Peptide stability in drug development. II. Effect of single amino acid substitution and glycosylation on peptide reactivity in human serum

The determination of peptide stability in human serum (HS) or plasma constitutes a powerful screening assay for eliminating unstable peptides from further development. Herein we report on the stability in HS of several major histocompatibility complex (MHC)-binding peptides. Some of these peptides are in development for the novel treatment of selected autoimmune disorders such as rheumatoid arthritis and insulin-dependent diabetes. For most of the l-amino acid peptides studied, the predominant degradation mechanism is exopeptidase-catalyzed cleavage. Peptides that were protected by d-amino acids at both termini were found to be more stable than predicted, based on additivity of single substitutions. In addition, N-acetylglucosamine glycopeptides were significantly stabilized, even when the glycosylation site was several amino acids from the predominant site(s) of cleavage. This indicates that long-range stabilization is possible, and likely due to altered peptide conformation. Finally, the effect of single amino acid substitutions on peptide stability in HS was determined using a model set of poly-Ala peptides which were protected from exopeptidase cleavage, allowing the study of endopeptidase cleavage pathways.

Role reversal for substrates and inhibitors. Slow inactivation of D-amino acid transaminase by its normal substrates and protection by inhibitors

D-Amino acid transaminase, which catalyzes the synthesis of D-alanine and D-glutamate for the bacterial cell wall, is a candidate for the design of specific inhibitors that could be novel antimicrobial agents. Under the experimental conditions usually employed for enzyme assays, kinetic parameters for its substrates were determined for short incubation periods, when intermediates and products do not accumulate and the enzyme activity is linear with time. Such kinetic analyses indicate that the enzyme accepts most D-amino acids but D-aspartate and D-glutamate are the best substrates. Under a different type of experimental conditions when the enzyme is exposed to D-alanine, intermediates, and products for periods of hours, it slowly becomes inactivated (Martinez del Pozo, A., Yoshimura, T., Bhatia, M. B., Futaki, S., and Manning, J. M. (1992) Biochemistry 31, 6018-6023). We now report that D-aspartate, D-glutamate, and L-alanine also lead to slow inactivation. Methylation or amidation of the alpha-COOH group of D-alanine prevents inactivation, indicating that decarboxylation is required for inactivation; the slow release of CO2 from substrate is demonstrated. The alpha-methyl analog of D-alanine, D-aspartate, and D-glutamate do not lead to inactivation, showing that the alpha-hydrogen of the substrate is required, i.e. that some processing is required. Lys145, which binds pyridoxal 5'-phosphate in the wild-type enzyme, is not involved in the inactivation since two active site mutant enzymes, K145Q and K145N, are also inactivated. Reactivation of the inactive enzyme at acidic pH is accompanied by the release of ammonia corresponding to 1 mol/mol of dimeric enzyme. Competitive inhibitors, amine-containing buffers, and thiols effectively impede the inactivation. This reversal in the roles of substrates and inhibitors, i.e. when a substrate can be an inactivator and an inhibitor can act as a protector, occurs during a time period not usually used to measure steady-state kinetics or initial velocities of enzyme reactions and could have physiological relevance in cells.

[The correlation between changes in EEG and eye movement (EOG) accompanied by lowered arousal level]

We investigated the correlation between the components of spontaneous eye movement (EOG) and EEG in six healthy individuals. The study was conducted in three periods; from the resting to drowsy stage, from the drowsy to spontaneous awakening stage, and in the forced waking stage. EEG, as registered from bipolar electrodes attached between the left parietal region (P3) and the left occipital region (O1), was monitored continuously by the FFT method with a segment of 12.8 seconds. Rapid eye movement and slow eye movement were observed simultaneously. In conclusion, even when the arousal level varies considerably, there is a close correlation between frequency of rapid eye movement and EEG patterns of 10.16 integral of 10.94 Hz & 17.97 integral of 19.53 Hz (simple correlation p < 0.01). Additionally, during the forced waking period (after stage 2), delta and theta bands increased in cases along with sleepiness. Even if the depth of sleep was the same, one's mood upon awakening was determined by the frequency of slow waves before awakening.

Site-directed mutagenesis of a hexapeptide segment involved in substrate recognition of phenylalanine dehydrogenase from Thermoactinomyces intermedius

Phenylalanine dehydrogenase from Thermoactinomyces intermedius and leucine dehydrogenase from Bacillus stearothermophilus show a 59% sequence similarity in their substrate-binding domains, although their substrate specificities are different. We prepared a phenylalanine dehydrogenase mutant enzyme whose inherent hexapeptide segment (124F-V-H-A-A-129R) in the substrate-binding domain was replaced by the corresponding part of leucine dehydrogenase (M-D-I-I-Y-Q) in order to investigate the mechanism of substrate recognition by phenylalanine dehydrogenase. The catalytic efficiencies (kcat/Km) of the mutant enzyme with aliphatic amino acids and aliphatic keto acids as substrates were 0.5 to 2% of those of the wild-type enzyme. In contrast, the efficiencies for L-phenylalanine and phenylpyruvate decreased to 0.008 and 0.035% of those of the wild-type enzyme, respectively. These results suggest that the hexapeptide segment plays an important role in the substrate recognition by phenylalanine dehydrogenase.

A new amino acid racemase with threonine alpha-epimerase activity from Pseudomonas putida: purification and characterization

We have found that Pseudomonas putida ATCC 17642 cells grown in a medium containing D-threonine as the sole nitrogen source produce an enzyme that catalyzes epimerization of threonine. Proton nuclear magnetic resonance analysis of the enzyme reaction in deuterium oxide clearly showed epimerization from L- to D-allo-threonine and also from D- to L-allo-threonine. This is the first example of an enzyme that was clearly shown to epimerize threonine. The enzyme has been purified to homogeneity, which was shown by polyacrylamide gel electrophoresis. The enzyme has a molecular weight of about 82,000 and consists of two subunits identical in molecular weight (about 41,000). The enzyme contains 1 mol of pyridoxal 5'-phosphate per mol of subunit as a cofactor, and its absorption spectrum exhibits absorption maxima at 280 and 420 nm. The enzyme catalyzes not only epimerization of threonine by stereoconversion at the alpha position but also racemization of various amino acids, except acidic and aromatic amino acids. The enzyme is similar to amino acid racemase with low substrate specificity (EC 5.1.1.10) in enzymological properties but is distinct from it in the action on threonine.

