Superstoichiometric binding of L-Phe to phenylalanine hydroxylase from Caenorhabditis elegans: evolutionary implications

Phenylalanine hydroxylase (PAH) catalyzes the hydroxylation of L-Phe to L-Tyr. Dysfunctional PAH results in phenylketonuria and mammalian PAH is therefore highly regulated and displays positive cooperativity for L-Phe (Hill coefficient (h) = 2). L-Phe does not bind to the regulatory ACT domain in full-length tetrameric human PAH and cooperativity is elicited by homotropic binding to the catalytic site (Thórólfsson et al. in Biochemistry 41:7573–7585, 2002). PAH from Caenorhabditis elegans (cePAH) is devoid of cooperativity for L-Phe (h = 0.9), and, as shown in this work, structural analysis reveal an additional L-Phe binding site at the regulatory domain of full-length cePAH. This site involves the GA(S)L/ISRP motifs, which are also found in ACT domains of other L-Phe binding proteins, such as prephenate dehydratase. Isothermal titration calorimetry further demonstrated 2 binding sites per subunit for cePAH versus ~1 for hPAH. Steric occlusion of the regulatory site, notably by residues Lys215/Tyr216 from the adjacent catalytic domain, appears to hinder regulatory binding in full-length hPAH. Accordingly, the humanized mutant Q215K/N216Y of cePAH binds ~1.4 L-Phe/subunit. This mutant also displays high catalytic activity and certain positive cooperativity for L-Phe (h = 1.4). Our results support that the acquisition of positive cooperativity in mammalian forms of PAH is accompanied by a closure of the regulatory L-Phe binding site. Concomitantly, the function of the regulatory ACT domain appears to be adapted from amino acid binding to serving the communication of conformational changes among catalytic subunits.     

Oxygen starvation induces cell death in Candida shehatae fermentations of d-xylose, but not d-glucose

Candida shehatae cells, cultivated on d-glucose and d-xylose, were subjected to a shift from fully aerobic to anaerobic fermentative conditions. After anaerobic conditions were imposed, growth was limited to approximately one doubling or less as C. shehatae rapidly entered a stationary phase of growth. Following the shift to anoxia, cell viability rapidly declined and the total cell volume declined in the d-xylose fermentations. Moreover, the cell volume distribution shifted to smaller volumes. Cell viability, measured by plate counts, declined nine times faster for d-xylose fermentations than for d-glucose fermentations. Anaerobic growth did not occur on either d-glucose or d-xylose. Selected vitamins and amino acids did not stimulate anaerobic growth in C. shehatae, but did enhance anaerobic growth on d-glucose in S. cerevisiae. The decline in cell viability and lack of anaerobic growth by C. shehatae were attributed to oxygen deficiency and not to ethanol inhibition. The results shed light on why C. shehatae anaerobic fermentations are not currently practical and suggest that research directed towards a biochemical understanding of why C. shehatae can not grow anaerobically will yield significant improvements in ethanol fermentations from d-xylose. Received 26 October 1998 / Received revision: 26 January 1999 / Accepted: 12 February 1999     

Hydrogen peroxide-induced chlorophyll a bleaching in the cytochrome b 6 f complex: a simple and effective assay for stability of the complex in detergent solutions

The instability of cytochrome b ₆ f complex in detergent solutions is a well-known problem that has been studied extensively, but without finding a satisfactory solution. One of the important reasons can be short of the useful method to verify whether the complex suspended in different detergent is in an intact state or not. In this article, a simple and effective assay for stability of the complex was proposed based on the investigation on the different effects of the two detergents, n-octyl-β-d-glucopyranoside (OG) and dodecyl-β-d-maltoside (DDM), on the properties of the complex. DDM stabilizes the complex preparation more effectively whereas OG denatures the interactions of the heme groups and pigment molecules with the protein environment, leading to the bleaching of chlorophyll a induced by addition of hydrogen peroxide. The assay of the use of hydrogen peroxide to characterize the complex by studying the bleaching of chlorophyll induced by hydrogen peroxide and the peroxidase activity of the complex was discussed. This simple method will probably be useful to study the stability of the complex. Xiao-Bo Chen and Xiao-Hui Zhao contributed equally to this work.     

