Identification and Phenotype of MAIT Cells in Cattle and Their Response to Bacterial Infections

Mucosal-associated invariant T (MAIT) cells are a population of innate-like T cells that utilize a semi-invariant T cell receptor (TCR) α chain and are restricted by the highly conserved antigen presenting molecule MR1. MR1 presents microbial riboflavin biosynthesis derived metabolites produced by bacteria and fungi. Consistent with their ability to sense ligands derived from bacterial sources, MAIT cells have been associated with the immune response to a variety of bacterial infections, such as Mycobacterium spp., Salmonella spp. and Escherichia coli. To date, MAIT cells have been studied in humans, non-human primates and mice. However, they have only been putatively identified in cattle by PCR based methods; no phenotypic or functional analyses have been performed. Here, we identified a MAIT cell population in cattle utilizing MR1 tetramers and high-throughput TCR sequencing. Phenotypic analysis of cattle MAIT cells revealed features highly analogous to those of MAIT cells in humans and mice, including expression of an orthologous TRAV1-TRAJ33 TCR α chain, an effector memory phenotype irrespective of tissue localization, and expression of the transcription factors PLZF and EOMES. We determined the frequency of MAIT cells in peripheral blood and multiple tissues, finding that cattle MAIT cells are enriched in mucosal tissues as well as in the mesenteric lymph node. Cattle MAIT cells were responsive to stimulation by 5-OP-RU and riboflavin biosynthesis competent bacteria in vitro. Furthermore, MAIT cells in milk increased in frequency in cows with mastitis. Following challenge with virulent Mycobacterium bovis, a causative agent of bovine tuberculosis and a zoonosis, peripheral blood MAIT cells expressed higher levels of perforin. Thus, MAIT cells are implicated in the immune response to two major bacterial infections in cattle. These data suggest that MAIT cells are functionally highly conserved and that cattle are an excellent large animal model to study the role of MAIT cells in important zoonotic infections.

Human embryoid bodies as a 3D tissue model of the extracellular matrix and α-dystroglycanopathies

The basal lamina is a specialized sheet of dense extracellular matrix (ECM) linked to the plasma membrane of specific cell types in their tissue context, which serves as a structural scaffold for organ genesis and maintenance. Disruption of the basal lamina and its functions is central to many disease processes, including cancer metastasis, kidney disease, eye disease, muscular dystrophies and specific types of brain malformation. The latter three pathologies occur in the α-dystroglycanopathies, which are caused by dysfunction of the ECM receptor α-dystroglycan. However, opportunities to study the basal lamina in various human disease tissues are restricted owing to its limited accessibility. Here, we report the generation of embryoid bodies from human induced pluripotent stem cells that model the basal lamina. Embryoid bodies cultured via this protocol mimic pre-gastrulation embryonic development, consisting of an epithelial core surrounded by a basal lamina and a peripheral layer of ECM-secreting endoderm. In α-dystroglycanopathy patient embryoid bodies, electron and fluorescence microscopy reveal ultrastructural basal lamina defects and reduced ECM accumulation. By starting from patient-derived cells, these results establish a method for the in vitro synthesis of patient-specific basal lamina and recapitulate disease-relevant ECM defects seen in the α-dystroglycanopathies. Finally, we apply this system to evaluate an experimental ribitol supplement therapy on genetically diverse α-dystroglycanopathy patient samples.This article has an associated First Person interview with the first author of the paper.

The Chemical Synthesis, Stability, and Activity of MAIT Cell Prodrug Agonists That Access MR1 in Recycling Endosomes

Mucosal-associated invariant T (MAIT) cells are antibacterial effector T cells that react to pyrimidines derived from bacterial riboflavin synthesis presented by the monomorphic molecule MR1. A major challenge in MAIT cell research is that the commonly used MAIT agonist precursor, 5-amino-6-d-ribitylaminouracil (5-A-RU), is labile to autoxidation, resulting in a loss of biological activity. Here, we characterize two independent autoxidation processes by LCMS. To overcome the marked instability, we report the synthesis of a 5-A-RU prodrug generated by modification of the 5-amino group with a cleavable valine-citrulline-p-aminobenzyl carbamate. The compound is stable in prodrug form, with the parent amine (i.e., 5-A-RU) released only after enzymatic cleavage. Analysis of the prodrug in vitro and in vivo showed an enhanced MAIT cell activation profile compared to 5-A-RU, which was associated with preferential loading within recycling endosomes, a route used by some natural agonists. This prodrug design therefore overcomes the difficulties associated with 5-A-RU in biological studies and provides an opportunity to explore different presentation pathways.