[Prevalence of hepatitis C virus and human immunodeficiency virus infection among Japanese female prostitutes]

To investigate the incidence of hepatitis C virus (HCV) and human immunodeficiency virus (HIV) infection in Japanese sexual workers, 191 out of 194 female prostitutes working in a certain district of the Tokyo metropolitan area were tested for anti-HCV, anti-hepatitis B virus (HBV), anti-HIV and anti-Treponema pallidum (TP). They were also interviewed with regard to their past history of blood transfusion, vaccination against HBV, liver diseases and repeated skin piercing treatments such as tattooing, acupuncture and intravenous drug use. Sera from 300 female blood donors (aged 20 s-40 s), of the same district, collected during the same year, were assessed for comparison. Incidence of seropositives for anti-HCV and anti-TP were found to be significantly high in the prostitute group (11.0% and 16.2%, respectively) compared to the control (0% for both). Anti-HBV seropositives were 14.1% in the prostitute group and 12.7% in the control group without significant difference and none of our subjects were found to be anti-HIV positive. Among HCV infected only 5 experienced repeated skin-piercing treatment and none underwent blood transfusion. From these results it is concluded that HCV has a potential for being transmitted by heterosexual contact, although the exact risk for infection needs to be determined by further investigation.

Kinetic and stereochemical comparison of wild-type and active-site K145Q mutant enzyme of bacterial D-amino acid transaminase

D-Amino acid transaminase (EC 2.6.1.21), from Bacillus sp. YM-1, a thermostable enzyme with pyridoxal 5'-phosphate as coenzyme and a target for the design of novel antimicrobial agents, catalyzes the reversible transfer of an amino group between D-alanine and alpha-ketoglutarate to form pyruvate and D-glutamate, respectively. To explore the catalytic role of Lys-145, which binds the coenzyme, a site-specific mutant enzyme, K145Q (in which Lys-145 had been mutated to glutamine) constructed earlier (Futaki, S., Ueno, H., Martinez del Pozo, A., Pospischil, M. A., Manning, J. M., Ringe, D., Stoddard, B., Tanizawa, K., Yoshimura, T., and Soda, K. (1990) J. Biol. Chem. 265, 22306-22312) was compared to the wild-type enzyme for its kinetic parameters. Initial velocity studies and partial reaction isotope exchange experiments showed that the low activity of the mutant enzyme (about 1.5% the activity of the wild-type enzyme with saturating substrates) is an intrinsic property, confirming that contaminating enzymes do not account for the low activity of the K145Q mutant enzyme. The rates of the forward reaction for both wild-type and mutant enzymes were 30-40 times higher than the rates of the reverse reaction. KM values for the four substrates were 10 to 100 higher for the mutant compared to the wild-type enzyme. Whereas D-alanine is preferred over L-alanine by the wild-type enzyme (10(3) higher kcat/KM for D- over L-alanine), the K145Q enzyme does not efficiently discriminate between L- and D-alanine. Both wild-type and mutant enzymes also catalyze the slow racemization of L- and D-alanine. Proton NMR studies showed that wild-type enzyme catalyzed a time-dependent exchange of the C alpha proton of D-alanine with solvent D2O and a slow exchange of the alpha proton of L-alanine; the latter slow exchange rate is the same for the C alpha proton of both L- and D-alanine with the K145Q mutant enzyme. Thus, in addition to binding pyridoxal 5'-phosphate, the active-site Lys-145 of D-amino acid transaminase is involved in several other important functions, i.e. it optimizes catalytic efficiency and it maintains stereochemical fidelity. The steady-state kinetic results on the K145Q mutant enzyme together with the findings on the relative racemization rates and the NMR protein exchange data suggest that an alternate base catalyzes abstraction of the alpha proton of substrate in this mutant D-amino acid transaminase.

Intestinal permeability in allergic rats: nerve involvement in antigen-induced changes

In vivo uptake of the probe 51Cr-labeled EDTA from the jejunum of egg albumin (EA)-sensitized rats was compared with controls at baseline and after intraluminal antigen challenge. Probe recovery in blood was 60-80% greater in sensitized animals during the baseline period, suggesting that sensitization resulted in increased intestinal permeability. Sensitized, but not control, rats demonstrated a 15-fold increase in 51Cr-EDTA uptake after intraluminal antigen; no change occurred with an unrelated protein. Macromolecular recovery was also enhanced in sensitized animals, since serum levels of immunoreactive EA were elevated 14-fold compared with controls. Antigen challenge was accompanied by biochemical (protease release) and morphological (reduced numbers) evidence of mast cell degranulation in sensitized rats. The neurotoxin tetrodotoxin (applied directly to ligated jejunal segments) inhibited EA-induced uptake of 51Cr-EDTA and antigen. In isolated jejunum from sensitized rats, tetrodotoxin reduced secretory responses to luminal, but not serosal, antigen. These results indicate that neural factors may influence the uptake of molecules from the gut lumen during intestinal anaphylaxis.

Valine dehydrogenase from a non-spore-forming bacterium, Alcaligenes faecalis: purification and characterization

An NAD-dependent valine dehydrogenase (L-valine:NAD oxidoreductase, deaminating, EC 1.4.1.-), was found in a non-spore-forming bacterium, Alcaligenes faecalis, and purified about 80-fold to be characterized. The molecular mass of the enzyme was estimated to be about 72 kDa and the enzyme consists of two identical subunits with a molecular mass of 40 kDa and is thermolabile. It loses its activity fully on incubation at 50 degrees C for 5 min. The enzyme catalyzes the reversible deamination of L-valine, which is the preferred substrate, and other branched-chain and straight-chain L-amino acids in the presence of NAD. The pH optima are about 10.8 and 8.8 for the oxidative deamination and reductive amination, respectively. The pro-S hydrogen at C-4 of the dihydronicotinamide ring of NADH was exclusively transferred to the substrate in the reductive amination. The amino-acid composition markedly differs from those of Bacillus sphaericus and Clostridium thermoaceticum leucine dehydrogenases. The enzyme did not react with the antibody of C. thermoaceticum leucine dehydrogenase. Therefore, the enzyme is clearly different from leucine dehydrogenases from spore-forming bacteria, which act on valine, and the first valine dehydrogenase to be characterized in detail.

