Peptide antibiotics

Contribution of macrolactin in Bacillus velezensis CLA178 to the antagonistic activities against Agrobacterium tumefaciens C58

Beneficial rhizobacteria can inhibit soilborne pathogens by secreting an array of polyketides, lipopeptides and dipeptides, but the effect of polyketides on crown gall disease caused by Agrobacterium tumefaciens C58 is unclear. In this study, the antagonistic compounds of the plant growth-promoting rhizobacterium Bacillus velezensis CLA178 was sorted with different organic phases, purified by high-pressure liquid chromatography, and detected by a liquid chromatography ionization-mass spectrometry system. Macrolactins were found to be the compounds with antagonistic activity against A. tumefaciens C58. When the macrolactin synthesis pathway was disrupted, the mutant △mlnA only showed slight antagonistic activity against A. tumefaciens C58. Transmission electron microscopy showed that the inhibition of C58 cell division by cell-free culture from the mutant △mlnA was weaker than that by cell-free culture from CLA178. The mutant deficient in production of macrolactin showed a weaker transcription of genes involved in attachment of C58 to plant and lower biocontrol of crown gall disease in rose than the wild-type strain CLA178. The effect of macrolactins on pathogen C58 has been also confirmed by the purified macrolactins. These results reveal that macrolactins contribute to the biocontrol activity of C58 by inhibiting cell division and downregulating the transcription of chvB and chvE.

Antifungal activity of endophytic Bacillus safensis B21 and its potential application as a biopesticide to control rice blast

Endophytic bacteria are potential biocontrol agents for the control of fungal diseases. Here, an endophyte strain, B21, was isolated from Osmanthus fragrans Lour. fruits and identified as Bacillus safensis by analysis of its 16S rDNA gene sequence and its biochemical and physiological characteristics. The culture filtrate showed antifungal activity against Magnaporthe oryzae, which causes rice blast disease, and the IC₅₀ of the methanol extract was 15.56 μg/mL, which was significantly lower than that of carbendazim (25.16 μg/mL). The antifungal activity of the methanol extract was stable at a wide range of pH values (1-9) and temperatures (40-100 °C). Two antifungal compounds were isolated by organic extraction, silica gel column chromatography and high-performance liquid chromatography (HPLC). Based on electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance spectrometry (NMR) analyses, the structures of the antifungal compounds were identified as iturin A2 and iturin A6. Additionally, the hyphae treated with iturin (iturin A2 or iturin A6) could be stained with the fluorescent dye propidium iodide (PI), indicating that these two compounds inhibited the growth of hyphae by changing the hyphal membrane permeability. In field experiments, spray treatment with fermentation broth resulted in a lower disease index than treatment with carbendazim, as did the culture filtrate. The results suggest that strain B21 and its bioactive compounds have the potential to be developed into a biopesticide for the biocontrol of rice blast.

Chiral checkpoints during protein biosynthesis

Protein chains contain only l-amino acids, with the exception of the achiral glycine, making the chains homochiral. This homochirality is a prerequisite for proper protein folding and, hence, normal cellular function. The importance of d-amino acids as a component of the bacterial cell wall and their roles in neurotransmission in higher eukaryotes are well-established. However, the wider presence and the corresponding physiological roles of these specific amino acid stereoisomers have been appreciated only recently. Therefore, it is expected that enantiomeric fidelity has to be a key component of all of the steps in translation. Cells employ various molecular mechanisms for keeping d-amino acids away from the synthesis of nascent polypeptide chains. The major factors involved in this exclusion are aminoacyl-tRNA synthetases (aaRSs), elongation factor thermo-unstable (EF-Tu), the ribosome, and d-aminoacyl-tRNA deacylase (DTD). aaRS, EF-Tu, and the ribosome act as "chiral checkpoints" by preferentially binding to l-amino acids or l-aminoacyl-tRNAs, thereby excluding d-amino acids. Interestingly, DTD, which is conserved across all life forms, performs "chiral proofreading," as it removes d-amino acids erroneously added to tRNA. Here, we comprehensively review d-amino acids with respect to their occurrence and physiological roles, implications for chiral checkpoints required for translation fidelity, and potential use in synthetic biology.