Detection, Expansion, and Isolation of Human MAIT Cells

Mucosal-associated invariant T (MAIT) cells are a novel subset of innate-like T cells that recognize vitamin B metabolites from a range of microbes presented by MHC class I-related molecules (MR1). The term mucosal-associated invariant T cells derives from the fact that MAIT cells are abundant in the liver and mucosal tissues, and human MAIT cells use a semi-invariant TCR Vα7.2 Jα33 paired with Vβ2 or Vβ13. Here, based on the interaction between MAIT cell and its ligand 5-OP-RU/MR1, we describe the protocols for identification, rapid expansion, and isolation of human MAIT cells.

Structure and inhibition of a quorum sensing target from Streptococcus pneumoniae

Streptococcus pneumoniae 5'-methylthioadenosine/S-adenosylhomocysteine hydrolase (MTAN) catalyzes the hydrolytic deadenylation of its substrates to form adenine and 5-methylthioribose or S-ribosylhomocysteine (SRH). MTAN is not found in mammals but is involved in bacterial quorum sensing. MTAN gene disruption affects the growth and pathogenicity of bacteria, making it a target for antibiotic design. Kinetic isotope effects and computational studies have established a dissociative S(N)1 transition state for Escherichia coli MTAN, and transition state analogues resembling the transition state are powerful inhibitors of the enzyme [Singh, V., Lee, J. L., Núñez, S., Howell, P. L., and Schramm, V. L. (2005) Biochemistry 44, 11647-11659]. The sequence of MTAN from S. pneumoniae is 40% identical to that of E. coli MTAN, but S. pneumoniae MTAN exhibits remarkably distinct kinetic and inhibitory properties. 5'-Methylthio-Immucillin-A (MT-ImmA) is a transition state analogue resembling an early S(N)1 transition state. It is a weak inhibitor of S. pneumoniae MTAN with a K(i) of 1.0 microM. The X-ray structure of S. pneumoniae MTAN with MT-ImmA indicates a dimer with the methylthio group in a flexible hydrophobic pocket. Replacing the methyl group with phenyl (PhT-ImmA), tolyl (p-TolT-ImmA), or ethyl (EtT-ImmA) groups increases the affinity to give K(i) values of 335, 60, and 40 nM, respectively. DADMe-Immucillins are geometric and electrostatic mimics of a fully dissociated transition state and bind more tightly than Immucillins. MT-DADMe-Immucillin-A inhibits with a K(i) value of 24 nM, and replacing the 5'-methyl group with p-Cl-phenyl (p-Cl-PhT-DADMe-ImmA) gave a K(i) value of 0.36 nM. The inhibitory potential of DADMe-Immucillins relative to the Immucillins supports a fully dissociated transition state structure for S. pneumoniae MTAN. Comparison of active site contacts in the X-ray crystal structures of E. coli and S. pneumoniae MTAN with MT-ImmA would predict equal binding, yet most analogues bind 10(3)-10(4)-fold more tightly to the E. coli enzyme. Catalytic site efficiency is primarily responsible for this difference since k(cat)/K(m) for S. pneumoniae MTAN is decreased 845-fold relative to that of E. coli MTAN.