Chiral resolution of phenylalanine by d-Phe imprinted membrane considering rejection property

An enantio-selective d-Phe imprinted P(AA-co-AN) membrane was prepared using the wet-phase inversion method. The membrane not only selectively adsorbed phenylalanine but also rejected it with a rejection selectivity of 0.82–0.64 and 0.91–0.63 during the filtration of 100 and 10 ppm (g m⁻³) racemate solutions, respectively. The fluxes of d-Phe and l-Phe during filtration of 10 ppm racemate solution were 0.0077–0.0229 and 0.0064–0.0208 mg m⁻² s⁻¹, respectively, and the fluxes of d-Phe and l-Phe during filtration of 100 ppm racemate solution were 0.1287–0.2522 and 0.1174–0.2458 mg m⁻² s⁻¹, respectively. The adsorption selectivity was higher at low concentration. The adsorption selectivities varied from 1.11 to 1.65 and from 1.64 to 2.78 during filtration of 100 and 10 ppm racemate solutions, respectively. In respect to desorption, the fractional difference between d-Phe and l-Phe in the recovered solution from 10 ppm was higher than that from 100 ppm.     

Properties of a cold-active protease from psychrotrophic Flavobacterium balustinum P104

Protease activity was detected in the culture medium of Flavobacterium balustinum P104 grown at 10 °C, which was isolated from salmon (Oncorhynchus keta) intestine. The enzyme, designated as CP-70 protease, was purified to homogeneity from the culture broth by ion exchange and gel filtration chromatographyies. The molecular mass of the protease was 70 kDa, and its isoelectric point was close to 3.5. Maximal activity toward azocasein was observed at 40 °C and from pH 7.0 to 9.0. The activity was strongly inhibited by phenylmethylsulfonyl fluoride, suggesting that the enzyme is a serine protease. The n-terminal amino acid sequence was Asp-Thr-Arg-Gln-Leu-Leu-Asn-Ala-Asn-Ser-Asp-Leu-Leu-Asn-Thr-Thr-Gly-Asn-Val-Thr-Gly-Leu-Thr-Gly-Ala-Phe-Asn-Gly-Glu-Asn. A search through the database for sequence homology yielded no significant match. The initial cleavage sites for oxidized insulin B-chain were found to be the Glu13-Ala14 and Phe24-Phe25 bonds. The result of the cleavage pattern of oxidized insulin B-chain suggests that CP-70 protease has a broader specificity than the other cold-active proteases against the peptide substrate. Received: 17 April 1998 / Received last revision: 17 June 1998 / Accepted: 10 July 1998     

d-Aspartate binding sites in rat Harderian gland

Radioligand binding of d-[³H]aspartic and l-[³H]glutamic acids to plasma membranes from rat Harderian gland was evaluated. Binding was optimal under physiological conditions of pH and temperature, and equilibrium was reached within 50 min. Specific binding for d-Asp and l-Glu was saturable, and Eadie–Hofstee analysis revealed interaction with a single population of binding sites (for d-Asp K _d = 860 ± 28 nM, B _max = 27.2 ± 0.5 pmol/mg protein; for l-Glu, K _d = 580 ± 15 nM and B _max = 51.3 ± 0.8 pmol/mg protein). l-[³H]glutamate had higher affinity and a greater percentage of specific binding than did d-[³H]aspartate. The pharmacological binding specificity of l-[³H]glutamate indicated an interaction with NMDA-type receptors. Specifically, the order of potency of the displacing compound tested was l-Glu > d-Asp > NMDA > MK801 > d-AP5 > glycine. For d-[³H]aspartate, the data revealed an interaction of d-Asp with either NMDA-type receptors or putative specific binding sites.     

Enhanced <Emphasis Type="SmallCaps">l</Emphasis>-phenylalanine production by recombinant <Emphasis Type="Italic">Escherichia coli</Emphasis> BR-42 (pAP-B03) resistant to bacteriophage BP-1 via a two-stage feeding approach