Distinctive Roles of D-Amino Acids in the Homochiral World: Chirality of Amino Acids Modulates Mammalian Physiology and Pathology

Living organisms enantioselectively employ L-amino acids as the molecular architecture of protein synthesized in the ribosome. Although L-amino acids are dominantly utilized in most biological processes, accumulating evidence points to the distinctive roles of D-amino acids in non-ribosomal physiology. Among the three domains of life, bacteria have the greatest capacity to produce a wide variety of D-amino acids. In contrast, archaea and eukaryotes are thought generally to synthesize only two kinds of D-amino acids: D-serine and D-aspartate. In mammals, D-serine is critical for neurotransmission as an endogenous coagonist of N-methyl D-aspartate receptors. Additionally, D-aspartate is associated with neurogenesis and endocrine systems. Furthermore, recognition of D-amino acids originating in bacteria is linked to systemic and mucosal innate immunity. Among the roles played by D-amino acids in human pathology, the dysfunction of neurotransmission mediated by D-serine is implicated in psychiatric and neurological disorders. Non-enzymatic conversion of L-aspartate or L-serine residues to their D-configurations is involved in age-associated protein degeneration. Moreover, the measurement of plasma or urinary D-/L-serine or D-/L-aspartate levels may have diagnostic or prognostic value in the treatment of kidney diseases. This review aims to summarize current understanding of D-amino-acid-associated biology with a major focus on mammalian physiology and pathology.

NMR-based assignment of isoleucine vs. allo-isoleucine stereochemistry

A simple ¹H and ¹³C NMR spectrometric analysis is demonstrated that permits differentiation of isoleucine and allo-isoleucine residues by inspection of the chemical shift and coupling constants of the signals associated with the proton and carbon at the α-stereocentre. This is applied to the estimation of epimerisation during metal-free N-arylation and peptide coupling reactions.

A new approach of indirect enzyme-linked immunosorbent assay for determination of D-glutamic acid through in situ conjugation

We propose a new approach of an indirect enzyme-linked immunosorbent assay (ELISA) for determination of D-glutamic acid (D-Glu) using a monoclonal antibody against D-glutamic acid (D-Glu-MAb), which recognizes D-Glu-glutaraldehyde (GA) molecule but not D-Glu molecule. Human serum albumin (HSA) was coated on an immunoplate and reacted with D-Glu via GA to produce D-Glu-GA-HSA conjugates in situ in the well to be recognized by D-Glu-MAb, which enabled the development of an indirect ELISA for the determination of free D-Glu. In this indirect ELISA, D-Glu can be specifically detected with limit of detection of 7.81 μ g/mL. Since anti-conjugate antibodies are often produced, even though anti-hapten antibodies are desired, this new approach could be very useful as an application of anti-conjugate antibodies to the development of quantitative analysis for detecting hapten.

D-Amino acids inhibit biofilm formation in Staphylococcus epidermidis strains from ocular infections

Biofilm formation on medical and surgical devices is a major virulence determinant for Staphylococcus epidermidis. The bacterium S. epidermidis is able to produce biofilms on biotic and abiotic surfaces and is the cause of ocular infection (OI). Recent studies have shown that d-amino acids inhibit and disrupt biofilm formation in the prototype strains Bacillus subtilis NCBI3610 and Staphylococcus aureus SCO1. The effect of d-amino acids on S. epidermidis biofilm formation has yet to be tested for clinical or commensal isolates. S. epidermidis strains isolated from healthy skin (n = 3), conjunctiva (n = 9) and OI (n = 19) were treated with d-Leu, d-Tyr, d-Pro, d-Phe, d-Met or d-Ala and tested for biofilm formation. The presence of d-amino acids during biofilm formation resulted in a variety of patterns. Some strains were sensitive to all amino acids tested, while others were sensitive to one or more, and one strain was resistant to all of them when added individually; in this way d-Met inhibited most of the strains (26/31), followed by d-Phe (21/31). Additionally, the use of d-Met inhibited biofilm formation on a contact lens. The use of l-isomers caused no defect in biofilm formation in all strains tested. In contrast, when biofilms were already formed d-Met, d-Phe and d-Pro were able to disrupt it. In summary, here we demonstrated the inhibitory effect of d-amino acids on biofilm formation in S. epidermidis. Moreover, we showed, for the first time, that S. epidermidis clinical strains have a different sensitivity to these compounds during biofilm formation.