Synthetic study on selenocystine-containing peptides

N-9-Fluorenylmethoxycarbonyl-Se-4-methoxybenzylselenocysteine++ + [Fmoc-Sec(MBzl)-OH] was synthesized from selenocystine and successfully applied to Fmoc-based solid-phase peptide synthesis. The stability and the deprotection conditions of the Se-MBzl group were examined. The diselenide bond of a peptide was directly and effectively established between Sec(MBzl) residues by treatment with iodine or the dimethyl sulfoxide-trifluoroacetic acid system. Reduction kinetics of diselenide and disulfide in model peptides by reduced glutathione were also studied comparatively.

Synthesis and characterization of the selenium analog of glutathione disulfide

We synthesized the selenium analog of glutathione disulfide by a liquid phase method and named it glutaselenone (i.e., gamma-L-glutamyl-L-selenocysteinylglycine) diselenide. The selenol of selenocysteine was protected by the p-methoxybenzyl group, which was removed by acidolysis with trifluoroacetic acid in the presence of thioanisol. The overall yield of the final product, glutaselenone diselenide, was about 9% based on the starting compound, Se-(p-methoxybenzyl)-L-selenocysteine. Glutaselenone diselenide showed a broad absorption band between 270 and 400 nm and circular dichroism bands around 270 nm (positive) and 330 nm (negative), which were attributable to diselenide bond.

Steroid-induced depletion of mucosal mast cells and eosinophils in intestine of athymic nude rats

In conventional rats, we have previously demonstrated that corticosteroid treatment caused macrophage engulfment and destruction of intestinal mucosal mast cells and eosinophils by 24 h without evidence of local tissue destruction, inflammation or secretion of rat mast cell protease II. As the growth and survival of these cells appear to be dependent on factors derived from T lymphocytes, we examined the response in congenitally athymic rnu/rnu rats and euthymic rnu/+ rats 35 days after parasitic infection. Rats were injected intraperitoneally with 1 mg dexamethasone and sections of jejunum were examined at 0, 7, 13 and 24 h. The numbers of mucosal mast cells significantly decreased in both groups and became less than 30% of the original values at 24 h. Tissue mast cell protease decreased similarly. However, protease in serum did not increase and there were no inflammatory changes at any time. The numbers of eosinophils also rapidly decreased and became less than 20% at 24 h in both rnu/rnu and rnu/+ rats. By electron microscopy, we saw granular changes (fusion) in mast cells and nuclear changes (apoptosis) in eosinophils by 7 h after corticosteroid in athymic rats. Macrophage engulfment of these cells was observed at 7 and 13 h. Our results suggest that inflammatory cell depletion by macrophages is not dependent on suppression of typical thymus-derived T lymphocytes, and may be due either to direct effects of steroids on the cells themselves, or indirectly upon cells other than T cells which normally supply maintenance and growth factors for them.

Thermolabile alanine racemase from a psychotroph, Pseudomonas fluorescens: purification and properties

A psychotrophic bacterium that produces a thermolabile alanine racemase was isolated from raw milk, and identified as Pseudomonas fluorescens TM5-2. The enzyme was purified to homogeneity from the cell extract, and characterized to be compared with enzymes from mesophiles (Bacillus subtilis and Salmonella typhimurium) and a thermophile (Bacillus stearothermophilus). The enzyme has a molecular weight of about 76,000 and consists of two subunits identical in molecular weight (38,000). The enzyme contains two mol of pyridoxal 5'-phosphate per mol as a coenzyme. The amino acid composition was different from those of other alanine racemases in content of valine. The amino acid sequence of the amino terminal region (from 1Met to 25Gly) had 21-33% homology with those of other alanine racemases. Kinetic parameters of the enzyme were similar to those of other alanine racemases. The enzyme is extremely labile over 30 degrees C, and shows the high catalytic activity even at 0 degrees C; it is thermolabile and psychotrophic.

[Evaluation of lidocaine metabolite (monoethylglycinexylidide) as a liver function test]

In order to determine whether or not the lidocaine metabolism, monoethylglycinexylidide (MEGX) formation, could be used as a liver function test, we measured the serum levels of MEGX in 38 patients. There were significant correlations between values of MEGX (MEGX15, MEGX30, AUC15-30, AUC0-180) and conventional liver function tests (ICG R15, AT III, T. Bil). It appeared that value of MEGX 30 had maximum factor loading on conventional liver function tests by using principal component analysis. The advantage of adapting the MEGX formation as a liver function test of drug metabolism is simplicity of the method. MEGX formation could be useful index of the total liver function.

Structural and thermodynamic aspects of the hydrophobic effect

From analyses of the solvation structure around HS solutes for HS solvents and various water models by the RISM integral equation method, the following conclusions are drawn: 1) Water has more small cavities than HS solvents, which makes it easier for water to accommodate small solutes with a radius less than about 1A. 2) With increasing solute radius from 0 to 3A, the average orientation of hydrating water varies from an inward to an outward orientation, which shows that some reorganization of water occurs in response to the change in solute size. 3) The hydration structure is formed as a network structure due to H-bonding interactions between water molecules being supported by the cooperation of repulsive forces between solute and water. Repulsive interactions of not only O atoms but also H atoms with solute are essential to the formation of such hydration structure. With regard to the physical mechanism of the hydrophobic effect, the following is concluded from an analysis of the physical meaning of the basic formula for the free energy of cavity formation: 4) It is predicted from the scaled particle theory that the solvent exclusion effect caused by the introduction of solute into solvent is an important factor of the hydrophobic effect. 5) The large negative transfer entropy at room temperature characteristic of the hydrophobic hydration results primarily from the decrease in the configuration entropy of water due to the solvent exclusion effect. 6) The structuralization of hydrating water results in exactly compensating changes in enthalpy and entropy, and a large positive change in heat capacity. As a result, the hydrophobic effect is dominated by the entropy effect at room temperature, while it is driven by enthalpy at temperatures higher than 110 degrees C. 7) Hydrating water is energetically similar to bulk water, and the term "highly structured" is not appropriate to describe it. The following descriptions can be made on estimating the free energy of transfer of biomolecules from gas or organic-liquid phase to water: 8) Derivation of the basic formula for the transfer free energy of solute with variable conformation was presented and physical meanings of the contributions to it were explained. 9) Applying the formula to the transfer of 40 organic molecules from gas phase to water, the two best models for diving all the constituent atoms into several hydration-thermodynamically independent groups and the atomic hydration parameters of respective groups were determined.(ABSTRACT TRUNCATED AT 400 WORDS)