The l-phenylalanine (l-Phe) production by Escherichia coli WSH-Z06 (pAP-B03) was frequently prevented by bacteriophage BP-1 infestation. To cope with the bacteriophage BP-1 problem for an improved l-Phe production, one bacteriophage BP-1-resistant mutant, E. coli BR-42, was obtained from 416 mutant colonies of E. coli WSH-Z06 after N-methyl-N’-nitro-N-nitrosoguanidine (NTG) mutagenesis by selection for resistance to bacteriophage BP-1. The recombinant E. coli BR-42-carrying plasmid pAP-B03 had a high capacity in l-Phe production and a remarkable tolerance to 1 × 10¹⁰ pfu (plaque-forming unit)/ml bacteriophage stock. For an enhanced l-Phe production by E. coli BR-42 (pAP-B03), the effects of different feeding strategies including pH–stat, constant rate feeding, linear decreasing rate feeding, and exponential feeding on l-Phe production were investigated; and a two-stage feeding strategy, namely exponential feeding at μ _set = 0.18 h⁻¹ in the first 20 h and a following linear varying rate feeding with F = (−0.55 × t + 18.6) ml/h, was developed to improve l-Phe production. With this two-stage feeding approach, a maximum l-Phe titer of 57.63 g/l with a high l-Phe productivity (1.15 g/l/h) was achieved, which was 15% higher than the highest level (50 g/l) reported so far according to our knowledge. The recombinant E. coli BR-42 (pAP-B03) is a potential l-Phe over-producer in substantial prevention of bacteriophage BP-1 infestation compared to its parent strain WSH-Z06 (pAP-B03).     

d-Xylose isomerases from a newly isolated strain, Paenibacillus sp., and from Alcaligenes ruhlandii : isolation, characterization and immobilisation to solid supports

d-Xylose/d-glucose isomerases from two strains, a newly isolated strain, Paenibacillus sp., and from Alcaligenes ruhlandii are described herein. The enzymes were purified to apparent homogeneity. Both of these d-xylose isomerases are homotetramers with relative subunit molecular masses of 45 000 and 53 000, respectively, as estimated by sodium dodecylsulphate-polyacrylamide gel electrophoresis. The native molecular masses determined on Superose 12 gel chromatography are 181 kDa for the enzyme from Paenibacillus sp. and 199 kDa for that from A. ruhlandii. The activity of both enzymes shows a requirement for divalent metal ions; the d-xylose isomerase from Paenibacillus sp. has the highest activity with Mn²⁺, while the enzyme from A. ruhlandii prefers Mg²⁺. Both enzymes also accept Co²⁺ with a somewhat lower efficiency, while Cu²⁺ inhibits the enzyme reaction. The binding of the metal ions obeys a biphasic characteristic, indicating the presence of two non-identical binding sites per subunit. d-Glucose is converted to d-fructose at a rate that is two- to three-fold slower than for the d-xylose isomerisation. d-Xylitol and d-lyxose are competitive inhibitors of both enzymes. Both enzymes have a pH optimum between 6.5 and 7.0, and they are active up to 60 °C. The enzyme from Paenibacillus sp. retained 50% of its activity after 4 days at 55 °C, whereas that from A. ruhlandii still retained 50% of its activity after 6 days at 55 °C. Polyacrylamide entrapment and immobilisation to both controlled pore glass and cyanogen-bromide-activated Sepharose were achieved for both enzymes with high efficiency. Received: 14 May 1998 / Received last revision: 29 July 1998 / Accepted: 29 July 1998     

Solubilization and partial purification of the rabbit parotid Na/K/Cl-dependent bumetanide binding site

Summary We demonstrate that the high affinity bumetanide binding site of the rabbit parotid acinar cell can be extracted from a basolateral membrane fraction using relatively low concentrations (0.07%, wt/vol; 1 mg membrane protein/ml) of the nonionic detergent Triton X-100. This extracted site cannot be sedimented by ultracentrifugation at 100,000 ×g × 1 hr. Bumetanide binding to this site retains the ionic characteristics of bumetanide binding to native membranes but shows a fivefold increase in binding affinity (K d=0.57±0.15 m vs.K d=3.3±0.7 m for native membranes). Inactivation of the extracted bumetanide binding site observed at detergent/protein ratios>1 can be prevented or (partially) reversed by the addition of exogenous lipid (0.2% soybean phosphatidylcholine). When the 0.07% Triton extract is fractionated by sucrose density gradient centrifugation in 0.24% Triton X-100, 0.2% exogenous lipid and 200mm salt, the high affinity bumetanide binding site sediments as a single band withS _20,w =8.8±0.8 S. This corresponds to a molecular weight 200 kDa for the bumetanide binding protein-detergent-lipid complex and represents a sevenfold purification of this site relative to the starting membrane fraction. In contrast to previous attempts to purify Na/K/Cl cotransport proteins and their associated bumetanide binding sites, the present method avoids harsh detergent treatment as well as direct covalent modification (inactivation) of the transporter itself. As a consequence, one can follow the still active protein through a series of extraction and purification steps by directly monitoring its bumetanide binding properties.     

