δ-amine at the C(α)-position. This is then followed by cleavage and loss of the α-NH(2) group to give the Δ1-pyrroline-2-carboxylate that is subsequently reduced to L-proline.

Glutamine: precursor or nitrogen donor for citrulline synthesis?

Although glutamine is considered the main precursor for citrulline synthesis, the current literature does not differentiate between the contribution of glutamine carbon skeleton vs. nonspecific nitrogen (i.e., ammonia) and carbon derived from glutamine oxidation. To elucidate the role of glutamine and nonspecific nitrogen in the synthesis of citrulline, l-[2-(15)N]- and l-[5-(15)N]glutamine and (15)N-ammonium acetate were infused intragastrically in mice. The amino group of glutamine labeled the three nitrogen groups of citrulline almost equally. The amido group and ammonium acetate labeled the ureido and amino groups of citrulline, but not the delta-nitrogen. D(5)-glutamine also infused in this arm of the study, which traces the carbon skeleton of glutamine, was utilized poorly, accounting for only 0.2-0.4% of the circulating citrulline. Dietary glutamine nitrogen (both N groups) incorporation was 25-fold higher than the incorporation of its carbon skeleton into citrulline. To investigate the relative contributions of the carbon skeleton and nonspecific carbon of glutamine, arginine, and proline to citrulline synthesis, U-(13)C(n) tracers of these amino acids were infused intragastrically. Dietary arginine was the main precursor for citrulline synthesis, accounting for approximately 40% of the circulating citrulline. Proline contribution was minor (3.4%), and glutamine was negligible (0.4%). However, the glutamine tracer resulted in a higher enrichment in the ureido group, indicating incorporation of nonspecific carbon from glutamine oxidation into carbamylphosphate used for citrulline synthesis. In conclusion, dietary glutamine is a poor carbon skeleton precursor for the synthesis of citrulline, although it contributes both nonspecific nitrogen and carbon to citrulline synthesis.

Pyrroline-5-carboxylate synthase and proline biosynthesis: from osmotolerance to rare metabolic disease

Pyrroline-5-carboxylate synthase (P5CS) is a bifunctional enzyme that exhibits glutamate kinase (GK) and gamma-glutamyl phosphate reductase (GPR) activities. The enzyme is highly relevant in humans because it belongs to a combined route for the interconversion of glutamate, ornithine and proline. The deficiency of P5CS activity in humans is associated with a rare, inherited metabolic disease. It is well established that some bacteria and plants accumulate proline in response to osmotic stress. The alignment of P5CSs from different species and analysis of the solved structures of GK and GPR reveal high sequence and structural conservation. The information acquired from different mutant enzymes with increased osmotolerant properties, together with the position of the insertion found in the longer human isoform, permit the delimitation of the regulatory site of GK and P5CS and the proposal of a model of P5CS architecture. Additionally, the GK moiety of the human enzyme has been modeled and the known clinical mutations and polymorphisms have been mapped.

A new method for accurately measuring Delta(1)-pyrroline-5-carboxylate synthetase activity

Proline is a key factor in plant adaptation to environmental stresses. The Delta(1)-pyrroline-5-carboxylate synthetase catalyzes the first committed step and the rate-limiting step for proline biosynthesis in both plants and mammals. This enzyme catalyzes the reduction of glutamate to pyrroline-5-carboxylate in two sequential steps including the phosphorylation and the reduction of its precursor. Several methods were established to assay P5CS activity but however none of them are fully reliable. Therefore, we developed a new simple and reliable assay which is based on the quantification of Pi. This assay allowed us to determine the optimal pH, the apparent K(m) and V(m) of P5CS with regard to ATP and glutamate.