Partial reactions of bacterial D-amino acid transaminase with asparagine substituted for the lysine that binds coenzyme pyridoxal 5'-phosphate

In bacterial D-amino acid transaminase (EC 2.6.1.21) replacement of Lys-145, which is covalently linked to the coenzyme pyridoxal 5'-phosphate in the wild-type enzyme, by an Asn residue gave a mutant enzyme (K145N) that slowly performed each half-reaction, as determined by spectral measurements. With the wild-type enzyme, the kinetics of these events were so rapid that pre-steady-state conditions were needed for their determination. The internal aldimine between coenzyme and Lys-145 was rapidly reduced with NaCNBH3 in the wild-type enzyme, whereas in the mutant enzyme the coenzyme, which is not covalently linked to the protein, was more resistant to reduction; the reduced forms of both wild-type and mutant enzymes were inactive. With large amounts of the K145N mutant enzyme and either amino acid or keto acid substrate alone, the formation of some reaction intermediates, i.e., the external aldimine with D-alanine and the ketimine with alpha-ketoglutarate, can be measured by conventional spectroscopy. Suicide substrates also induced slow spectral shifts of the E-PLP form of the enzyme. For the K145N enzyme, exogenous amines affected only the rate of the transaldimination but not the removal of the alpha-proton of the substrate. These results suggest that in the mutant enzyme some amino acid side chain other than Lys-145 performs this function. In order to identify this site, the K145N mutant enzyme was completely inactivated by the radiolabeled suicide substrate D-serine. Peptide mapping of tryptic digests showed that Lys-267 was the modified site.(ABSTRACT TRUNCATED AT 250 WORDS)

[Clinical study of bronchial inhalation challenge in summer-type hypersensitivity pneumonitis induced by Trichosporon cutaneum]

To evaluate the methods and criteria of judgement for the bronchial antigen inhalation challenge test, the test was performed with culture filtrate antigen of serotype I and II of Trichosporon cutaneum in 18 patients with summer-type hypersensitivity pneumonitis from 15 families. The quantity of 15 mg of culture filtrate antigen was adequate, and had no side effects. In the tests, 17 of 18 patients showed a positive reaction to both or either, serotype of antigen. In 36 performances of inhalation, there were 21 positive reactions and 15 negative reactions. According to the criteria of judgment for inhalation challenge test, the positive response rates of observation items were 75% for symptoms and signs, and 51% for laboratory data. Items with a high positive rate were cough, crepitant rales, and decrease of PaO2. On the other hand, low positive rates were observed for decrease of DLco, VC and positive CRP. Items with both high sensitivity and high specificity were cough, crepitant rales and decrease of PaO2. The low positive rate of decreased DLco was due to insufficient improvement before inhalation challenge. It was concluded that our methods and criteria of judgment for bronchial inhalation challenge test are useful.

Properties of aspartate racemase, a pyridoxal 5'-phosphate-independent amino acid racemase

Aspartate racemase from Streptococcus thermophilus contains no pyridoxal 5'-phosphate or other cofactors such as FAD, NAD+, and metal ions. It was affected by neither carbonyl reagents such as hydroxylamine nor sodium borohydride but was strongly inhibited by iodoacetamide and other thiol reagents. Aspartate, cysteate, and cysteine sulfinate were the only substrates. The Km values for L- and D-aspartate were 35 and 8.7 mM, respectively. The enzyme catalyzed the exchange of alpha-hydrogen of the substrate with the solvent hydrogen. Racemization of L-aspartate in 2H2O showed an overshooting in the optical rotation of aspartate before the substrate was fully racemized. This shows that the removal of alpha-hydrogen of the substrate is at least partially rate-determining. When L- or D-aspartate was incubated with aspartate racemase in tritiated water, tritium was incorporated preferentially into the product enantiomer. The results strongly suggest that aspartate racemase contains two hydrogen acceptors.

Leucine dehydrogenase from Bacillus stearothermophilus: identification of active-site lysine by modification with pyridoxal phosphate

We have constructed an efficient expression plasmid for the leucine dehydrogenase gene previously cloned from Bacillus stearothermophilus. The recombinant enzyme was overproduced in Escherichia coli cells to a level of more than 30% of the total soluble protein upon induction with isopropyl beta-D-thiogalactopyranoside. The enzyme could be readily purified to homogeneity by heat treatment and a single step of ion-exchange chromatography. The purified enzyme was inactivated in a time-dependent manner upon incubation with pyridoxal 5'-phosphate (PLP) followed by reduction with sodium borohydride. The inactivation was completely prevented in the copresence of L-leucine and NAD+. Concomitantly with the inactivation, several molecules of PLP were incorporated into each subunit of the hexameric enzyme. Sequence analysis of the fluorescent peptides isolated from a proteolytic digest of the modified protein revealed that Lys80, Lys91, Lys206, and Lys265 were labeled. Among these residues, Lys80 was predominantly labeled and, in the presence of L-leucine and NAD+, was specifically protected from the labeling. Furthermore, a linear relationship of about 1:1 was observed between the extent of inactivation and the amount of PLP incorporated into Lys80. A slightly active mutant enzyme, in which Lys80 is replaced by Ala, was not inactivated at all by incubation with PLP, showing that the inactivation is correlated with the labeling of only Lys80. Lys80is conserved in the corresponding regions of all the amino acid dehydrogenase sequences reported to date. These results suggest that Lys80 is located at the active site and plays an important role in the catalytic function of leucine dehydrogenase.