Genome-wide identification of genes conferring energy related resistance to a synthetic antimicrobial peptide (Bac8c)

A fundamental issue in the design and development of antimicrobials is the lack of understanding of complex modes of action and how this complexity affects potential pathways for resistance evolution. Bac8c (RIWVIWRR-NH₂) is an 8 amino acid antimicrobial peptide (AMP) that has been shown to have enhanced activity against a range of pathogenic Gram-positive and Gram-negative bacteria, as well as yeast. We have previously demonstrated that Bac8c appears to interfere with multiple targets, at least in part through the disruption of cytoplasmic membrane related functions, and that resistance to this peptide does not easily develop using standard laboratory methods. Here, we applied a genomics approach, SCalar Analysis of Library Enrichement (SCALEs), to map the effect of gene overexpression onto Bac8c resistance in parallel for all genes and gene combinations (up to ∼ 10 adjacent genes) in the E. coli genome (a total of ∼ 500,000 individual clones were mapped). Our efforts identified an elaborate network of genes for which overexpression leads to low-level resistance to Bac8c (including biofilm formation, multi-drug transporters, etc). This data was analyzed to provide insights into the complex relationships between mechanisms of action and potential routes by which resistance to this synthetic AMP can develop.

Spatiotemporal localization of D-amino acid oxidase and D-aspartate oxidases during development in Caenorhabditis elegans

Recent investigations have shown that a variety of D-amino acids are present in living organisms and that they possibly play important roles in physiological functions in the body. D-Amino acid oxidase (DAO) and D-aspartate oxidase (DDO) are degradative enzymes stereospecific for D-amino acids. They have been identified in various organisms, including mammals and the nematode Caenorhabditis elegans, although the significance of these enzymes and the relevant functions of D-amino acids remain to be elucidated. In this study, we investigated the spatiotemporal localization of C. elegans DAO and DDOs (DDO-1, DDO-2, and DDO-3) and measured the levels of several D- and L-amino acids in wild-type C. elegans and four mutants in which each gene for DAO and the DDOs was partially deleted and thereby inactivated. Furthermore, several phenotypes of these mutant strains were characterized. The results reported in this study indicate that C. elegans DAO and DDOs are involved in egg-laying events and the early development of C. elegans. In particular, DDOs appear to play important roles in the development and maturation of germ cells. This work provides novel and useful insights into the physiological functions of these enzymes and D-amino acids in multicellular organisms.

Emerging knowledge of regulatory roles of D-amino acids in bacteria

The d-enantiomers of amino acids have been thought to have relatively minor functions in biological processes. While l-amino acids clearly predominate in nature, d-amino acids are sometimes found in proteins that are not synthesized by ribosomes, and d-Ala and d-Glu are routinely found in the peptidoglycan cell wall of bacteria. Here, we review recent findings showing that d-amino acids have previously unappreciated regulatory roles in the bacterial kingdom. Many diverse bacterial phyla synthesize and release d-amino acids, including d-Met and d-Leu, which were not previously known to be made. These noncanonical d-amino acids regulate cell wall remodeling in stationary phase and cause biofilm dispersal in aging bacterial communities. Elucidating the mechanisms by which d-amino acids govern cell wall remodeling and biofilm disassembly will undoubtedly reveal new paradigms for understanding how extracytoplasmic processes are regulated as well as lead to development of novel therapeutics.