Exploring the systematic effect of N-substituted PxxP motifs on peptoid affinity to ARHGEF5/TIM SH3 domain and its relationship with ARHGEF5/TIM activation

The TIM protein is a short isoform of full-length Rho guanine nucleotide exchange factor 5 (ARHGEF5), which acts as a functional regulator of Rho-dependent signaling pathways by activating the Rho family of GTPases. The activation is auto-inhibited by a putative helix N-terminal to the DH domain of TIM, which is stabilized by the intramolecular interaction of C-terminal SH3 domain with a proline-rich region ⁴⁷SSPRQP RKAL⁵⁶ (termed as SSP peptide) between the putative helix and the DH domain. Previously, we demonstrate that the auto-inhibitory state of TIM protein can be relieved by targeting its SH3 domain with rationally designed peptide ligands. However, the designed natural peptides have only a moderately increased affinity (~2-fold) as compared to the cognate SH3-SSP interaction and are susceptible to protease degradation. Here, considering that proline is the only endogenous N-substituted amino acid that plays a critical role in SH3-peptide recognition, the two key proline residues Pro49 and Pro52 in the core ⁴⁹PxxP ⁵² motif of SSP peptide are systematically replaced by 19 N-substituted amino acid types to derive a variety of nonnatural peptoid ligands for TIM SH3 domain. Dynamics and energetics analyses reveal that the replacement would impair the active polyproline II (PPII) helical conformation of SSP peptide due to lack of structural constraint introduced by the five-membered ring of native proline side-chains, thus increasing the peptide flexibility that could incur a large entropy penalty upon binding to the domain. However, the impairment is not very significant and the peptide affinity may also be restored and improved if the N-substituted motif of derived peptiod ligands can effectively interact with the PxxP-binding site of TIM SH3 domain. Consequently, a number of potent peptoids are successfully designed by fluorescence spectroscopy confirmation, in which three (ie, SSP[N-Ile49, N-Asn52], SSP[N-Phe49, N-Gln52], and SSP[N-Tyr49, N-Asn52]) exhibit considerably increased affinity (K_d = 0.09, 0.07, and 0.04 μM, respectively) relative to the native SSP peptide (K_d = 0.87 μM). In addition, guanine nucleotide exchange assays also substantiate that the designed SH3-targeted peptiods can effectively enhance TIM-catalyzed RhoA exchange activity (EA), which is observed to present an exponential relationship with the measured SH3-peptoid binding affinity (pK_d).     

Uptake and conversion of <Emphasis Type="SmallCaps">d</Emphasis>-amino acids in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

The d-enantiomers of proteinogenic amino acids fulfill essential functions in bacteria, fungi and animals. Just in the plant kingdom, the metabolism and role of d-amino acids (d-AAs) still remains unclear, although plants have to cope with significant amounts of these compounds from microbial decay in the rhizosphere. To fill this gap of knowledge, we tested the inhibitory effects of d-AAs on plant growth and established a method to quantitate 16 out of 19 proteinogenic amino acids and their d-enantiomers in plant tissue extracts. Therefore, the amino acids in the extracts were derivatized with Marfey’s reagent and separated by HPLC–MS. We used two ecotypes (Col-0 and C24) and a mutant (lht1) of the model plant Arabidopsis thaliana to determine the influence and fate of exogenously applied d-AAs. All of them were found in high concentrations in the plant extracts after application, even in lht1, which points to additional transporters facilitating the import of d-AAs. The addition of particular amino acids (d-Trp, d-Phe, d-Met and d-His) led to the accumulation of the corresponding l-amino acid. In almost all cases, the application of a d-AA resulted in the accumulation of d-Ala and d-Glu. The presented results indicate that soil borne d-AAs can actively be taken up and metabolized via central metabolic routes.     

