THE SUCCINATE-GLYCINE CYCLE

Metabolic trends of Chinese hamster ovary cells in biopharmaceutical production under batch and fed-batch conditions

Extensive knowledge of Chinese hamster ovary (CHO) cell metabolism is required to improve process productivity and culture performance in biopharmaceutical manufacturing. However, CHO cells show a dynamic metabolism during culturing in batch and fed-batch bioreactors. CHO cell metabolism is generally described as taking place in three stages: exponential growth phase, stationary phase, and death phase. This review aims to summarize the trends of central metabolism for CHO cells during each stage. Additional insights into how culture conditions are related to phase transitions and force metabolic rewiring are provided. Understanding of CHO cell metabolism lends itself to improving culture qualities by, for example, identifying sources of toxic byproducts and pathways for cellular engineering. In summary, this review describes the changes in CHO cell central metabolism over the course of the culture.

[Development of Medicines for Infectious Diseases -Malaria]

In developed countries, it is said that "threats of infectious diseases are already thought as things of the past". However, as you can see in the case of Ebola hemorrhagic fever that occurred in West Africa, this is a big mistake. Among infectious diseases, only smallpox has been successfully eradicated worldwide. In addition to the three major infectious diseases of HIV/AIDS, tuberculosis, and malaria, there is another group called emerging and reemerging infectious diseases. Recently, neglected tropical diseases (NTDs) have been listed as threats by the WHO, as have drug-resistant bacteria. The spread of these pathogens is increasing due to an increase in global travel. Malaria and more than half of the NTDs are parasitic diseases, such as trypanosomiasis and soil-borne helminthiasis. These are caused by parasites, with eukaryotes similar to their host mammals. In the case of these NTDs, protective immune responses induced by differences between a pathogen and host do not work well, and there is no vaccine against parasites. As for drugs developed to treat these diseases, because the properties of enzymes and target receptors are very similar, and effective drugs simultaneously show efficacy against both the disease and the host, severe side effects often occur. Therefore, the search for targets specifically present in parasites, and screening for drugs that inhibit their physiological functions, is extremely important. Here, as an example of the development of antiparasitic drugs, I will introduce a study on malaria.

The heme biosynthesis pathway is essential for Plasmodium falciparum development in mosquito stage but not in blood stages

Heme is an essential cofactor for aerobic organisms. Its redox chemistry is central to a variety of biological functions mediated by hemoproteins. In blood stages, malaria parasites consume most of the hemoglobin inside the infected erythrocytes, forming nontoxic hemozoin crystals from large quantities of heme released during digestion. At the same time, the parasites possess a heme de novo biosynthetic pathway. This pathway in the human malaria parasite Plasmodium falciparum has been considered essential and is proposed as a potential drug target. However, we successfully disrupted the first and last genes of the pathway, individually and in combination. These knock-out parasite lines, lacking 5-aminolevulinic acid synthase and/or ferrochelatase (FC), grew normally in blood-stage culture and exhibited no changes in sensitivity to heme-related antimalarial drugs. We developed a sensitive LC-MS/MS assay to monitor stable isotope incorporation into heme from its precursor 5-[(13)C4]aminolevulinic acid, and this assay confirmed that de novo heme synthesis was ablated in FC knock-out parasites. Disrupting the FC gene also caused no defects in gametocyte generation or maturation but resulted in a greater than 70% reduction in male gamete formation and completely prevented oocyst formation in female Anopheles stephensi mosquitoes. Our data demonstrate that the heme biosynthesis pathway is not essential for asexual blood-stage growth of P. falciparum parasites but is required for mosquito transmission. Drug inhibition of pathway activity is therefore unlikely to provide successful antimalarial therapy. These data also suggest the existence of a parasite mechanism for scavenging host heme to meet metabolic needs.

Cloning of the δ-aminolevulinic acid synthase structural gene in yeast

HEM1, the structural gene for δ-aminolevulinic acid synthase, has been isolated on recombinant plasmids. A yeast genomic pool constructed in the E. coli - yeast shuttle vector YEp13 was used to clone the HEM1 gene by complementation. A leu2 hem1 yeast mutants was transformed with the yeast genomic pool and hybrid YEp13 plasmids carrying the HEM1 gene were cloned by their ability to complement both the leu2 and hem1 mutations in the recipient strain. The yeast transformants, bearing the HEM1-containing plasmids pYe(HEM1), showed a 24-28 fold increase in δ-aminolevulinic acid synthase activity and in the intracellular content of δ-aminolevulinic acid (5-8 fold) as compared to wild type strains, suggesting that the p(HEM1) gene is being expressed as a catalytically active enzyme which can be transported into the mitochondria. However, the transformant strains did not present higher-than-normal content of heme or cytochromes either in glucose or in glycerol media, indicating that the production of δ-aminolevulinic acid is not the rate-limiting step in heme biosynthesis in yeast.