Study of the antifungal activity of Bacillus vallismortis ZZ185 in vitro and identification of its antifungal components

An endophytic Bacillus vallismortis ZZ185 was isolated from healthy stems of the plant Broadleaf Holly (Ilex latifolia Thunb) collected in Nanjing, China. Both the culture filtrate and the n-butanol extract of strain ZZ185 showed strong growth inhibition activity in vitro against the phytopathogens Fusarium graminearum, Alternaria alternata, Rhizoctonia solani, Cryphonectria parasitica and Phytophthora capsici. The results showed that the filtrate and extract reduced the symptoms of wheat seedlings infected with A. alternata and F. graminearum by about 90% and 50%, respectively, based on the comparison of the lengths of zones on the seminal roots showing cortical browning with those of the roots of uninfected controls. The antifungal activity of the culture filtrate was significantly correlated with cell growth of strain ZZ185. The active metabolite in the filtrate was relatively thermally stable with more than 50% of the antifungal activity of the culture filtrate being retained even after being held at 121 degrees C for 30 min. Meanwhile, the antifungal activity of the filtrate against the growth of A. alternata and F. graminearum remained almost unchanged (>75%) when the culture was exposed to a pH ranging from 1 to 8, but significantly reduced after the filtrate had been exposed to basic conditions. From the n-butanol extract of the filtrate, the antifungal compounds were isolated as a mixture of Bacillomycin D (n-C14) and Bacillomycin D (iso-C15). The strong antifungal activity implied that the endophytic B. vallismortis ZZ185 and its bioactive components might provide an alternative resource for the biocontrol of plant diseases.

Study of the antifungal activity of Acinetobacter baumannii LCH001 in vitro and identification of its antifungal components

An Acinetobacter strain, given the code name LCH001 and having the potential to be an endophytic antagonist, has been isolated from healthy stems of the plant Cinnamomum camphora (L.) Presl, guided by an in vitro screening technique. The bacterium inhibited the growth of several phytopathogenic fungi such as Cryphonectria parasitica, Glomerella glycines, Phytophthora capsici, Fusarium graminearum, Botrytis cinerea, and Rhizoctonia solani. Biochemical, physiological, and 16S rDNA sequence analysis proved that it is Acinetobacter baumannii. When the filtrate from the fermentation broth of strain LCH001 was tested in vitro and in vivo, it showed strong growth inhibition against several phytopathogens including P. capsici, F. graminearum, and R. solani, indicating that suppression of the growth of the fungi was due to the presence of antifungal compounds in the culture broth. Moreover, the antifungal activity of the culture filtrate was significantly correlated with the cell growth of strain LCH001. The active metabolites in the filtrate were relatively thermally stable, but were sensitive to acidic conditions. Three antifungal compounds were isolated from the culture broth by absorption onto macropore resin, ethanol extraction, chromatography on silica gel or LH-20 columns, and crystallization. The structures of the bioactive compounds were identified by spectroscopic methods as isomers of iturin A, namely, iturin A2, iturin A3, and iturin A6. The characterization of an unusual endophytic bacterial strain LCH001 and its bioactive components may provide an alternative resource for the biocontrol of plant diseases.

Asymmetric, anti-Selective Scandium-Catalyzed Sakurai Additions to Glyoxyamide. Applications to the Syntheses of N-Boc d-Alloisoleucine and d-Isoleucine

[reaction: see text] An enantio- and diastereoselective Sakurai-Hosomi reaction, catalyzed by chiral scandium pyridyl-bis(oxazoline) (pybox) complexes, has been developed. Both alkyl- and aryl-substituted allylsilanes are effective coupling partners with N-phenylglyoxamide. Applications of this reaction to the asymmetric syntheses of N-Boc D-alloisoleucine and D-isoleucine are described.