Glutamine metabolism and function in relation to proline synthesis and the safety of glutamine and proline supplementation

At normal intakes, dietary glutamine and glutamate are metabolized by the small intestine and essentially all glutamine within the body is synthesized de novo through the action of glutamine synthetase. The major sites of net glutamine synthesis are skeletal muscle, lung, and adipose tissue and, under some conditions, the liver. In addition to the small intestine, where glutamine is the major respiratory fuel, other sites of net glutamine utilization include the cells of the immune system, the kidneys, and the liver. The intestine expresses pyrroline 5-carboxylate (P5C) synthase, which means that proline is an end product of intestinal glutamine catabolism. Proline can also be synthesized from ornithine and the exact contribution of the 2 pathways is not certain. Infusion of proline i.v. to increase circulating concentrations is associated with increased proline oxidation and decreased proline synthesis. In contrast, conditions of proline insufficiency, after feeding low-proline diets or in response to high rates of proline catabolism in burn patients, do not result in increased proline synthesis. Glutamine supplementation is widespread and up to 0.57-0.75 g.kg(-1).d(-1) is well tolerated. Similarly, the only study of proline supplementation, in which patients with gyrate atrophy were given 488 mg.kg(-1).d(-1), reported no deleterious side effects. In the absence of controlled trials, it is currently not possible to estimate a safe upper limit for either of these 2 amino acids.

Pipecolic acid in microbes: biosynthetic routes and enzymes

Pipecolic acid is an important precursor of many useful microbial secondary metabolites. Pipecolic acid-derived moieties are often crucial for the biological activities of some microbial natural products with pharmaceutical applications. Understanding the biogenesis of pipecolic acid in microorganisms would be a significant step toward the mutasynthesis of novel analogs of choice. This review focuses on various microbial pathways and enzymes for pipecolic acid synthesis, especially those related to the origination of pipecolic acid moieties in secondary metabolites.

Crystal structures of delta1-pyrroline-5-carboxylate reductase from human pathogens Neisseria meningitides and Streptococcus pyogenes

L-proline is an amino acid that plays an important role in proteins uniquely contributing to protein folding, structure, and stability, and this amino acid serves as a sequence-recognition motif. Proline biosynthesis can occur via two pathways, one from glutamate and the other from arginine. In both pathways, the last step of biosynthesis, the conversion of delta1-pyrroline-5-carboxylate (P5C) to L-proline, is catalyzed by delta1-pyrroline-5-carboxylate reductase (P5CR) using NAD(P)H as a cofactor. We have determined the first crystal structure of P5CR from two human pathogens, Neisseria meningitides and Streptococcus pyogenes, at 2.0 angstroms and 2.15 angstroms resolution, respectively. The catalytic unit of P5CR is a dimer composed of two domains, but the biological unit seems to be species-specific. The N-terminal domain of P5CR is an alpha/beta/alpha sandwich, a Rossmann fold. The C-terminal dimerization domain is rich in alpha-helices and shows domain swapping. Comparison of the native structure of P5CR to structures complexed with L-proline and NADP+ in two quite different primary sequence backgrounds provides unique information about key functional features: the active site and the catalytic mechanism. The inhibitory L-proline has been observed in the crystal structure.

Molecular enzymology of mammalian Delta1-pyrroline-5-carboxylate synthase. Alternative splice donor utilization generates isoforms with different sensitivity to ornithine inhibition