Structural and thermodynamic insights into the binding mode of five novel inhibitors of lumazine synthase from Mycobacterium tuberculosis

Recently published genomic investigations of the human pathogen Mycobacterium tuberculosis have revealed that genes coding the proteins involved in riboflavin biosynthesis are essential for the growth of the organism. Because the enzymes involved in cofactor biosynthesis pathways are not present in humans, they appear to be promising candidates for the development of therapeutic drugs. The substituted purinetrione compounds have demonstrated high affinity and specificity to lumazine synthase, which catalyzes the penultimate step of riboflavin biosynthesis in bacteria and plants. The structure of M. tuberculosis lumazine synthase in complex with five different inhibitor compounds is presented, together with studies of the binding reactions by isothermal titration calorimetry. The inhibitors showed the association constants in the micromolar range. The analysis of the structures demonstrated the specific features of the binding of different inhibitors. The comparison of the structures and binding modes of five different inhibitors allows us to propose the ribitylpurinetrione compounds with C4-C5 alkylphosphate chains as most promising leads for further development of therapeutic drugs against M. tuberculosis.

Mulberry latex rich in antidiabetic sugar-mimic alkaloids forces dieting on caterpillars

Since ancient times, mulberry leaves (Morus spp.) have been used to rear the silkworm Bombyx mori. Because the silkworm grows well on mulberry leaves, the toxicities and defensive activities of these leaves against herbivorous insects have been overlooked. Here we show that mulberry leaves are highly toxic to caterpillars other than the silkworm B. mori, because of the ingredients of the latex, a milky sap exuded from mulberry leaf veins. The toxicity of mulberry leaves was lost when the latex was eliminated from the leaves, and artificial diets containing latex showed toxicity. Mulberry latex contained very high concentrations of alkaloidal sugar-mimic glycosidase inhibitors reported to have antidiabetic activities, such as 1,4-dideoxy-1,4-imino-D-arabinitol, 1-deoxynojirimycin, and 1,4-dideoxy-1,4-imino-D-ribitol. The overall concentrations of these inhibitors in latex reached 1.5-2.5% (8-18% dry weight) in several mulberry varieties, which were approximately 100 times the concentrations previously reported from whole mulberry leaves. These sugar-mimic alkaloids were toxic to caterpillars but not to the silkworm B. mori, indicating that the silkworm can circumvent the mulberry tree's defense. Our results suggest that latex ingredients play key roles in defense of this tree and of other plants against insect herbivory, and they imply that plant latexes are treasuries of bioactive substances useful as medicines and pesticides.

Design, Synthesis, and Evaluation of 9-d-Ribitylamino-1,3,7,9-tetrahydro-2,6,8-purinetriones Bearing Alkyl Phosphate and α,α-Difluorophosphonate Substituents as Inhibitors of Riboflavin Synthase and Lumazine Synthase

Lumazine synthase and riboflavin synthase catalyze the last two steps in the biosynthesis of riboflavin, an essential metabolite that is involved in electron transport processes. To obtain structural probes of these two enzymes, as well as inhibitors of potential value as antibiotics, a series of ribitylpurinetriones bearing alkyl phosphate and alpha,alpha-difluorophosphonate substituents were synthesized. Since the purinetrione ring system and the ribityl hydroxyl groups can be alkylated, the synthesis required the generation of these two moieties in protected form before the desired alkylation reaction could be carried out. These substances were designed as intermediate analogue inhibitors of lumazine synthase that would bind to its phosphate-binding site. All of the compounds were found to be effective inhibitors of both Bacillus subtilis lumazine synthase as well as Escherichia coli riboflavin synthase. Molecular modeling of the binding of 3-(1,3,7,9-tetrahydro-9-D-ribityl-2,6,8-trioxopurin-7-yl)propane 1-phosphate provided a structural explanation for how these compounds are able to effectively inhibit both enzymes. Interestingly, the enzyme kinetics of these new compounds in comparison with the parent purinetrione demonstrated unexpectedly that the phosphate and phosphonate substituents contributed negatively to the binding. A possible explanation for these effects on lumazine synthase would be that the inorganic phosphate in the assay buffer competes with the substituted purinetriones for binding to the enzyme. This would be consistent with the observed increase in K(m) of the 3,4-dihydroxybutanone-4-phosphate substrate from 5.2 microM in Tris buffer or from 6.7 microM in MOPS buffer to 50 microM in phosphate buffer when tested on Bacillus subtilis lumazine synthase. However, when tested in Tris buffer vs Mycobacterium tuberculosis lumazine synthase, three of the phosphate inhibitors displayed inhibition constants in the 4-5 nM range, indicating that they are much more potent than the parent purinetrione. Under these conditions, the phosphate moieties of the inhibitors do contribute positively to their binding. The alpha,alpha-difluorophosphonate analogue, which is expected to have enhanced metabolic stability relative to the phosphates, was also found to be an inhibitor of Mycobacterium tuberculosis lumazine synthase with a K(i) of 60 nM.