Structural and functional differences in two cyclic bacteriocins with the same sequences produced by lactobacilli

Lactobacillus gasseri LA39 and L. reuteri LA6 isolated from feces of the same human infant were found to produce similar cyclic bacteriocins (named gassericin A and reutericin 6, respectively) that cannot be distinguished by molecular weights or primary amino acid sequences. However, reutericin 6 has a narrower spectrum than gassericin A. In this study, gassericin A inhibited the growth of L. reuteri LA6, but reutericin 6 did not inhibit the growth of L. gasseri LA39. Both bacteriocins caused potassium ion efflux from indicator cells and liposomes, but the amounts of efflux and patterns of action were different. Although circular dichroism spectra of purified bacteriocins revealed that both antibacterial peptides are composed mainly of alpha-helices, the spectra of the bacteriocins did not coincide. The results of D- and L-amino acid composition analysis showed that two residues and one residue of D-Ala were detected among 18 Ala residues of gassericin A and reutericin 6, respectively. These findings suggest that the different D-alanine contents of the bacteriocins may cause the differences in modes of action, amounts of potassium ion efflux, and secondary structures. This is the first report that characteristics of native bacteriocins produced by wild lactobacillus strains having the same structural genes are influenced by a difference in D-amino acid contents in the molecules.

Identification and characterization of a new gene from Variovorax paradoxus Iso1 encoding N-acyl-D-amino acid amidohydrolase responsible for D-amino acid production

An N-acyl-d-amino acid amidohydrolase (N-D-AAase) was identified in cell extracts of a strain, Iso1, isolated from an environment containing N-acetyl-d-methionine. The bacterium was classified as Variovorax paradoxus by phylogenetic analysis. The gene was cloned and sequenced. The gene consisted of a 1467-bp ORF encoding a polypeptide of 488 amino acids. The V. paradoxusN-D-AAase showed significant amino acid similarity to the N-acyl-d-amino acid amidohydrolases of the two eubacteria Alcaligenes xylosoxydans A-6 (44-56% identity), Alcaligenes facelis DA1 (54% identity) and the hyperthermophilic archaeon Pyrococcus abyssi (42% identity). After over-expression of the N-D-AAase protein in Escherichia coli, the enzyme was purified by multistep chromatography. The native molecular mass was 52.8 kDa, which agreed with the predicted molecular mass of 52 798 Da and the enzyme appeared to be a monomer protein by gel-filtration chromatography. A homogenous protein with a specific activity of 516 U.mg-1 was finally obtained. After peptide sequencing by LC/MS/MS, the results were in agreement with the deduced amino acid sequence of the N-D-AAase. The pI of the enzyme was 5.12 and it had an optimal pH and temperature of 7.5 and 50 degrees C, respectively. After 30 min heat treatment at 45 degrees C, between pH 6 and pH 8, 80% activity remained. The N-D-AAase had higher hydrolysing activity against N-acetyl-d-amino acid derivates containing d-methionine, d-leucine and d-alanine and against N-chloroacetyl-d-phenylalanine. Importantly, the enzyme does not act on the N-acetyl-l-amino acid derivatives. The enzyme was inhibited by chelating agents and certain metal ions, but was activated by 1 mm of Co2+ and Mg2+. Thus, the N-D-AAase from V. paradoxus can be considered a chiral specific and metal-dependent enzyme.

Occurrence of D-amino acids in a few archaea and dehydrogenase activities in hyperthermophile Pyrobaculum islandicum

The contents of D-enantiomers of serine, alanine, proline, glutamate (glutamine) and aspartate (asparagine) were examined in the membrane fractions, soluble proteins and free amino acids from some species of archaea, Pyrobaculum islandicum, Methanosarcina barkeri and Halobacterium salinarium. Around 2% (D/D+L) of D-aspartate was found in the membrane fractions. In the soluble proteins, the D-amino acid content was higher in P. islandicum than that in the other archaeal cells: the concentrations in P. islandicum were 3 and 4% for D-serine and D-aspartate, respectively. High concentrations of free D-amino acids were found in P. islandicum and H. salinarium; the concentrations of D-serine (12-13%), D-aspartate (4-7%) and D-proline (3-4%) were higher than those of D-alanine and D-glutamate. This result showed a resemblance between these archaea and not bacterial, but eukaryotic cells. The presence of D-amino acids was confirmed by their digestion with D-amino acid oxidase and D-aspartate oxidase. The occurrence of D-amino acids was also confirmed by the presence of activities catalyzing catabolism of D-amino acids in the P. islandicum homogenate, as measured by 2-oxo acid formation. The catalytic activities oxidizing D-alanine, D-aspartate and D-serine at 90 degrees C were considerably high. Under anaerobic conditions, dehydrogenase activities of the homogenate were 69, 84 and 30% of the above oxidase activities toward D-alanine, D-aspartate and D-serine, respectively. Comparable or higher dehydrogenase activities were also detected with these D-amino acids as substrate by the reduction of 2, 6-dichlorophenolindophenol. No D-amino acid oxidase activity was detected in the homogenates of M. barkeri and H. salinarium.