Delta1-Pyrroline-5-carboxylate synthase (P5CS; EC not assigned), a mitochondrial inner membrane, ATP- and NADPH-dependent, bifunctional enzyme, catalyzes the reduction of glutamate to Delta1-pyrroline-5-carboxylate, a critical step in the de novo biosynthesis of proline and ornithine. We utilized published plant P5CS sequence to search the expressed sequence tag data base and cloned two full-length human P5CS cDNAs differing in length by 6 base pairs (bp) in the open reading frame. The short cDNA has a 2379-bp open reading frame encoding a protein of 793 residues; the long cDNA, generated by "exon sliding," a form of alternative splicing, contains an additional 6-bp insert following bp +711 of the short form resulting in inclusion of two additional amino acids in the region predicted to be the gamma-glutamyl kinase active site of P5CS. The long form predominates in all tissues examined except gut. We also isolated the corresponding long and short murine P5CS transcripts. To confirm the identity of the putative P5CS cDNAs, we expressed both human forms in gamma-glutamyl kinase- and gamma-glutamyl phosphate reductase-deficient strains of Saccharomyces cerevisiae and showed that they conferred the proline prototrophy. Additionally, we found expression of the murine putative P5CS cDNAs conferred proline prototrophy to P5CS-deficient Chinese hamster ovary cells (CHO-K1). We utilized stable CHO-K1 cell transformants to compare the biochemical characteristics of the long and short murine P5CS isoforms. We found that both confer P5CS activity and that the short isoform is inhibited by L-ornithine with a Ki of approximately 0.25 mM. Surprisingly, the long isoform is insensitive to ornithine inhibition. Thus, the two amino acid insert in the long isoform abolishes feedback inhibition of P5CS activity by L-ornithine.

Endogenous synthesis of arginine plays an important role in maintaining arginine homeostasis in postweaning growing pigs

This study was conducted to determine whether endogenous synthesis of arginine plays a role in regulating arginine homeostasis in postweaning pigs. Pigs were fed a sorghum-based diet containing 0. 98% arginine and were used for studies at 75 d of age (28.4 kg body weight). Mitochondria were prepared from the jejunum and other major tissues for measuring the activities of Delta1-pyrroline-5-carboxylate (P5C) synthase and proline oxidase (enzymes catalyzing P5C synthesis from glutamate and proline, respectively) and of ornithine aminotransferase (OAT) (the enzyme catalyzing the interconversion of P5C into ornithine). For metabolic studies, jejunal enterocytes were incubated at 37 degrees C for 30 min in Krebs-Henseleit bicarbonate buffer containing 2 mmol/L L-glutamine, 2 mmol/L L-[U-14C]proline, and 0-200 micromol/L gabaculine (an inhibitor of OAT). The activities of P5C synthase, proline oxidase and OAT were greatest in enterocytes among all of the tissues studied. Incubation of enterocytes with gabaculine resulted in decreases (P < 0.05) in the synthesis of ornithine and citrulline from glutamine and proline. When gabaculine was orally administered to pigs (0.83 mg/kg body weight) to inhibit intestinal synthesis of citrulline from glutamine and proline, plasma concentrations of citrulline (-26%) and arginine (-22%) decreased (P < 0.05), whereas those of alanine (+21%), ornithine (+17%), proline (+107%), taurine (+56%) and branched-chain amino acids (+21-40%) increased (P < 0.05). On the basis of dietary arginine intake and estimated arginine utilization, the endogenous synthesis of arginine in the 28-kg pig provided >/=50.2% of total daily arginine requirement. Taken together, our results suggest an important role for endogenous synthesis of arginine in regulating arginine homeostasis in postweaning growing pigs, as previously shown in neonatal pigs.

Molecular characterization of the proline-1 (pro-1) locus of Neurospora crassa, which encodes delta 1-pyrroline-5-carboxylate reductase

delta 1-pyrroline-5-carboxylate reductase (P5CR; [L-proline: NAD(P+) 5-oxidoreductase]; EC 1.5.1.2) catalyzes the final step in proline biosynthesis. We have shown that the proline-1 (pro-1) locus of Neurospora crassa encodes P5CR. The pro-1 gene was localized to a 3.2 kb region by complementation of (restoration of proline-independent growth to) a proline auxotroph carrying a recessive mutation at the pro-1 locus. The nucleotide sequence of this 3.2 kb region contains an open reading frame with coding capacity of 311 amino acids. The deduced polypeptide shows significant similarity to P5CR amino acid sequences. Similarity of N. crassa P5CR is greatest to that of the yeast, Saccharomyces cerevisiae, but is also strong to P5CR sequences from archaea, eubacteria, plants, and humans. In N. crassa, amino acid imbalance, including deficiency or excess of a single amino acid, such as histidine, induces expression of many amino acid biosynthetic genes that are under cross-pathway control, a general regulatory system analogous to general amino acid control in Saccharomyces. Although P5CR catalyzes the only committed step in proline biosynthesis, pro-1 expression was unaltered by histidine starvation and independent of CPC1, a positively acting transcription factor that mediates cross-pathway control in N. crassa.