Inactivation of dimeric D-amino acid transaminase by a normal substrate through formation of an unproductive coenzyme adduct in one subunit

D-amino acid transaminase, which contains pyridoxal 5'-phosphate (vitamin B6) as coenzyme, catalyzes the formation of D-alanine and D-glutamate from their corresponding alpha-keto acids; these D-amino acids are required for bacterial cell wall biosynthesis. Under conditions usually used for kinetic assay of enzyme activity, i.e., short incubation times with dilute enzyme concentrations, D-alanine behaves as one of the best substrates. However, the enzyme slowly loses activity over a period of hours when exposed to substrates, intermediates, and products at equilibrium. The rate of inactivation is dependent on enzyme concentration but independent of substrate concentration greater than Km values. Continuous removal of the product pyruvate by enzymic reduction precludes the establishment of equilibrium and prevents inactivation. The formation of small but detectable amounts of a quinonoid intermediate absorbing at 493 nm is proportional to inactivation. Studies with [14C]-D-alanine labeled on different carbon atoms indicate that the alpha-carboxyl group of the substrate is absent in the inactive enzyme; such decarboxylation is not a usual function of this enzyme. The inactive transaminase contains 1.1 mol of [14C]-D-alanine-derived adduct per mole of dimeric enzyme; this finding is consistent with the 50% reduction in the fluorescence intensity at 390 nm (due to the PMP form of the coenzyme) for the inactive enzyme. Thus, inactivation of one subunit of the dimeric enzyme renders the entire molecule inactive. Inactivation may occur when a coenzyme intermediate, perhaps the ketimine, is slowly decarboxylated and then undergoes a conformational change from its catalytically competent location.(ABSTRACT TRUNCATED AT 250 WORDS)

Reaction Mechanism of Glutamate Racemase, a Pyridoxal Phosphate-Independent Amino Acid Racemase

Glutamate racemase of Pediococcus pentosaceus contained no cofactor, and was completely inactivated by a thiol reagent. The role of a cysteine residue in the enzyme reaction was studied by chemical modification. The modification of this cysteine residue resulted in a concomitant loss of activity. DL-Glutamate protected the enzyme from inactivation. The inactivated enzyme was reactivated by addition of dithiothreitol. The racemization in 2H2O showed an overshoot in the optical rotation of glutamate before the substrate was completely racemized. This indicates that the removal of alpha-hydrogen is the rate determining step. During the racemization of D- or L-glutamate in 3H2O, tritium was incorporated preferentially into the product. Glutamate is racemized by the enzyme probably through a two base mechanism.

[A comparative study of the physico-chemical properties of lysine oxidase from Trichoderma sp. and Trichoderma viride Y 244-2]

Some physicochemical properties of L-lysine oxidase from two strains of Trichoderma were studied. Content of metals and cofactors (Se, Zn, Cu, Fe, Co, Mn, Mo), amino acid analysis, secondary structure were estimated. The enzyme molecule from Trichoderma sp was found to contain both FAD and PQQ cofactors.

[Systems analytical investigation of the effect of countermeasures against the AIDS epidemic among homosexuals]

In Japan, the AIDS (Acquired Immunodeficiency Syndrome) epidemic among haemophiliacs has been the most urgent issue, because of the large number of haemophiliacs with AIDS. However, after governmental approval of the production of heated coagulating agents in 1985 and 1986, the prime object of prevention against the AIDS epidemic shifted from transmission through coagulation agents to that through sexual contacts. In order to investigate the most appropriate countermeasures against the AIDS epidemic among homosexuals in Japan, the numbers of HIV (Human Immunodeficiency Virus)-infected cases and AIDS cases in the future were estimated, and changes of the future numbers of HIV-infected persons and AIDS cases by behavior of homosexuals were compared using a systems analytical method. The methods of estimation and comparison are similar to those of R. M. Anderson and others, using numerical analysis of a mathematical model consisting of differential equations. They assumed a closed homosexual group whose members seldom have contact with members of other homosexual groups, but we assumed an open homosexual group whose members are being infected by other groups, because this assumption was more appropriate to the situation of homosexual society in Japan. The results showed that the prevalence number of HIV-infected cases would be about 1,800, that the prevalence number of AIDS cases would be about 100 among 100,000 homosexuals at 20 years after the beginning of the AIDS epidemic, and that the most effective countermeasure was reducing the frequency of sexual contacts among members or taking prophylactic measures during sexual contacts. These prevalence numbers of HIV-infected cases and AIDS cases would be reduced to between 1/4 and 1/2 of the above-mentioned calculated values by promotion of that countermeasure.

Purification and characterization of a new NAD(+)-dependent enzyme, L-tartrate decarboxylase, from Pseudomonas sp. group Ve-2

A new enzyme, L-tartrate decarboxylase, was found in cells of Pseudomonas sp. group Ve-2. The enzyme was purified to homogeneity and characterized. The enzyme requires K+, Mg2+, and NAD+ for L-tartrate decarboxylation. The dependence of the enzymatic decarboxylation on NAD+ suggests that the decarboxylation involves redox reactions of the substrate. The enzyme catalyzes NAD(+)-linked oxidative decarboxylation of D-malate as well. The enzyme is composed of four subunits with identical molecular weight (Mr 40,000). The apparent Michaelis constants for L-tartrate and NAD+ are 1.1 mM, respectively. The cofactor requirements and the physical properties of the enzyme were similar to those of L-tartrate dehydrogenase-D-malate dehydrogenase from Rhodopseudomonas sphaeroides, and tartrate dehydrogenase from P. putida.

Thermostable alanine racemase of Bacillus stearothermophilus: subunit dissociation and unfolding

The guanidine hydrochloride-induced subunit dissociation and unfolding of thermostable alanine racemase from Bacillus stearothermophilus have been studied by circular dichroism, fluorescence and absorption spectroscopies, and gel filtration. The overall process was found to be reversible: more than 75% of the original activity was recovered upon reduction of the denaturant concentration. In the range of 0.6 to 1.5 M guanidine hydrochloride, the dimeric enzyme was dissociated into a monomeric form, which was catalytically inactive. The monomeric enzyme appeared to bind the cofactor pyridoxal phosphate by a non-covalent linkage, although the native dimeric enzyme binds the cofactor through an aldimine Schiff base linkage. The monomer was mostly unfolded, with the transition occurring in the range of 1.8 to 2.2 M guanidine hydrochloride.