Occurrence of peptidyl D-amino acids in soluble fractions of several eubacteria, archaea and eukaryotes

The occurrence of peptidyl D-amino acids in the aqueous soluble fractions was investigated in various eubacteria, some archaea and some eukaryotes. The contents of the D-enantiomers of serine, alanine, proline, glutamate (glutamine), aspartate (asparagine) and phenylalanine were determined with cell- and tissue-extracts, by means of acid hydrolysis and high-performance liquid chromatography. The rate of D-enantiomer (%, the ratio in molar concentration of a D-amino acid to the total of the D-amino acid and the corresponding L-amino acid) of alanine and glutamate were high in some Gram-positive eubacteria: 11.7% in Staphylococcus epidermidis and 10.3% in Streptococcus pyogenes for alanine, and 22.3% for glutamate in Bacillus YN-1. The D-glutamate content was also high (8.0%) in the Gram-negative eubacterium, Thiobacillus ferrooxidans. D-Aspartate was common, as was D-glutamate: the highest D-aspartate content was detected in an archaeum, Pyrobaculum islandicum (4.0%). However, the content of D-aspartate was low, 0.2-1.8% in most other bacteria. The presence of D-serine was shown in some organisms, but that of D-proline was scarce. The D-enantiomer of phenylalanine was not detected in any of the organisms examined. These results indicate that of the bacteria examined herein most Gram-negative and some Gram-positive eubacteria, as well as archaea contain only low levels of D-amino acids in the soluble peptidyl fraction, and the levels were comparable to those in eukaryotes examined. To our knowledge, the general presence of peptidyl D-amino acids in these organisms, especially archaea and eukaryotic cells including those from rat liver tissues, has been shown here for the first time.

Bacitracin production by a new strain of Bacillus subtilis. Extraction, purification, and characterization

A new strain of Bacillus subtilis C 126 was isolated from sugar cane fermentation and produced an antibiotic that inhibited the growth of Micrococcus flavus. The production of the antibiotic in culture medium followed to extraction with n-butanol, thin layer chromatography, and microbiological tests indicated that a polypeptide antibiotic was produced. The fraction obtained by Sephadex G-25 column and analyzed by HPLC indicated that bacitracin complex was produced.

Production and Purification of d -Aminoacylase from Alcaligenes denitrificans and Taxonomic Study of the Strain

A d -aminoacylase-producing microorganism, strain DA181, isolated from soil was identified as Alcaligenes denitrificans subsp. denitrificans . This strain produced about 29,300 units (micromoles of product formed per hour) of d -aminoacylase and 2,300 units of l -aminoacylase per gram of cells (wet weight) when cultivated in a medium containing 1% N -acetyl- dl -leucine as the carbon source. The d -aminoacylase was purified 345-fold. The specific activity of the purified enzyme was 108,600 units per mg of protein when N -acetyl- d -methionine was used as a substrate. The apparent molecular weight was 58,000, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. N -Acetyl- d -methionine was the favored substrate, followed by N -acetyl- d -phenylalanine. This enzyme had a high stereospecificity, and its hydrolysis of N -acetyl- l -amino acids was almost negligible.