A bifunctional enzyme (delta 1-pyrroline-5-carboxylate synthetase) catalyzes the first two steps in proline biosynthesis in plants

Many plants synthesize and accumulate proline in response to osmotic stress. Despite the importance of this pathway, however, the exact metabolic route and enzymes involved in the synthesis of proline in plants have not been unequivocally identified. We report here the isolation of a mothbean (Vigna aconitifolia) cDNA clone encoding a bifunctional enzyme, delta 1-pyrroline-5-carboxylate synthetase (P5CS), with both gamma-glutamyl kinase and glutamic-gamma-semialdehyde dehydrogenase activities that catalyzes the first two steps in proline biosynthesis. The two enzymatic domains of P5CS correspond to the ProB and ProA proteins of Escherichia coli and contain a leucine zipper in each domain, which may facilitate inter- or intramolecular interaction of this protein. The Vigna P5CS enzyme activity is feedback regulated by proline but is less sensitive to end-product inhibition than is the E. coli gamma-glutamyl kinase. The P5CS gene is expressed at high levels in Vigna leaves and is inducible in roots subjected to salt stress, suggesting that P5CS plays a key role in proline biosynthesis, leading to osmoregulation in plants.

Proline biosynthesis in Saccharomyces cerevisiae: molecular analysis of the PRO1 gene, which encodes gamma-glutamyl kinase

The PRO1 gene of Saccharomyces cerevisiae encodes the 428-amino-acid protein gamma-glutamyl kinase (ATP:L-glutamate 5-phosphotransferase, EC 2.7.2.11), which catalyzes the first step in proline biosynthesis. Amino acid sequence comparison revealed significant homology between the yeast and Escherichia coli gamma-glutamyl kinases throughout their lengths. Four close matches to the consensus sequence for GCN4 protein binding and one close match to the RAP1 protein-binding site were found in the PRO1 upstream region. The response of the PRO1 gene to changes in the growth medium was analyzed by measurement of steady-state mRNA levels and of beta-galactosidase activity encoded by a PRO1-lacZ gene fusion. PRO1 expression was not repressed by exogenous proline and was not induced by the presence of glutamate in the growth medium. Although expression of the PRO1 gene did not change in response to histidine starvation, both steady-state PRO1 mRNA levels and beta-galactosidase activities were elevated in a gcd1 strain and reduced in a gcn4 strain. In addition, a pro1 bradytrophic strain became completely auxotrophic for proline in a gcn4 strain background. These results indicate that PRO1 is regulated by the general amino acid control system.

Effect of exogenous amino acids on the intracellular content of proline and other amino acids in Daucus carota cells

The effect of tryptophan on the biosynthesis of proline has been investigated. Cells of Daucus carota grown in B5 medium supplemented with 5×10(-4)M tryptophan acquired the ability to grow in the presence of inhibitory concentrations of azetidine-2-carboxylic acid, an analog of proline. When trp was added to carrot cell cultures at sub-growth inhibiting concentrations, overproduction of intracellular free proline was observed. An increase was also observed for lys, his, ala, leu and phe. Likewise, the addition of asparagine, glutamic acid and phenylalanine to the medium stimulated the intracellular increase of free proline and other amino acids.