Thermostable alanine racemase of Bacillus stearothermophilus. Construction and expression of active fragmentary enzyme

Limited proteolysis studies on alanine racemase suggested that the enzyme subunit is composed of two domains (Galakatos, N. G., and Walsh, C. T. (1987) Biochemistry 26, 8475-8480). We have constructed a mutant gene that tandemly encodes the two polypeptides of the Bacillus stearothermophilus enzyme subunit cleaved at the position corresponding to the predicted hinge region. The mutant gene product purified was shown to be composed of two sets of the two polypeptide fragments and was immunologically identical to the wild-type enzyme. The mutant enzyme, i.e. the fragmentary alanine racemase, was active in both directions of the racemization of alanine. The maximum velocity (Vmax) was about half that of the wild-type enzyme, and the Km value was about double. Absorption and circular dichroism spectra of the fragmentary enzyme were similar to those of the wild-type enzyme. An attempt was made to separately express in Escherichia coli a single polypeptide corresponding to each domain, but no protein reactive with the antibody against the wild-type alanine racemase was produced. Therefore, it is suggested that the two polypeptide fragments can fold into an active structure only when they are co-translated and that they correspond to structural folding units in the parental polypeptide chain.

Distribution, purification, and characterization of thermostable phenylalanine dehydrogenase from thermophilic actinomycetes

Phenylalanine dehydrogenase (L-phenylalanine:NAD oxidoreductase, deaminating; EC 1.4.1.-) was found in various thermophilic actinomycetes. We purified the enzyme to homogeneity from Thermoactinomyces intermedius IFO 14230 by heat treatment and by Red Sepharose 4B, DEAE-Toyopearl, Sepharose CL-4B, and Sephadex G-100 chromatographies with a 13% yield. The relative molecular weight of the native enzyme was estimated to be about 270,000 by gel filtration. The enzyme consists of six subunits identical in molecular weight (41,000) and is highly thermostable: it is not inactivated by incubation at pH 7.2 and 70 degrees C for at least 60 min or in the range of pH 5 to 10.8 at 50 degrees C for 10 min. The enzyme preferably acts on L-phenylalanine and its 2-oxo analog, phenylpyruvate, in the presence of NAD and NADH, respectively. Initial velocity and product inhibition studies showed that the oxidative deamination proceeds through a sequential ordered binary-ternary mechanism. The Km values for L-phenylalanine, NAD, phenylpyruvate, NADH, and ammonia were 0.22, 0.078, 0.045, 0.025, and 106 mM, respectively. The pro-S hydrogen at C-4 of the dihydronicotinamide ring of NADH was exclusively transferred to the substrate.

Lowered responsiveness of bronchoalveolar lavage T lymphocytes in hypersensitivity pneumonitis

We previously reported that Trichosporon cutaneum was the major causative antigen of summer-type hypersensitivity pneumonitis (HP) in Japan. In summer-type HP patients, we noticed that the proliferative responses of bronchoalveolar lavage (BAL) lymphocytes to phytohemagglutinin (PHA) and concanavalin A were significantly lower than those of the peripheral blood lymphocytes of the same patients. It was shown in this study that the low response of BAL lymphocytes was due to an intrinsic lowering of the responsiveness of the T cells. Results of the mixed culture experiments, in which the responses to mitogens of BAL and peripheral blood T cells mixed with either alveolar macrophages or blood monocytes were compared, indicated that the decreased proliferative response was due neither to the suppressive effect nor to defects in accessory function of the alveolar macrophages. BAL T cells did not act as suppressor cells when they were added to the culture of peripheral T cells. The decreased proliferative response was not due to the dominance of CD8+ T cells frequently seen in BAL cells of HP patients, because both CD4+ and CD8+ T cells separated from BAL cells of HP patients showed lower responsiveness than those of peripheral blood T cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of substitution of a lysyl residue that binds pyridoxal phosphate in thermostable D-amino acid aminotransferase by arginine and alanine

Lys-145 of the thermostable D-amino acid aminotransferase, which binds pyridoxal phosphate, was replaced by Ala or Arg by site-directed mutagenesis. Both mutant enzymes were purified to homogeneity; their absorption spectra indicated that both mutant enzymes contained pyridoxal phosphate bound non-covalently. Even though the standard assay method did not indicate any activity with either mutant, addition of an amino donor, D-alanine, to the Arg-145 mutant enzyme led to a slow decrease in absorption at 392 nm with a concomitant increase in absorption at 333 nm. This result suggests that the enzyme was converted into the pyridoxamine phosphate form. The amount of pyruvate formed was almost equivalent to that of the reactive pyridoxal phosphate in the mutant enzyme. Thus, the Arg-145 mutant enzyme is able to catalyze slowly the half-reaction of transamination. Exogenous amines, such as methylamine, had no effect on the half-reaction with the Arg-145 mutant enzyme. In contrast, the Ala-145 mutant enzyme neither underwent the spectral change by addition of D-alanine nor catalyzed pyruvate formation, in the absence of added amine. However, the Ala-145 mutant enzyme catalyzed the half-reaction significantly in the presence of added amine. These findings suggest that a basic amino acid residue, such as lysine or arginine, is required at position 145 for catalysis of the half-reaction. The role of the exogenous amines differs with various active-site mutant enzymes.

Metalloselenonein, the selenium analogue of metallothionein: synthesis and characterization of its complex with copper ions

We used an automated peptide synthesizer to produce a peptide, metalloselenonein, that contains selenocysteine residues substituted for all cysteine residues in Neurospora crassa copper metallothionein. Metalloselenonein binds 3 mol of Cu(I) per mol. This adduct shows a broad absorption band between 230 and 400 nm and a fluorescence band at 395 nm, which can be attributed to copper-selenolate coordination. The circular dichroism spectrum of the copper-metalloselenonein complex shows a positive band around 245 nm attributable to asymmetry in metal coordination.

Macrophage engulfment of mucosal mast cells in rats treated with dexamethasone

The effects of corticosteroid treatment on mucosal mast cells in rat jejunal mucosa were examined. Rats previously infected with Nippostrongylus brasiliensis received a single IP injection of 1 mg dexamethasone. Three hours later, one third of mucosal mast cells demonstrated minor granular changes (fusion or peripheral clear zones) by electron microscopy. At 7 hours, by light microscopy, the majority of mucosal mast cells appeared abnormal with clustering of granules. By electron microscopy, 151 of 233 (65%) mucosal mast cells had been engulfed by enlarged macrophages and were in various stages of degeneration inside large phagosomes. By 24 hours, the number of mucosal mast cells had decreased to less than 10% of the initial number with parallel decreases in tissue rat mast cell protease II and histamine levels. Serum levels of rat mast cell protease II did not increase, and intestinal morphology was invariably normal with no evidence of inflammatory changes up to and including 24 hours. Observations were similar in uninfected animals. In contrast, in rats undergoing antigen-induced anaphylaxis, a significant elevation of serum rat mast cell protease II level was evident at 3 and 7 hours, and macrophage engulfment of mucosal mast cells was never seen, although tissue edema, enterocyte loss, and hemorrhage were observed. It is concluded that dexamethasone treatment results in macrophage engulfment and destruction of mucosal mast cells that occurs without granular mediator release and local inflammatory effects.

