Amino acid catalyzed direct asymmetric aldol reactions: a bioorganic approach to catalytic asymmetric carbon-carbon bond-forming reactions

Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with commercially available chiral cyclic secondary amines as catalysts. Structure-based catalyst screening identified L-proline and 5,5-dimethyl thiazolidinium-4-carboxylate (DMTC) as the most powerful amino acid catalysts for the reaction of both acyclic and cyclic ketones as aldol donors with aromatic and aliphatic aldehydes to afford the corresponding aldol products with high regio-, diastereo-, and enantioselectivities. Reactions employing hydroxyacetone as an aldol donor provide anti-1,2-diols as the major product with ee values up to >99%. The reactions are assumed to proceed via a metal-free Zimmerman-Traxler-type transition state and involve an enamine intermediate. The observed stereochemistry of the products is in accordance with the proposed transition state. Further supporting evidence is provided by the lack of nonlinear effects. The reactions tolerate a small amount of water (<4 vol %), do not require inert reaction conditions and preformed enolate equivalents, and can be conveniently performed at room temperature in various solvents. In addition, reaction conditions that facilitate catalyst recovery as well as immobilization are described. Finally, mechanistically related addition reactions such as ketone additions to imines (Mannich-type reactions) and to nitro-olefins and alpha,beta-unsaturated diesters (Michael-type reactions) have also been developed.

Overexpression of glyoxalase-I in bovine endothelial cells inhibits intracellular advanced glycation endproduct formation and prevents hyperglycemia-induced increases in macromolecular endocytosis

Methylglyoxal (MG), a dicarbonyl compound produced by the fragmentation of triose phosphates, forms advanced glycation endproducts (AGEs) in vitro. Glyoxalase-I catalyzes the conversion of MG to S-D-lactoylglutathione, which in turn is converted to D-lactate by glyoxalase-II. To evaluate directly the effect of glyoxalase-I activity on intracellular AGE formation, GM7373 endothelial cells that stably express human glyoxalase-I were generated. Glyoxalase-I activity in these cells was increased 28-fold compared to neo-transfected control cells (21.80+/-0.1 vs. 0. 76+/-0.02 micromol/min/mg protein, n = 3, P < 0.001). In neo-transfected cells, 30 mM glucose incubation increased MG and D-lactate concentration approximately twofold above 5 MM (35.5+/-5.8 vs. 19.6+/-1.6, P < 0.02, n = 3, and 21.0+/-1.3 vs. 10.0+/-1.2 pmol/ 10(6) cells, n = 3, P < 0.001, respectively). In contrast, in glyoxalase-I-transfected cells, 30 mM glucose incubation did not increase MG concentration at all, while increasing the enzymatic product D-lactate by > 10-fold (18.9+/-3.2 vs. 18.4+/- 5.8, n = 3, P = NS, and 107.1+/-9.0 vs. 9.4+/-0 pmol/10(6) cells, n = 3, P < 0.001, respectively). After exposure to 30 mM glucose, intracellular AGE formation in neo cells was increased 13.6-fold (2.58+/-0.15 vs. 0.19+/-0.03 total absorbance units, n = 3, P < 0.001). Concomitant with increased intracellular AGEs, macromolecular endocytosis by these cells was increased 2.2-fold. Overexpression of glyoxalase-I completely prevented both hyperglycemia-induced AGE formation and increased macromolecular endocytosis.

SHMT2 drives glioma cell survival in ischaemia but imposes a dependence on glycine clearance

Cancer cells adapt their metabolic processes to support rapid proliferation, but less is known about how cancer cells alter metabolism to promote cell survival in a poorly vascularized tumour microenvironment. Here we identify a key role for serine and glycine metabolism in the survival of brain cancer cells within the ischaemic zones of gliomas. In human glioblastoma multiforme, mitochondrial serine hydroxymethyltransferase (SHMT2) and glycine decarboxylase (GLDC) are highly expressed in the pseudopalisading cells that surround necrotic foci. We find that SHMT2 activity limits that of pyruvate kinase (PKM2) and reduces oxygen consumption, eliciting a metabolic state that confers a profound survival advantage to cells in poorly vascularized tumour regions. GLDC inhibition impairs cells with high SHMT2 levels as the excess glycine not metabolized by GLDC can be converted to the toxic molecules aminoacetone and methylglyoxal. Thus, SHMT2 is required for cancer cells to adapt to the tumour environment, but also renders these cells sensitive to glycine cleavage system inhibition.