Compartmentalization of proline pools and apparent rates of collagen and non-collagen protein synthesis in arterial smooth muscle cells in culture

Rates of collagen and non-collagen protein synthesis in rabbit arterial smooth muscle cells (SMC) were determined by using the specific (radio)activity of [3H]proline in the extracellular, intracellular, and prolyl-tRNA pools. The intracellular free proline specific activity was only 25% of the extracellular value in cultures incubated for 12 h in 0.25 mM-proline. The specific activity of prolyl-tRNA was less than 10% of the extracellular specific activity. Increasing the extracellular proline concentration 10-fold (to 2.5 mM), while keeping the extracellular specific activity of proline constant, resulted in equilibration of the specific activities of intracellular and extracellular free proline, but the specific activity of prolyl-tRNA remained at less than 10% of the extracellular specific activity. Therefore, calculated rates of collagen and non-collagen protein synthesis were greatly underestimated using the intracellular or extracellular specific activity of proline. SMC were also incubated with 0.1 mM-[14C]ornithine in 0.25 nM or 2.5 mM non-labelled proline to examine synthesis de novo of proline and prolyl-tRNA from ornithine. In SMC cultures containing 0.25 mM unlabelled proline, the specific activity of intracellular ornithine was approx. 45% of the extracellular specific activity, due to the production of unlabelled ornithine. The specific activity of ornithine-derived intracellular free proline in SMC incubated with 2.5 mM-proline was significantly lower than in SMC incubated in 0.25 mM-proline, due to the influx of unlabelled proline. However, a corresponding difference in the specific activity of [14C]prolyl-tRNA between SMC in 0.25 mM- or 2.5 mM-proline was not observed. Ornithine-derived [14C]proline was incorporated into proteins in a manner different from that of exogenously added radiolabelled proline. A much higher proportion of the proline synthesized de novo was channelled into collagen synthesis relative to total protein synthesis. Together, these results show that intracellular proline pools are highly compartmentalized in arterial SMC. They also suggest that proline synthesized from ornithine may enter a prolyl-tRNA pool separate from that of proline entering from the extracellular medium.

High-frequency induction by 5-azacytidine of proline independence in CHO-K1 cells

Proline independence in CHO-K1 Chinese hamster cells has in previous studies been characterized as an auxotrophic gene mutation. In the absence of direct proof, an alternative model must be considered, based on suppression of proline synthesis by DNA methylation changes at one or more loci concerned. This concept receives strong support from the present study, in which we show that treatment of CHO-K1 cells with 5-azacytidine induces a 10(5)-10(6) increase in background conversion to the proline-independent state. Revertants thus obtained, as well as those arising spontaneously or after treatment with ethyl methane sulfonate, are stable phenotypically in the presence or absence of proline. Proline independence in all variants examined was correlated with increased activity of pyrroline-5-carboxylate synthase. Four of five variants induced with 5-azacytidine showed simultaneous increases in activity of ornithine aminotransferase as well. Our data suggest that epigenetic, rather than genetic changes, underlie the transitions between proline dependence and independence in CHO-K1 cells.

Mutant cell lines resistant to azetidine-2-carboxylic acid: alterations in the synthesis of proline from glutamic acid

Two mutant Chinese hamster lung fibroblast lines have been isolated that are resistant to the toxic proline analog L-azetidine-2-carboxylic acid. The line designated AZCA-1 has 30-fold elevated activity of pyrroline-5-carboxylate synthase and a large increase in the rate of proline production and release compared to controls. Pyrroline-5-carboxylate synthase activity is not elevated in the resistant line designated AZCA-4, but the enzyme is less sensitive to inhibition by ornithine and proline than control enzyme. Intracellular proline is elevated in AZCA-4 cells, with no change in the rate of release of proline synthesized from glutamate. Resistance to azetidine carboxylic acid in both mutant lines is attributed to the expanded intracellular proline pool that results from alterations in pyrroline-5-carboxylate synthase. These results indicate that intracellular proline levels are determined at least in part by the regulated activity of pyrroline-5-carboxylate synthase.