Synergy of ferrous ion on 5-aminolevulinic acid-mediated growth inhibition of Plasmodium falciparum

Haem biosynthesis appeared to be a target of malaria therapy because 5-aminolevulinic acid (ALA), a haem biosynthesis starting material, with light exposure or a high amount of ALA alone reduced Plasmodium falciparum growth to undetectable level. However, the administration of a high dose of ALA is unrealistic for clinical therapy. We found that Fe(2+) enhanced P. falciparum-killing potency of ALA and significantly inhibited the parasite growth. The intermediates of haem biosynthesis localized to the parasite organelles, and coproporphyrin III was the most accumulated intermediate. These novel findings may lead to development of a new anti-malarial drug using ALA and Fe(2+).

Closing in on complete pathways of biotin biosynthesis

Biotin is an enzyme cofactor indispensable to metabolic fixation of carbon dioxide in all three domains of life. Although the catalytic and physiological roles of biotin have been well characterized, the biosynthesis of biotin remains to be fully elucidated. Studies in microbes suggest a two-stage biosynthetic pathway in which a pimelate moiety is synthesized and used to begin assembly of the biotin bicyclic ring structure. The enzymes involved in the bicyclic ring assembly have been studied extensively. In contrast the synthesis of pimelate, a seven carbon α,ω-dicarboxylate, has long been an enigma. Support for two different routes of pimelate synthesis has recently been obtained in Escherichia coli and Bacillus subtilis. The E. coli BioC-BioH pathway employs a methylation and demethylation strategy to allow elongation of a temporarily disguised malonate moiety to a pimelate moiety by the fatty acid synthetic enzymes whereas the B. subtilis BioI-BioW pathway utilizes oxidative cleavage of fatty acyl chains. Both pathways produce the pimelate thioester precursor essential for the first step in assembly of the fused rings of biotin. The enzymatic mechanisms and biochemical strategies of these pimelate synthesis models will be discussed in this review.

Discovery of genes essential for heme biosynthesis through large-scale gene expression analysis

Heme biosynthesis consists of a series of eight enzymatic reactions that originate in mitochondria and continue in the cytosol before returning to mitochondria. Although these core enzymes are well studied, additional mitochondrial transporters and regulatory factors are predicted to be required. To discover such unknown components, we utilized a large-scale computational screen to identify mitochondrial proteins whose transcripts consistently coexpress with the core machinery of heme biosynthesis. We identified SLC25A39, SLC22A4, and TMEM14C, which are putative mitochondrial transporters, as well as C1orf69 and ISCA1, which are iron-sulfur cluster proteins. Targeted knockdowns of all five genes in zebrafish resulted in profound anemia without impacting erythroid lineage specification. Moreover, silencing of Slc25a39 in murine erythroleukemia cells impaired iron incorporation into protoporphyrin IX, and vertebrate Slc25a39 complemented an iron homeostasis defect in the orthologous yeast mtm1Delta deletion mutant. Our results advance the molecular understanding of heme biosynthesis and offer promising candidate genes for inherited anemias.