Sodium-cotransport systems in intestine and kidney of the winter flounder

Summary Brush-border membrane vesicles were isolated from the intestine and kidney of the winter flounder,Pseudopleuronectes americanus, and the transport ofd-glucose,l-alanine and sodium was examined by a rapid filtration technique.d-glucose,l-alanine, and sodium entered the same osmotically reactive space suggesting that uptake into vesicles represents transport across rather than binding to the membrane. d-glucose andl-alanine uptake by intestinal and renal brush-border membrane vesicles was stimulated by sodium as compared to potassium or choline. In the presence of a sodium chloride gradient, overshooting uptake was observed indicating a transient intravesicular accumulation ofd-glucose andl-alanine. The sodium-dependentd-glucose uptake was inhibited by phlorizin andd-galactose while the transport ofl-alanine was inhibited byl-phenylalanine. The sodium-dependent transport ofd-glucose andl-alanine was affected by the electrical potential difference across the vesicle membrane; the addition of valinomycin in the presence of an inwardly directed potassium chloride gradient inhibited sodium-dependent solute uptake, whereas replacing chloride or gluconate with more permeant anions, such as SCN^–, stimulated uptake. Similar results were obtained with intestinal and renal membranes; they document the presence of sodium/d-glucose and sodium/l-alanine cotransport systems in the brush-border membrane of intestine and kidney.Sodium uptake into brush border membrane vesicles from the flounder intestine and kidney was saturable (tracer replacement) and trans-stimulated (tracer coupling), indicating transport via facilitated diffusion systems. Additionally, sodium uptake was only slightly affected by superimposing diffusion potentials demonstrating that the majority of sodium transport was by electroneutral coupled processes. In both the intestinal and kidney brush-border membrane vesicles sodium uptake was inhibited by an inwardly directed proton gradient suggesting the presence of a sodium/proton exchange mechanism. In intestinal, but not in renal membrane preparations, sodium uptake was stimulated by chloride. Chloride stimulation was abolished after preincubation with furosemide indicating the presence of an additional coupled sodium-chloride transport in the intestinal brush-border membranes.The experiments were carried out at the Mount Desert Island Biological Laboratory, Salsbury Cove, Maine 04672, USAAddress effective February 1, 1980: Albert Einstein College of Medicine, Department of Physiology, 1300 Morris Park Avenue, Bronx, New York 10461, USA     

Permeabilization and lysis of Pseudomonas pseudoalcaligenes cells by Triton X-100 for efficient production of d-malate

Pseudomonas pseudoalcaligenes can only form d-malate from maleate after incubation of the cells with a solvent or a detergent. The effect of the detergent Triton X-100 on d-malate production was studied in more detail. The longer the cells were incubated with Triton X-100, the higher was the d-malate production activity, until the maximal malease activity was reached. Incubation of P. pseudoalcaligenes cells with Triton X-100 also resulted in an increase in the protein concentration of the supernatant, indicating that cell lysis had occurred. The rate at which the d-malate production activity increased was dependent on the Triton X-100 concentration and on the cell density. Also the rate at which lysis occurred depended on the Triton X-100 concentration.     

Nuclear magnetic resonance investigation of lysine–oligopeptides and a complex with d(pA)3pGpC(pT)3

We report proton magnetic resonance studies of a series of lysine oligopeptides in H₂O solution. At pH 5 the protonated ε-amino groups are seen as broad resonances; the peptide NH proton resonances are split by spin–spin coupling with the C^α-H proton, and appear at positions which depend on position in the chain and on chain length. Assignments were made by the europium shift method, and we observed the expected effect of catalysis by the terminal —NH₃⁺ of exchange of the adjacent peptide NH. Coupling constants and the temperature coefficient of chemical shift values were consistent with a non-hydrogen-bonded structure for the oligolysines. The rate and mechanism of NH hydrogen exchange were investigated by line-broadening measurements of the peptide protons as a function of pH. Exchange was found to be OH^? catalyzed, with large differences in the rate depending on position in the chain. Preliminary studies of the complex between double-helical d(pA)₃pGpC(pT)₃ and tetra(L -lysine) were performed using ¹H- and ³¹P-nmr techniques. Pmr spectra of the complex at pH values ranging from 3.98 to 8.15 showed very complicated patterns. Downfield shifts and reduction in exchange rates were observed for several tetra(L -lysine) protons. ³¹P-nmr spectra of the complex reveal an upfield shift of 1 ppm for 3′-5′ phosphate diester resonances on complexation. ³¹P T₁ relaxation times change little on complex formation at low temperature but are altered at higher temperature.     