Studies on the pathway from ornithine to proline in cultured skin fibroblasts with reference to the defect in hyperornithinaemia with hyperammonaemia and homocitrullinuria

The reduction in the conversion of ornithine to proline by fibroblasts from a patient with hyperornithinaemia with hyperammonaemia and homocitrullinuria cannot be explained by a reduced uptake of exogenous ornithine, an altered total intracellular ornithine content, or reduced conversion of gamma-glutamate semialdehyde to proline. However, neither could the postulated defect in mitochondrial ornithine uptake be demonstrated using the digitonin method. Increasing the ornithine concentration in the medium increased the incorporation of 14C label from ornithine into protein in both the patient's and control cells. In the patient's cells the apparent Km for ornithine was ten times that of the controls, although the Vmax values were comparable. This result parallels the clinical response to ornithine supplementation.

A new synaptosomal biosynthetic pathway of glutamate and GABA from ornithine and its negative feedback inhibition by GABA

In sonicates of mouse brain synaptosomes, we demonstrated that gamma-aminobutyric acid (GABA) can be formed when L-ornithine (Orn) through L-glutamic acid (Glu), but not through putrescine (Put). Incubation of these sonicates with [3H]ORN yielded not only [3H]Glu and [3H]L-proline (Pro) but also produced [3H]GABA from the [3H]Glu. Formation of each of these three major amino acids from [3H]Orn was strongly inhibited by the addition of GABA (1-5 mM). The likely enzymatic site of this negative feedback inhibition by GABA appeared to be ornithine delta-aminotransferase (OAT). A radiometric procedure was employed to study the effects of the three amino acids cited above and of others found in the free form in brain on the activity of a 30-fold-purified OAT from rat brain. Enzyme activity was measured in the presence of low concentrations of Orn, such as might occur in vivo. OAT was inhibited by GABA to a considerably greater extent than by Glu, L-glutamine, or Put; no inhibition was found with Pro, glycine, aspartarte, taurine, or beta-alanine. The inhibition of GABA was competitive with Orn. These results clearly show that one of the molecular mechanisms underlying the negative feedback inhibition of synaptosomal GABA biosynthesis from Orn is a competitive inhibition by GABA of the brain OAT activity that is responsible for the formation of L-glutamic-gamma-semialdehyde in equilibrium with L-delta 1-pyrroline-5-carboxylic acid from Orn. Thus, the results suggest that GABA may play an important role in restricting the metabolic flow from Orn to Glu and thence to GABA. It is confirmed that L-canaline (delta-aminooxy-L-alpha-aminobutyric acid) is a potent and specific inhibitor of brain OAT whereas much weaker inhibition was observed with two other carbonyl-trapping agents, aminooxyacetic acid and hydrazine.

Effects of arginine-free meals on ureagenesis in cats

Cats given a single arginine-free meal have been reported to develop severe hyperammonemia, attributed to impaired function of ornithine aminotransferase (OAT). We found that cats that developed hyperammonemia following an arginine-free meal had low hepatic ornithine levels. However, the average sum of hepatic ornithine plus arginine plus citrulline rose, indicating that some ornithine synthesis via OAT took place, and hyperammonemia failed to occur in cats with higher hepatic ornithine levels. OAT activity and kinetic constants were comparable to values reported in the rat. Furthermore, dietary supplementation with ornithine caused only occasional and transient hyperornithinemia. Thus, OAT can function in the cat. The Ka of N-acetylglutamate (AGA) synthetase for arginine was 5 times higher in cats than in rats, but AGA content and citrullinogenesis by intact mitochondria were the same following arginine-free or arginine-containing meals. Other kinetic parameters of AGA synthetase and carbamoylphosphate synthetase were similar to values in the rat. We conclude that low levels of hepatic ornithine are probably responsible for making some cats susceptible to hyperammonemia following this stimulus.