ALAD porphyria is a conformational disease

ALAD porphyria is a rare porphyric disorder, with five documented compound heterozygous patients, and it is caused by a profound lack of porphobilinogen synthase (PBGS) activity. PBGS, also called "delta-aminolevulinate dehydratase," is encoded by the ALAD gene and catalyzes the second step in the biosynthesis of heme. ALAD porphyria is a recessive disorder; there are two common variant ALAD alleles, which encode K59 and N59, and eight known porphyria-associated ALAD mutations, which encode F12L, E89K, C132R, G133R, V153M, R240W, A274T, and V275M. Human PBGS exists as an equilibrium of functionally distinct quaternary structure assemblies, known as "morpheeins," in which one functional homo-oligomer can dissociate, change conformation, and reassociate into a different oligomer. In the case of human PBGS, the two assemblies are a high-activity octamer and a low-activity hexamer. The current study quantifies the morpheein forms of human PBGS for the common and porphyria-associated variants. Heterologous expression in Escherichia coli, followed by separation of the octameric and hexameric assemblies on an ion-exchange column, showed that the percentage of hexamer for F12L (100%), R240W (80%), G133R (48%), C132R (36%), E89K (31%), and A274T (14%) was appreciably larger than for the wild-type proteins K59 and N59 (0% and 3%, respectively). All eight porphyria-associated variants, including V153M and V275M, showed an increased propensity to form the hexamer, according to a kinetic analysis. Thus, all porphyria-associated human PBGS variants are found to shift the morpheein equilibrium for PBGS toward the less active hexamer. We propose that the disequilibrium of morpheein assemblies broadens the definition of conformational diseases beyond the prion disorders and that ALAD porphyria is the first example of a morpheein-based conformational disease.

Biological significance and development of practical synthesis of biotin

Biotin (1), a water-soluble B series vitamin, distributes widely in microorganisms, plants, and animals. Biosynthesis of 1 involves five steps sequence starting from pimelic acid. The last step, a transformation from dethiobiotin (DTB) to 1, includes an iron clusters-mediated radical process. The compound 1 is a cofactor of carboxylation enzymes and plays crucial roles in the metabolism of fatty acids, sugars, and alpha-amino acids. In addition to the increasing application to feed additives, recent reports have revealed that 1 enhances insulin secretion in animals, suggesting it for a promising therapeutic candidate for an anti-diabetes drug. The remarkably strong affinity of 1 with avidin and streptavidin has been extensively applied for such technologies as photoaffinity labeling. Among the number of approaches to 1 so far developed in 50 years, a synthesis using L-cysteine and thiolactone as a starting material and a key intermediate, respectively, represents one of the best routes leading to 1, because of short steps, high yield, use of inexpensive reagents, and ease of operation.

URINARY AMINO-ACETONE EXCRETION IN LEAD WORKERS

Urinary amino-acetone concentrations were determined in a group of 51 persons with industrial exposure to lead and the findings compared with those obtained from a control group of 27 persons having no exposure to lead.A good correlation was found between delta-aminolaevulinic acid and amino-acetone excretion in the control group but this correlation was not so well defined in the lead-exposed group. In this group excretion of amino-acetone seemed to be better correlated with excretion of delta-aminolaevulinic acid for values less than 0.6 mg.% than with values greater than 0.6 mg.%. All the men concerned in the investigation were employed in dusty occupations. Hypotheses are put forward to attempt to explain the differences in excretion of amino-acetone noted amongst lead workers with high excretion of delta aminolaevulinic acid.Little correlation was found between amino-acetone excretion and that of either porphobilinogen or coproporphyrin.

The common origins of the pigments of life-early steps of chlorophyll biosynthesis

The complex pathway of tetrapyrrole biosynthesis can be dissected into five sections: the pathways that produce 5-aminolevulinate (the C-4 and the C-5 pathways), the steps that transform ALA to uroporphyrinogen III, which are ubiquitous in the biosynthesis of all tetrapyrroles, and the three branches producing specialized end products. These end products include corrins and siroheme, chlorophylls and hemes and linear tetrapyrroles. These branches have been subjects of recent reviews. This review concentrates on the early steps leading up to uroporphyrinogen III formation which have been investigated intensively in recent years in animals, in plants, and in a wide range of bacteria.

Isolation and characterization of a new mutant of Saccharomyces cerevisiae with altered synthesis of 5-aminolevulinic acid

A new gene, RHM1, required for normal production of 5-aminolevulinic acid by Saccharomyces cerevisiae, was identified by a novel screening method. Ethyl methanesulfonate treatment of a fluorescent porphyric strain bearing the pop3-1 mutation produced nonfluorescent or weakly fluorescent mutants with defects in early stages of tetrapyrrole biosynthesis. Class I mutants defective in synthesis of 5-aminolevulinate regained fluorescence when grown on medium supplemented with 5-aminolevulinate, whereas class II mutants altered in later biosynthetic steps did not. Among six recessive class I mutants, at least three complementation groups were found. One mutant contained an allele of HEM1, the structural gene for 5-aminolevulinate synthase, and two mutants contained alleles of the regulatory gene CYC4. The remaining mutants contained genes complementary to both hem1 and cyc4. Mutant strain DA3-RS3/68 contained mutant gene rhm1, which segregated independently of hem1 and cyc4 during meiosis. 5-Aminolevulinate synthase activity of the rhm1 mutant was 35 to 40% of that of the parental pop3-1 strain, whereas intracellular 5-aminolevulinate concentration was only 3 to 4% of the parental value. Transformation of an rhm1 strain with a multicopy plasmid containing the cloned HEM1 gene restored normal levels of 5-aminolevulinate synthase activity, but intracellular 5-aminolevulinate was increased to only 9 to 10% of normal. We concluded that RHM1 could control either targeting of 5-aminolevulinate synthase to the mitochondrial matrix or the activity of the enzyme in vivo.