Enzymatic production of l-phenylalanine from the racemic mixture of d,l-phenyllactate

Summary The production of l-phenylalanine from the racemate d,l-phenyllactate in an enzyme membrane reactor has been examined. In a first step the racemate is dehydrogenated to the prochiral intermediate phenylpyruvate by the enzymes d-and l-hydroxyisocaproate dehydrogenase. In a second step phenylpyruvate is reductively aminated to l-phenylalanine by l-phenylalanine dehydrogenase. Both steps are dependent on coenzyme, the first one requires NAD, the second one NADH in stoichiometric amounts; in this way the coenzyme is regenerated and only required catalytically. The coenzyme is covalently bound to polyethylene glyco-20 000 and can thus be retained in the reactor analogously to the three enzymes. In order to optimize the continuous production of l-phenylalanine from d,l-phenyllactate, models of the reaction kinetics and of the reactor system have been set up. By means of the reactor model, we can calculate the optimum ratio of the three enzymes, the optimum coenzyme concentration and the optimum phenylpyruvate concentration in the feed.In this process, at a substrate concentration of 50 mM d,l-phenyllactate we reached a spacetime-yield of 28 g l-Phe/(l*d).Abbreviations PEG polyethylene glycol - d-HicDH d-hydroxyisocaproate dehydrogenase - l-HicDH l-hydroxyisocaproate dehydrogenase - PheDH l-phenylalanine dehydrogenase - V _max maximum velocity - K _M Michaelis-Menten constant - K _l inhibition constant - R₁ reaction rate of the d-HicDH forward reaction - R₂ reaction rate of the d-HicDH reverse reaction - R₃ reaction rate of the l-HicDH forward reaction - R₄ reaction rate of the l-HicDH reverse reaction - R₅ reaction rate of the PheDH forward reaction - R₆ reaction rate of the PheDH reverse reaction - d-PLac d-phenyllactate - l-PLac l-phenyllactate - PPy phenylpyruvate - l-Phe l-phenylalanine - NH₄ ammonium - residence time     

Eryngase: a Pleurotus eryngii aminopeptidase exhibiting peptide bond formation activity

An aminopeptidase that has peptide bond formation activity was identified in the cell-free extract of carpophore of Pleurotus eryngii. The enzyme, redesignated as eryngase, was purified for homogeneity and characterized. Eryngase had a molecular mass of approximately 79 kDa. It showed somewhat high stability with respect to temperature and pH; it was inhibited by iodoacetate. Among hydrolytic activities toward aminoacyl-p-nitroanilides (aminoacyl-pNAs), eryngase mainly hydrolyzed hydrophobic l-aminoacyl-pNAs and exhibited little activity toward d-Ala-pNA and d-Leu-pNA. In terms of peptide bond formation activity, eryngase used various aminoacyl derivatives as acyl donors and acceptors. The products were all dipeptidyl derivatives. Investigation of time dependence on peptide synthesis revealed that some peptides that are not recognized as substrates for hydrolytic activity of eryngase could become good targets for synthesis. Furthermore, eryngase has produced opioid dipeptides––l-kyotorphin (l-Tyr-l-Arg) and d-kyotorphin (l-Tyr-d-Arg)––using l-Tyr-NH₂ and d- and l-Arg-methyl ester respectively as an acyl donor and acceptor. Yield evaluation of kyotorphin synthesis indicated that the conversion ratio of substrate to kyotorphin was moderate: the value was estimated as greater than 20%.     

New Drugs for the Na+/H+ Exchanger. Influence of Na+ Concentration and Determination of Inhibition Constants with a Microphysiometer

The NHE-1 isoform of the Na⁺/H⁺ exchanger is excessively activated in cardiac cells during ischemia. Hence NHE-1 specific inhibitors are being developed since they could be of beneficial influence under conditions of cardiac ischemia and reperfusion. In this study, the Cytosensor™ microphysiometer was used to measure the potency of four new drug molecules, i.e., EMD 84021, EMD 94309, EMD 96785 and HOE 642 which are inhibitors of the isoform 1 of the Na⁺/H⁺ exchanger. The experiments were performed with Chinese hamster ovary cells (CHO K1) which are enriched in the NHE-1 isoform of the Na⁺/H⁺ antiporter. The Na⁺/H⁺ exchanger was stimulated with NaCl and the rate of extracellular acidification was quantified with the Cytosensor. The proton exchange rate was measured as a function of the NaCl concentration in the range of 10–138 mm NaCl stimulation. The proton exchange rate followed Michaelis-Menten kinetics with a K _M = 30 ± 4 mm for Na⁺. Addition of either one of the four inhibitors decreased the acidification rate. The IC₅₀ values of the four compounds could be determined as 23 ± 7 nm for EMD 84021, 5 ± 1 nm for EMD 94309, 9 ± 2 nm for EMD 96785 and 8 ± 2 nm for HOE 642 at 138 mm NaCl, in good agreement with more elaborate biological assays. The IC₅₀ values increased with the NaCl concentration indicating competitive binding of the inhibitor. The microphysiometer approach is a fast and simple method to measure the activity of the Na⁺/H⁺ antiporter and allows a quantitative kinetic analysis of the proton excretion rate. Received: 3 September 1998/Revised: 20 November 1998     