Stimulation of Tentoxin Synthesis by Aged-Culture Filtrates and Continued Synthesis in the Presence of Protein Inhibitors

Tentoxin, a cyclic tetrapeptide produced by Alternaria alternata (Fries) Keissler, induces chlorosis in certain seedling plants. It can be extracted from culture filtrates of the fungus. Tentoxin production is stimulated and increased by using a mixture of aged culture filtrates and modified Richards solution. Aged culture filtrates can be obtained from 3-week-old or older cultures of A. alternata in modified Richards solution or Pratts solution. A mixture of aged culture filtrate and fresh medium in the ratio 2:3 gives the maximal enhancement of tentoxin production. This growth system provided us with a model for studying the effects of protein synthesis inhibitors on tentoxin production. Two antibiotics which inhibit protein synthesis at the ribosomal level were tested on growth, protein synthesis, and tentoxin production in A. alternata cultures. Cycloheximide at concentrations of 500 μg/ml or emetine at concentrations of 250 μg/ml did not inhibit tentoxin synthesis, although they stopped mycelial growth and protein synthesis of the fungus at the logarithmic growth stage in the enhancement medium. These results led us to conclude that tentoxin, like certain other bioactive cyclic peptides, is synthesized by a nonribosomal peptide synthesis mechanism.

The purification and some properties of a stereospecific D-asparaginase from an extremely thermophilic bacterium, Thermus aquaticus

A specific D-asparaginase was isolated and crystallized from Thermus aquaticus strain T351. It is present in larger amounts than the L-asparaginase. The enzyme has a molecular weight of 60 000, an isoelectric point of 4.8 and a Km of 2 mM. It has 6 disulphide bonds/molecule, and a histidine residue at the active site. It is inhibited by keto acids and by high salt concentrations.

Effect of topical application of amino acids on gastric pepsin secretion in the rat. Part III: effect of L- and D-isomers of amino acids on gastric secretion in reperfusion system

This study was undertaken to compare the potency of each L- and D-isomers of 5 amino acids in stimulating gastric acid and pepsin secretion by the intragastric reperfusion preparation in rats. Gastric basal and glycine-stimulated secretions in this preparation were lower than those Ghosh-Lai preparation, but not to a statistically significant extent. All D- and L-isomers of amino acids tested were found to markedly stimulate pepsin secretion, but to only slightly stimulate acid secretion. Pepsin stimulatory response to each D and L-isomer of amino acid was similar. The stimulatory effects of amino acids thus seem to be unrelated to the optical stereoisomeric configuration.

Purification and characterisation of D-amino acid aminotransferase from Rhizobium japonicum

Rhizobium japonicum has D-amino acid aminotransferase and alanine racemase activities. The D-amino-acid aminotransferase has been partially purified and characterized. This enzyme has a broad specificity and is very active with D-alpha-aminobutyrate and D-aspartate as well as D-alanine and D-glutamate. The stereospecificity of the enzyme for D-amino acids was apparently absolute with respect to product inhibition, pyridoxamine formation as well as catalytic activity. The apparent molecular weight was 58,000 and the pH optimum was 7.8-7.9. The equilibrium constant in the direction of D-glutamate formation was 1.9. Initial-velocity kinetic studies indicate the enzyme acts by a ping-pong mechanism. The dissociation constant for pyridoxal phosphate and the Michaelis constants (+/- standard errors) for D-alanine and 2-oxoglutarate were determined to be 0.51 +/- 0.06 micrometer, and 2.13 +/- 0.18 and 0.058 +/- 0.005 mM respectively. The enzyme is moderately inhibited (30%) by 4 mM p-chloromercuribenzoate.

On the function of the polypeptide antibiotic bacitracin in the producer strain Bacillus licheniformis

The growth of the bacitracin producing strain Bacillus licheniformis AL and the bacitracin-negative mutant SB 319 have been compared at different cultural conditions. Concentrations of the metal chelator EDTA which strongly inhibited the growth of the non-producer only slightly inhibited the growth of the bacitracin producer. The inhibitory effect of EDTA upon SB 319 was reversed by the addition of excess manganese(II)ions, cobalt(II)ions, or zinc(II)ions to the culture. The addition of several other ions had no such effect. The addition of bacitracin to the EDTA inhibited mutant also promoted growth. When the non-producer was mutated back to bacitracin production, the inhibitory effect of EDTA was lost. It is suggested that bacitracin may normally promote the uptake of several trace metals during growth of the producer organism.