Metabolism of [U-14C]-4-hydroxybutyric acid to intermediates of the tricarboxylic acid cycle in extracts of rat liver and kidney mitochondria

The metabolism of [U-14C]-4-hydroxybutyric acid (GHB) was investigated in sonicates of mitochondria of rat heart, kidney and liver. The conversion of this precursor to 14C-organic acids was monitored and quantified by sequential liquid-partition chromatography on hydrated columns of silicic acid using a concave-upward gradient of 2-methylbutan-2-ol in chloroform. Sonicates of liver and kidney mitochondria, but not heart, readily converted [U-14C]-GHB to 14C organic acids via a pathway of conversion to 14C-succinic acid, followed by further metabolism through the tricarboxylic acid cycle. This conversion was facilitated by exogenous NAD+ and NADP+. No evidence for the beta-oxidation of GHB was obtained in any of the mitochondrial sonicates. Studies with exogenous non-labelled succinic semialdehyde indicated that this compound was an intermediate in the conversion of GHB to succinic acid.

Evolution of the control of pigment and plastid development in photosynthetic organisms

How do bioenergetic organelles relate to the cells they are in and how was this relationship established over the course of evolution? Plastids and mitochondria are viewed as prokaryotic residents in eukaryotic cells. These organelles are semiautonomous: they perpetuate themselves by division but regulate and are subject to regulation by the cell in which they are residents. Although these organelles are usually constitutive, their development is arrested in certain organisms when an inducing substrate is absent (light, for example, in the case of the chloroplast) with the formation of precursor organelles such as proplastids. Various trends in the evolution of photo-control systems are discussed including those concerned with photoperception and photomorphogenesis. The photocontrol of chloroplast development by blue and red light is discussed in relation to its possible evolutionary origins in a system for finding the right light for photosynthesis. Models for various types of cellular regulation by light during chloroplast development are discussed. Also considered is the evolution of plastid pigments in response to available light. A parallel evolution of accessory pigments and chlorophylls is suggested which led to chlorophyll reaction centers serving as energy sinks for light absorbed by accessory pigments and, therefore, having their absorptions pushed to the longest possible wavelengths as accessory pigments evolved to fill the middle of the spectrum in response to ecological selection. An endosymbiotic origin of bioenergetic organelles is suggested based on polyphyletic origins of chloroplasts from a number of oxygenic procaryotic precursors. The similarity between proplastids and these oxygenic procaryotes suggests that the original invading organelle may have resembled a modern proplastid rather than a mature chloroplast.

D-2-hydroxyglutaric aciduria: case report and biochemical studies

A patient with protein-losing gastroenteropathy and egg allergy has been shown to have a previously unrecognized organic aciduria, D-2-hydroxyglutaric aciduria. The observations made are consistent with an inherited metabolic disorder in the catabolism of 5-aminolaevulinate possibly due to deficient activity of a specific D-2-hydroxyglutarate dehydrogenase.

Synthesis of the pyrrole porphobilinogen by sepharose-linked -aminolevulinic acid dehydratase

delta-Aminolevulinic acid dehydratase from Rhodopseudomonas spheroides was covalently linked to Sepharose 4B, which had been activated with cyanogen bromide. A column containing this enzyme gel readily catalyzed the synthesis of the pyrrole porphobilinogen on continuous passage of a solution of delta-aminolevulinic acid. Under the conditions of the procedures, product inhibition was minimized and a 50 to 94 percent yield was attained. A column containing about 1 milligram of enzyme was continuously operated for 27 days. Although its total activity appeared to be reduced about 30 percent at the end of this time, the bound enzyme produced approximately 200 milligrams of porphobilinogen each day, and about 5 grams of the pyrrole were isolated.