The ncgl1108 (PheP (Cg)) gene encodes a new L-Phe transporter in Corynebacterium glutamicum

Corynebacterium glutamicum played a central role in the establishment of fermentative production of amino acids, and it is a model for genetic and physiological studies. The general aromatic amino acid transporter, AroP _Cg , was the sole functionally identified aromatic amino acid transporter from C. glutamicum. In this study, the ncgl1108 (named as pheP _Cg ), which is located upstream of the genetic cluster (ncgl1110 ∼ ncgl1113) for resorcinol catabolism, was identified as a new l-Phe specific transporter from C. glutamicum RES167. The disruption of pheP _Cg resulted in RES167∆ncgl1108, and this mutant showed decreased growth on l-Phe (as nitrogen source) but not on l-Tyr or l-Trp. Uptake assays with unlabeled and ¹⁴C-labeled l-Phe and l-Tyr indicated that the mutants RES167∆ncgl1108 showed significant reduction in l-Phe uptake than RES167. Expression of pheP _Cg in RES167∆ncgl1108/pGXKZ1 or RES167∆(ncgl1108-aroP _Cg )/pGXKZ1 restored their ability to uptake for l-Phe and growth on l-Phe. The uptake of l-Phe was not inhibited by nine amino acids but by l-Tyr. The K _m and V _max values of RES167∆(ncgl1108-aroP _Cg )/pGXKZ1 for l-Phe were determined to be 10.4 ± 1.5 μM and 1.2 ± 0.1 nmol min⁻¹ (mg DW)⁻¹, respectively, which are different from K _m and V _max values of RES167∆(ncgl1108-aroP _Cg ) for l-Phe [4.0 ± 0.4 μM and 0.6 ± 0.1 nmol min⁻¹ (mg DW)⁻¹]. In conclusion, this PheP _Cg is a new l-Phe transporter in C. glutamicum.     

Characterization of a glucose 3-dehydrogenase from the cultivated mushroom (Agaricus bisporus )

An extracellular enzyme with glucose dehydrogenase activity was purified from liquid cultures of the basidiomycete Agaricus bisporus after growth with d-cellobiose or d-glucose as carbon source. The molecular mass was measured as 57 kDa by gel filtration and 55 kDa by sodiumdodecyl sulphate/polyacrylamide gel electrophoresis, while the isoelectric point was at pH 3.6. By analysis of ¹H-NMR spectra in D₂O, the product of d-glucose oxidation was identified as 3-ketoglucose. The substrates oxidized included d-cellobiose, l-arabinose, d-xylose and sucrose, but the specificity parameter (k _cat/K _m) was highest for d-glucose. Two electron acceptors were identified, namely 2,6-dichloroindophenol and p-benzoquinone, but reduction of dioxygen, ferricyanide or cytochrome c was not detectable. The selective C-3 oxidation of d-glucose is well-characterized for Agrobacterium and Flavobacterium, but this is the first report for a fungus. Received: 19 June 1998 / Received revision: 15 September 1998 / Accepted: 17 September 1998     

Murein structure of Acetobacterium woodii

The isolated cell walls of Acetobacterium woodii contain a murein of the crosslinkage type B. d-Orinithinyl residues function as interpeptide bridges between the -carboxyl group of d-glutamic acid and the carboxyl group of the terminal d-analyl residue of an adjacent peptide subunit. The usual l-alanyl residue in position 1 of the peptide subunit is replaced by a l-seryl residue. As yet this murein type was only found in Eubacterium limosum, an organism which was supposed to be related to Acetobacterium because of some metabolic similarities.