Design, synthesis, and evaluation of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-ribitylaminolumazines as inhibitors of riboflavin synthase and lumazine synthase

A series of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-D-ribityllumazine were synthesized as inhibitors of both Escherichia coli riboflavin synthase and Bacillus subtilis lumazine synthase. The compounds were designed to bind to both the ribitylpurine binding site and the phosphate binding site of lumazine synthase. In the carboxyalkyl series, maximum activity against both enzymes was observed with the 3'-carboxypropyl compound 22. Lengthening or shortening the chain linking the carboxyl group to the lumazine by one carbon resulted in decreased activity. In the phosphonoxyalkyl series, the 3'-phosphonoxypropyl compound 33 was more potent than the 4'-phosphonoxybutyl derivative 39 against lumazine synthase, but it was less potent against riboflavin synthase. Molecular modeling suggested that the terminal carboxyl group of 6-(3'-carboxypropyl)-7-oxo-8-D-ribityllumazine (22) may bind to the side chains of Arg127 and Lys135 of the enzyme. A hypothetical molecular model was also constructed for the binding of 6-(2'-carboxyethyl)-7-oxolumazine (15) in the active site of E. coli riboflavin synthase, which demonstrated that the active site could readily accommodate two molecules of the inhibitor.

Computer simulations of trypanosomal nucleoside hydrolase: determination of the protonation state of the bound transition-state analogue

Inosine-uridine nucleoside hydrolase (IU-NH) catalyzes the hydrolysis of nucleosides into base and ribose moieties via a ribooxocarbenium ion transition state, which has been characterized using kinetic isotope effects. Protozoan parasites lack de novo purine and pyrimidine biosynthesis and depend on the purine salvage from the host. Vern Schramm and co-workers characterized p-aminophenyliminoribitol (pAPIR) to be a potent inhibitor of IU-NH from Crithidia fasciculata with K(d) of 30 nM. The cyclic amine function of the iminoribitol ring can be either protonated (pAPIRH(+)) or unprotonated (pAPIR). pAPIRH(+) resembles the charge and geometry of the ribooxocarbenium ion transition state and can be looked upon as a transition-state analogue inhibitor; however, it is known that the pAPIR species is initially bound to the enzyme. We have characterized the pAPIRH(+) species as resident of the active site using ab initio calculations and molecular dynamics simulations. This is a novel use of molecular dynamics to investigate the protonation state of the bound ligand to the active site. Nanosecond molecular dynamics simulations reveal a short hydrogen-bonding network between pAPIRH(+)-O2'-Asp14-His241 triad, which is not seen in the crystal structure. Other features discussed are: hydrogen bonding between pAPIRH(+) and Asn168, unusual geometry of the iminoribitol ring, and hydrophobic interactions.

Cryoprotective role of polyols independent of the increase in supercooling capacity in diapausing adults of Pyrrhocoris apterus (Heteroptera: Insecta)

Diapausing cold-acclimated adults of the bug Pyrrhocoris apterus accumulate four 'winter' polyols, ribitol, sorbitol, mannitol and arabinitol, in total concentrations of up to 100 mM. The accumulation started only when the temperatures dropped below a threshold of +5 degrees C in laboratory acclimated insects. The supercooling capacity (SCP) was not affected by polyol accumulation and remained constant at approximately -17 degrees C. Cold hardiness, measured as survival time (Lt50) at -15 degrees C, increased from approximately 1 day to approximately 1 week in parallel with polyol accumulation. There was a tight correlation (r=0.98) between the concentration of 'winter' polyols in haemolymph and Lt50(-15). When a mixture of ribitol and sorbitol was injected into the haemolymph of the bugs acclimated to +5 degrees C, the concentration of polyols increased from 2.5 to 83.1 mM in haemolymph, or from 0.07 to 6.61 microg/mg of fresh weight in the whole body, the SCP remained unchanged and survival after exposure to -14 degrees C for 3 days increased approximately three-fold in comparison to untreated controls. Such results were interpreted as evidence for the cryoprotective role of accumulated polyols independent of the depression of SCP. Acclimation protocol using thermoperiod, mimicking daily temperature oscillations, resulted in moderately lower SCP, higher sum of polyols accumulated and significantly longer Lt50(-15) than at acclimation protocol with constant temperatures.