Degradation of L-djenkolate catalyzed by S-alkylcysteine alpha,beta-lyase from Pseudomonas putida

S-Alkylcysteine alpha, beta-lyase [EC 4.4.1.6] of Pseudomonas putida catalyzes alpha,beta-elimination of L-djenkolate [3,3'-methylenedithiobis(2-aminopropionic acid)] to produce pyruvate, ammonia, and S-(mercaptomethyl)cysteine initially. Secondly, S-(mercaptomethyl)-cysteine, which was identified in the form of S-(mercaptomethyl)cysteine thiolactone and S-(2-thia-3-carboxypropyl)cysteine in the absence and presence of iodoacetic acid, respectively, is decomposed enzymatically to pyruvate, ammonia, and bis(mercapto)methane, or spontaneously to cysteine, formaldehyde, and hydrogen sulfide. Balance studies showed that 1.3 mol each of pyruvate and ammonia and 0.2 mol each of formaldehyde and cysteine were produced with consumption of 1 mol of L-djenkolate. 1,2,4,5-Tetrathiane, 1,2,4-trithiolane, 1,2,4,6-tetrathiepane, and 1,2,3,5,6-pentathiepane, which are derivatives of bis(mercapto)methane, were also produced during the alpha,beta-elimination of L-djenkolate. In addition, a polymer with the general formula of -(CH2S)n- was produced as a white precipitate. When the alpha,beta-elimination of L-djenkolate was carried out in the presence of 20 mM iodoacetic acid, neither formaldehyde, cysteine, hydrogen sulfide, or the polymer were formed. Instead, the S-carboxymethyl derivatives of bis(mercapto)methane and S-(mercaptomethyl)cysteine were produced in addition to pyruvate and ammonia.

Thermostable phenylalanine dehydrogenase of Thermoactinomyces intermedius: cloning, expression, and sequencing of its gene

The gene encoding the thermostable phenylalanine dehydrogenase [EC 1.4.1.-] of a thermophile, Thermoactinomyces intermedius, was cloned and its complete DNA sequence was determined. The phenylalanine dehydrogenase gene (pdh) consists of 1,098 nucleotides and encodes 366 amino acid residues corresponding to the subunit (Mr 41,000) of the hexameric enzyme. The amino acid sequence deduced from the nucleotide sequence of the pdh gene of T. intermedius was 56.0 and 42.1% homologous to those of the phenylalanine dehydrogenases of Bacillus sphaericus and Sporosarcina ureae, respectively. It shows 47.5% homology to that of the thermostable leucine dehydrogenase from B. stearothermophilus. The pdh gene was highly expressed in E. coli JM109, the amount of phenylalanine dehydrogenase produced amounting up to about 8.3% of that of the total soluble protein. We purified the enzyme to homogeneity from transformant cells in a day, with a 58% recovery.

Thermostable aspartate aminotransferase from a thermophilic Bacillus species. Gene cloning, sequence determination, and preliminary x-ray characterization

The gene encoding aspartate aminotransferase of a thermophilic Bacillus species, YM-2, has been cloned and expressed efficiently in Escherichia coli. The primary structure of the enzyme was deduced from nucleotide sequences of the gene and confirmed mostly by amino acid sequences of tryptic peptides. The gene consists of 1,176 base pairs encoding a protein of 392 amino acid residues; the molecular mass of the enzyme subunit is estimated to be 42,661 daltons. The active site lysyl residue that binds the coenzyme, pyridoxal phosphate, was identified as Lys-239. Comparison of the amino acid sequence with those of aspartate aminotransferases from other organisms revealed very low overall similarities (13-14%) except for the sequence of the extremely thermostable enzyme from Sulfolobus solfataricus (34%). Several amino acid residues conserved in all the compared sequences include those that have been reported to participate in binding of the coenzyme in three-dimensional structures of the vertebrate and E. coli enzymes. However, the strictly conserved arginyl residue that is essential for binding of the distal carboxyl group of substrates is not found in the corresponding region of the sequences of the thermostable enzymes from the Bacillus species and S. solfataricus. The Bacillus aspartate aminotransferase has been purified from the E. coli clone cell extracts on a large scale and crystallized in the buffered ammonium sulfate solution by the hanging drop method. The crystals are monoclinic with unit cell dimensions a = 121.2 A, b = 110.5 A, c = 81.8 A, and beta = 97.6 degrees, belonging to space group C2, and contain two molecules in the asymmetric unit. The crystals of the enzyme-alpha-methylaspartate complex are isomorphous with those without the substrate analog.

The dynamics of intestinal eosinophil depletion in rats treated with dexamethasone

We examined the effects of corticosteroid treatment on eosinophils in the jejunal mucosa of rats previously infected with Nippostrongylus brasiliensis. Rats received a single intraperitoneal injection of 1 mg of dexamethasone. At a light microscopic level, the number of eosinophils with typical nuclei and granules was significantly decreased as early as 3 hours after injection, and had diminished to 17% of starting values at 24 hours. Pyknotic cells containing eosinophilic granules or fragments were observed scattered in the subepithelial interstitial space 3 and 7 hours after injection. By electron microscopy, more than 20% of eosinophils demonstrated nuclear abnormalities. Degenerating eosinophils without granular changes (27 of 127, 14.1%) or with or with granular changes (9 of 127, 7.1%) increased at 3 hours compared with untreated rats (8 of 233, 3.4%; 4 of 233, 1.7%). At 7 hours, 47 of 96 (49.0%) eosinophils were located inside phagocytic vacuoles of macrophages. Single macrophages occasionally engulfed two or more eosinophils. Only a few degeneration eosinophils (7 of 96) were observed outside macrophages. At 13 hours, the percentage of degenerating eosinophils and eosinophils inside macrophages was decreased; at 24 hours, few eosinophils were seen and eosinophil structures could not be identified inside macrophages. The epithelium and lamina propria did not show structural damage typical of an inflammatory reaction at any time. Eosinophil numbers in mesenteric lymph nodes, spleens, and peripheral blood were also reduced by dexamethasone. Similar observations were made in the jejunal mucosa of noninfected rats. We observed the slow restoration of eosinophil numbers in the intestinal wall, finally reaching preinjection numbers after 14 days. We conclude that dexamethasone has important effects on eosinophils causing (a) nuclear degeneration and (b) changes in the granular matrix. Subsequently, these damaged eosinophils are engulfed by macrophages and swiftly disappear from the intestinal mucosa. These effects appear to be due to the induction of apoptosis. Our findings offer an explanation for one of the significant antiinflammatory effects observed with the use of corticosteroids.