Kelletinin A, from the marine mollusc Buccinulum corneum, promotes differentiation in Hydra vulgaris

The effects of kelletinin A [ribityl pentakis (p-hydroxybenzoate)] (KA), a natural compound isolated from the marine gastropod Buccinulum corneum, were studied in vivo in Hydra vulgaris during regeneration. KA caused a marked increase of regenerated tentacle numbers (ATN) and promoted transdifferentiation of epithelial cells into battery cells, and nematocyte differentiation. Morphological data were correlated to changes in acid and alkaline phosphatase levels, enzymes that have been described as regeneration markers.

Ricin A-chain: kinetics, mechanism, and RNA stem-loop inhibitors

Ricin A-chain (RTA) catalyzes the depurination of a single adenine at position 4324 of 28S rRNA in a N-ribohydrolase reaction. The mechanism and specificity for RTA are examined using RNA stem-loop structures of 10-18 nucleotides which contain the required substrate motif, a GAGA tetraloop. At the optimal pH near 4.0, the preferred substrate is a 14-base stem-loop RNA which is hydrolyzed at 219 min-1 with a kcat/Km of 4.5 x 10(5) M-1 s-1 under conditions of steady-state catalysis. Smaller or larger stem-loop RNAs have lower kcat values, but all have Km values of approximately 5 microM. Both the 10- and 18-base substrates have kcat/Km near 10(4) M-1 s-1. Covalent cross-linking of the stem has a small effect on the kinetic parameters. Stem-loop DNA (10 bases) of the same sequence is also a substrate with a kcat/Km of 0.1 that for RNA. Chemical mechanisms for enzymatic RNA depurination reactions include leaving group activation, stabilization of a ribooxocarbenium transition state, a covalent enzyme-ribosyl intermediate, and ionization of the 2'-hydroxyl. A stem-loop RNA with p-nitrophenyl O-riboside at the depurination site is not a substrate, but binds tightly to the enzyme (Ki = 0.34 microM), consistent with a catalytic mechanism of leaving group activation. The substrate activity of stem-loop DNA eliminates ionization of the 2'-hydroxyl as a mechanism. Incorporation of the C-riboside formycin A at the depurination site provides an increased pKa of the adenine analogue at N7. Binding of this analogue (Ki = 9.4 microM) is weaker than substrate which indicates that the altered pKa at this position is not an important feature of transition state recognition. Stem-loop RNA with phenyliminoribitol at the depurination site increases the affinity substantially (Ki = 0.18 microM). The results are consistent with catalysis occurring by leaving group protonation at ring position(s) other than N7 leading to a ribooxocarbenium ion transition state. Small stem-loop RNAs have been identified with substrate activity within an order of magnitude of that reported for intact ribosomes.

Synthesis of 2,6-dioxo-(1H,3H)-9-N-ribitylpurine and 2,6-dioxo-(1H,3H)-8-aza-9-N-ribitylpurine as inhibitors of lumazine synthase and riboflavin synthase

2,6-Dioxo-(1H,3H)-9-N-ribitylpurine (6) and 2,6-dioxo-(1H,3H)-8-aza-9-N-ribitylpurine (7) have been synthesized and evaluated as inhibitors of lumazine synthase and riboflavin synthase. Reaction of 5-amino-6-ribitylaminouracil hydrochloride (8) with diethoxymethyl acetate (9) afforded the purine 6, while diazotization of 8 afforded the 8-aza purine 7. Compounds 6 and 7 were evaluated against lumazine synthase of Bacillus subtilis and riboflavin synthase of Escherichia coli. Both 6 and 7 were better inhibitors of lumazine synthase than riboflavin synthase. The 8-azapurine 7 had a lower KI (0.33 and 0.39 mM) than the purine 6 (0.47 and 0.54 mM) when evaluated with lumazine synthase and riboflavin synthase, respectively.

Isozyme-specific transition state inhibitors for the trypanosomal nucleoside hydrolases

Protozoan parasites lack de novo purine biosynthesis and require purine salvage from the host. Nucleoside hydrolases are involved in nucleoside salvage and are not found in mammals, making them protozoan-specific targets for inhibitor design. Several protozoan nucleoside hydrolase isozymes with distinct substrate specificities have been characterized. Novel substituted iminoribitols have been synthesized to resemble the transition state structure of the nonspecific inosine-uridine nucleoside hydrolase from Crithidia fasciculata (IU-nucleoside hydrolase). These inhibitors have been characterized for this enzyme and for a purine-specific nucleoside hydrolase (IAG-nucleoside hydrolase) from Trypanosoma brucei brucei. Inhibitors which provide nanomolar inhibition constants for IU-nucleoside hydrolase exhibit micromolar inhibition constants for the IAG-enzyme. For example, p-bromophenyliminoribitol inhibits the IU- and IAG-enzymes with dissociation constants of 28 nM and 190 microM, respectively. Substrate specificity, the action of transition state inhibitors and the pH-dependence of the kinetic constants establish that the catalytic mechanisms and transition state structures are fundamentally different for the IU- and IAG-isozymes. The finding is remarkable since these isozymes share significant homology at the catalytic sites and both use inosine as a preferred substrate. The specificity of the transition state analogues indicates that logically-designed transition state inhibitors are isozyme-specific, with (Km/Ki IU-nucleoside hydrolase)/(Km/Ki IAG-nucleoside hydrolase) values up to 39,000. The mechanism of the differential inhibition is based on the relative leaving group activation and ribosyl-oxocarbenium-forming abilities of these enzymes. In addition to providing isozyme-specific inhibitors, the novel molecules described here have diagnostic value for the nature of the transition states for N-ribohydrolase enzymes.

Inhibition mechanisms of HIV-1, Mo-MuLV and AMV reverse transcriptases by Kelletinin A from Buccinulum corneum

Kelletinin A [ribity pentakis (p-hydroxybenzoate)] (KA), an inhibitor of HTLV-1 replication isolated from Buccinulum corneum, showed a noncompetitive inhibitory activity with respect to the template primer and to dTTP in the poly(rA).oligo(dT)12-18-directed reaction of HIV-1, Mo-MuLV and AMV reverse transcriptases (RT). Analysis of natural and synthetic KA-related compounds showed that the inhibitory activity was strictly related to the structural peculiarities of the molecule. In the presence of DNA as template primer the inhibition mechanism was drastically modified: HIV-1 RT activity was stimulated by low concentrations of KA and was inhibited by increasing the concentration of the compound, while Mo-MuLV and AMV activities were irreversibly inhibited by the formation of a non-reactive complex. The RNase H activities of these RTs were not affected by KA. The results of this study suggest a different mechanism of interaction of Kelletinins with HIV-1 RT compared with other non-nucleoside inhibitors. A possible use of these drugs in combination therapy and in the design of structure-based reverse transcriptase inhibitors is discussed.

Antimitotic and antiviral activities of Kelletinin A in HTLV-1 infected MT2 cells

Kelletinin A [ribityl-pentakis (p-hydroxybenzoate)] (KA), a natural compound isolated from the marine gastropod Buccinulum corneum, showed antiviral activity on the human T-cell leukemia virus type-1 (HTLV-1) and antimitotic activity on HTLV-1-infected MT2 cells. KA inhibited cellular DNA and RNA synthesis, without influencing protein synthesis, and interfered with viral transcription by reducing the levels of high molecular weight transcripts. Finally, the compound inhibited HTLV-1 reverse transcriptase in vitro.