Overproduction of glutamate racemase of Pediococcus pentosaceus in Escherichia coli clone cells and its purification

We previously isolated a 6.0-kb DNA fragment that specifies glutamate racemase activity from the chromosomal DNA of Pediococcus pentosaceus by digestion with HindIII (N. Nakajima, K. Tanizawa, H. Tanaka, and K. Soda, 1986), Agric. Biol. Chem. 50, 2823-2830). We digested it further with EcoRI to obtain a fragment of 1.8 kb, which was blunt-ended and ligated into the SmaI site of vector plasmid pKK223-3. The recombinant plasmid showed a high glutamate racemase activity upon transformation of Escherichia coli W3110 cells with it; the plasmid was named pICR223. Glutamate racemase was overproduced in the clone cells and occurred in inclusion bodies in the cells. The enzyme was solubilized with 6 M urea, renatured by dialysis to remove urea, and purified to homogeneity with an overall yield of about 70% after a single DEAE-cellulose column chromatography. The amount of enzyme produced by the clone cells corresponded to about 38% of the total insoluble protein.

Acyclic monoterpene primary alcohol:NADP+ oxidoreductase of Rauwolfia serpentina cells: the key enzyme in biosynthesis of monoterpene alcohols

Acyclic monoterpene primary alcohol:NADP+ oxidoreductase, a key enzyme in the biosynthesis of monoterpene alcohols in plants, is unstable and has been only poorly characterized. However we have established conditions which stabilize the enzyme from Rauwolfia serpentina cells, and then purified it to homogeneity. It is a monomer with a molecular weight of about 44,000 and contains zinc ions. Various branched-chain allylic primary alcohols such as nerol, geraniol, and 10-hydroxygeraniol were substrates, but ethanol was inert. The enzyme exclusively requires NADP+ or NADPH as the cofactor. Steady-state kinetic studies showed that the nerol dehydrogenation proceeds by an ordered Bi-Bi mechanism. NADP+ binds the enzyme first and then NADPH is the second product released from it. Gas chromatography-mass spectrometric analysis of the reaction products showed that 10-hydroxygeraniol undergoes a reversible dehydrogenation to produce 10-oxogeraniol or 10-hydroxygeranial, which are oxidized further to give 10-oxogeranial, the direct precursor of iridodial. The enzyme has been found to exclusively transfer the pro-R hydrogen of NADPH to neral. The N-terminal sequence of the first 21 amino acids revealed no significant homology with those of various other proteins including the NAD(P)(+)-dependent alcohol dehydrogenases registered in a protein data bank.

Mechanistic studies of two amino acid racemases of broad substrate specificity from Pseudomonas striata and Aeromonas caviae

The conversion of L-[alpha-2H]alanine in H2O and unlabeled L-alanine in 2H2O into D-alanine, under nearly irreversible conditions, with the amino acid racemase from Pseudomonas striata showed significant internal transfer of the alpha-hydrogen. This result has been interpreted as being indicative of a single base mechanism for the racemization. The relative rates of deuterium incorporation into unlabeled D- and L-methionine by the two amino acid racemases of broad substrate specificity from P. striata and Aeromonas caviae, were measured in 2H2O. The results showed a markedly different pattern, dependent upon the configuration of the initial substrate; with D-methionine as substrate deuterium is incorporated into both enantiomers at approximately the same rate, but with L-methionine as substrate deuterium is incorporated considerably faster into the D than the L enantiomer. These results argue against a single base mechanism of racemization for these enzymes and are best rationalized in terms of a double base model where only one of the bases undergoes proton (deuterium) exchange with the solvent while the amino acid is enzyme-bound. The interpretation of the earlier experiment needs to be considered in light of these results.

Characterization of the half and overall reactions catalyzed by L-lysine:2-oxoglutarate 6-aminotransferase

Significant differences were found in the reaction rate, and the substrate and reaction specificities between the half reactions and the overall reactions catalyzed by L-lysine: 2-oxoglutarate 6-aminotransferase. The half reactions between an amino donor and the enzyme-bound pyridoxal 5'-phosphate, and also between an amino acceptor and the bound pyridoxamine 5'-phosphate followed first order reaction kinetics. The extrapolated first order rate constants and dissociation constants of the substrates were determined for the half reactions: lysine, 0.87 min-1 and 5.5 mM; glutamate, 1.1 min-1 and 10.5 mM; alanine, 0.66 min-1 and 6.6 mM; 6-aminohexanoate, 0.43 min-1 and 13.3 mM; and 2-oxoglutarate, 0.33 min-1 and 2.5 mM. As compared with the values reported for the overall reactions [Soda, K., Misono, H., & Yamamoto, T. (1968) Biochemistry 7, 4102-4109], the reactivity of the inherent substrates was lower by over 4 orders in the half reaction than that in the overall reaction, and the reactivity of alanine with the bound pyridoxal 5'-phosphate was reduced to 10% of that in the overall reaction. The substrate specificity in the half reaction was much lower than that in the overall reaction, which was re-examined in a reaction system containing the same concentration of the enzyme as that for the half reactions. Lysine 6-aminotransferase catalyzes the transfer of only the terminal amino group of lysine to 2-oxoglutarate in the overall reaction. However, in the half reaction, the 2-amino group as well as the terminal one was transferred to the bound pyridoxal 5'-phosphate. The ratio of reactivity of the 2-amino group to that of the 6-amino group was considerably influenced by the pH of the reaction mixture.(ABSTRACT TRUNCATED AT 250 WORDS)