Binding modes for substrate and a proposed transition-state analogue of protozoan nucleoside hydrolase

The transition-state structure for inosine-uridine nucleoside hydrolase (IU-nucleoside hydrolase) from Crithidia fasciculata is characterized by oxycarbonium character in the ribosyl and weak bonds to the departing hypoxanthine and incipient water nucleophile [Horenstein, B. A., Parkin, D. W., Estupiñán, B., & Schramm, V. L. (1991) Biochemistry 30, 10788-10795]. Inhibitors designed to resemble the transition state are slow-onset, tight-binding inhibitors with observed Km/Ki values up to 2 x 10(5) [Schramm, V. L., Horenstein, B. H., & Kline, P. C. (1994) J. Biol. Chem. 269, 18259-18262]. Although slow-onset, tight binding is consistent with transition-state stabilization, more direct evidence can be obtained by comparing the groups which interact with the substrate to provide binding and catalysis with those which interact with the putative transition-state inhibitor. The Km value for inosine binding to IU-nucleoside hydrolase is independent of pH over the range 5.6-10.5. Dependencies of Vmax and Vmax/Km on pH result in pH optima near 8.0. A single group with pK of 9.1 must be protonated for catalytic activity, and protonation of a second group with a pK of 7.1 results in loss of activity. 1-(S)-Phenyl-1,4-dideoxy-1,4-imino-D-ribitol (phenyliminoribitol) binds with an equilibrium Kd of 30 nM and has been proposed to be a transition-state inhibitor. The pH dependence for the competitive inhibition by phenyliminoribitol resembles the Vmax profile with the protonation of a single group, pK 7.5, required for inhibitor binding and the protonation of a subsequent group, pK 6.6, causing loss of binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanosine-inosine-preferring nucleoside N-glycohydrolase from Crithidia fasciculata

Protozoan parasites are incapable of de novo purine biosynthesis and obtain purines by salvage pathways. A nucleoside hydrolase which prefers inosine and uridine as substrates (IU-nucleoside hydrolase) has been characterized and implicated in purine salvage in Crithidia fasciculata (Parkin, D. W., Horenstein, B. A., Abdulah, D. R., Estupiñán, B., and Schramm, V. L. (1991) J. Biol. Chem. 31, 20658-20665). Treatment of C. fasciculata with inhibitors of the IU-nucleoside hydrolase did not prevent cell growth, suggesting alternative enzymes. A guanosine-inosine-preferring enzyme (GI-nucleoside hydrolase) has been purified from extracts of C. fasciculata and characterized. The enzyme is an oligomer of M(r) 38,500 subunits. The Vmax/Km for guanosine, inosine, and adenosine are 3.2 x 10(6), 6.2 x 10(6), and 9.8 M-1 S-1, respectively. Deoxynucleosides, nucleotides, and pyrimidine nucleosides are poor substrates. The pH profile for Km is independent of pH, whereas both Vmax and Vmax/Km demonstrate that a single protonated base, pKa 7.7 is required for activity. The transition state inhibitors of IU-nucleoside hydrolase, 1,4-dideoxy-1,4-imino-1-(S)-phenyl-D-ribitol (Horenstein, B. A., and Schramm, V. L. (1993) Biochemistry 32, 9917-9925) and p-nitrophenylriboamidrazone (Boutellier, M., Horenstein, B. A., Semenyaka, A., Schramm, V.L., and Ganem, B. (1994) Biochemistry 33, 3994-4000), are unexceptional inhibitors of the GI-nucleoside hydrolase. The enzyme is inhibited by 3-deazaadenosine and 2-iodoadenosine with Km/Ki values of 145 and 61, respectively. The results demonstrate that this previously uncharacterized enzyme has distinct structure, kinetic, and chemical mechanisms relative to IU-nucleoside hydrolase. Metabolic studies with labeled inosine as the sole purine source indicated that the GI-enzyme is efficient for purine salvage in vivo.

Nonenzymatic glycosylation-induced modifications of intact bovine kidney tubular basement membrane

We examined structural changes in bovine kidney tubular basement membrane (TBM) following in vitro nonenzymatic glycosylation (NEG). Isolated TBM was incubated for 2 wk at 37 degrees C in the absence of sugar or in the presence of either glucose or ribitol under conditions that minimized degradation and oxidative damage. NEG and crosslink formation in glycated TBM were confirmed by decreased solubility, increased amounts of low mobility material by SDS-PAGE, and increased specific fluorescence compared to controls. Morphological analysis using high resolution, low voltage scanning electron microscopy (LV-SEM) revealed a complex three-dimensional meshwork of interconnecting strands with intervening openings. Glycated TBM underwent distinct morphological changes, including a 58% increase in the amount of image surface area occupied by openings. This was due to an apparent increase in the number of large openings (diameters > 12.5 nm), whereas the number of small openings (diameters < 12.5 nm) remained unchanged. These findings corroborate earlier physiological studies, which established that the loss of glomerular permselectivity seen in patients with diabetic nephropathy is due to the formation of large pores in the kidney filtration barrier of which the BM is a major component. We conclude that NEG and crosslink formation among BM components lead to modifications of BM ultrastructure, which could play a role in loss of barrier function in diabetic microangiopathy and nephropathy.

Further studies on the growth inhibition of some oral bacteria by xylitol

The growth in Brain Heart Infusion medium containing glucose or sucrose of Streptococcus mutans strain OMZ 176, two strains of Strep. sanguis (ATCC 10556 and No. 39) and Actinomyces viscosus type ATCC 27044 was inhibited by xylitol but not by D-arabitol or ribitol. When fructose was substituted for glucose, xylitol had no growth-inhibiting effect. It was also shown that the bacteria produced less acid in the presence of xylitol, as measured by the pH of the cultures or by neutralization with alkali.

Protection against heat-induced cell killing by polyols in vitro

The polyols erythritol and adonitol reduced 45 degrees heat killing in asynchronous Chinese hamster ovary cells. Heat protection by glycerol and erythritol increased with the apparent intracellular concentration, as inferred from cell volume measurements, and the number of hydroxyl groups per alcohol molecule. The nonlinear tetrahydroxy alcohol pentaerythritol did not protect but sensitized to heat killing. On cell survival curves, the reduced cell killing of protected cells was expressed by an increased Do for the pentahydroxy alcohol adonitol (0.3 M), whereas equimolar concentrations of glycerol increased primarily the Dq (quasithreshold dose) with little increase in Do. The distribution of Chinese hamster ovary cells within the cell cycle was unaffected by the presence of 0.3 M glycerol in the culture medium. However, the polyols erythritol and sorbitol caused a small but significant loss of cells from the heat-resistant G1 compartment. The cell cycle redistribution with prolonged incubation (6 hr) in polyol-supplemented medium is expected to increase the heat sensitivity of the perturbed cell population; the observed heat protection by polyols suggests that heat resistance in the presence of polyols is not an artifact of an asynchronous cell system. Instead, the data identify a family of heat-protective compounds that may occur naturally in mammalian cells.

Protective effect of adonitol on lactic acid bacteria subjected to freeze-drying

The protective effects of glycerol, adonitol, and four other related polyhydric alcohols on lactic acid bacteria subjected to freeze-drying were examined. The presence of adonitol in the suspending medium markedly protected the viabilities of the 12 stains tested. Dulcitol, mannitol, m-inositol, and sorbitol were found to provide little or no protection.

Selective inhibition of Klebsiella aerogenes growth on pentoses by pentitols

Selective inhibition of growth by pentitols was observed when Klebsiella aerogenes M-7 which could not utilize pentitols was grown on pentoses. D-Arabitol inhibited the growth on D-arabinose as a sole carbon source, but had no effect on the growth on L-arabinose, D-xylose, and D-ribose. Similarly, L-arabitol inhibited the growth on D-arabinose and L-arabinose, ribitol inhibited the growth on D-arabinose and L-arabinose, and xylitol inhibited the growth on D-xylose. From the following reasons, we postulated that the selective growth inhibition by pentitols was due to the competitive inhibition of pentose isomerase reaction by the cell by pentitols. (i) D-Arabinose transport activity was not inhibited by pentitols. (ii) Induction of D-arabinose and L-arabinose isomerases was not inhibited by D- and L-arabitol, respectively. (iii) The specificity of growth inhibition by pentitols was the same as that of competitive inhibition of pentose isomerases by pentitols.