Field experiments with transformed plants reveal the sense of floral scents

Plants use many means to attract pollinators, including visual cues and odor. We investigated how nonpigment floral chemistry influences nectar removal, floral visitation, florivory, rates of outcrossing, and fitness through both male and female functions. We blocked expression of biosynthetic genes of the dominant floral attractant [benzyl acetone (Nachal1)] and nectar repellent [nicotine (Napmt1/2)] in all combinations in the native tobacco Nicotiana attenuata and measured their effects on plants in their native habitat. Both repellent and attractant were required to maximize capsule production and seed siring in emasculated flowers and flower visitation by native pollinators, whereas nicotine reduced florivory and nectar robbing.

Crystal structure of a Baeyer-Villiger monooxygenase

Flavin-containing Baeyer-Villiger monooxygenases employ NADPH and molecular oxygen to catalyze the insertion of an oxygen atom into a carbon-carbon bond of a carbonylic substrate. These enzymes can potentially be exploited in a variety of biocatalytic applications given the wide use of Baeyer-Villiger reactions in synthetic organic chemistry. The catalytic activity of these enzymes involves the formation of two crucial intermediates: a flavin peroxide generated by the reaction of the reduced flavin with molecular oxygen and the "Criegee" intermediate resulting from the attack of the flavin peroxide onto the substrate that is being oxygenated. The crystal structure of phenylacetone monooxygenase, a Baeyer-Villiger monooxygenase from the thermophilic bacterium Thermobifida fusca, exhibits a two-domain architecture resembling that of the disulfide oxidoreductases. The active site is located in a cleft at the domain interface. An arginine residue lays above the flavin ring in a position suited to stabilize the negatively charged flavin-peroxide and Criegee intermediates. This amino acid residue is predicted to exist in two positions; the "IN" position found in the crystal structure and an "OUT" position that allows NADPH to approach the flavin to reduce the cofactor. Domain rotations are proposed to bring about the conformational changes involved in catalysis. The structural studies highlight the functional complexity of this class of flavoenzymes, which coordinate the binding of three substrates (molecular oxygen, NADPH, and phenylacetone) in proximity of the flavin cofactor with formation of two distinct catalytic intermediates.

Small Peptides Catalyze Highly Enantioselective Direct Aldol Reactions of Aldehydes with Hydroxyacetone: Unprecedented Regiocontrol in Aqueous Media

[reaction: see text] L-Proline-based small peptides have been developed as efficient catalysts for the asymmetric direct aldol reactions of hydroxyacetone with aldehydes. Chiral 1,4-diols 7, which are disfavored products in similar aldol reactions catalyzed by either aldolases or L-proline, were obtained in high yields and enantioselectivities of up to 96% ee with peptides 3 and 4 in aqueous media.

In-depth mechanistic study on the formation of acrylamide and other vinylogous compounds by the maillard reaction

The formation of acrylamide was studied in low-moisture Maillard model systems (180 degrees C, 5 min) based on asparagine, reducing sugars, Maillard intermediates, and sugar degradation products. We show evidence that certain glycoconjugates play a major role in acrylamide formation. The N-glycosyl of asparagine generated about 2.4 mmol/mol acrylamide, compared to 0.1-0.2 mmol/mol obtained with alpha-dicarbonyls and the Amadori compound of asparagine. 3-Hydroxypropanamide, the Strecker alcohol of asparagine, generated only low amounts of acrylamide ( approximately 0.23 mmol/mol), while hydroxyacetone increased the acrylamide yields to more than 4 mmol/mol, indicating that alpha-hydroxy carbonyls are much more efficient than alpha-dicarbonyls in converting asparagine into acrylamide. The experimental results are consistent with the reaction mechanism based on (i) a Strecker type degradation of the Schiff base leading to azomethine ylides, followed by (ii) a beta-elimination reaction of the decarboxylated Amadori compound to afford acrylamide. The beta-position on both sides of the nitrogen atom is crucial. Rearrangement of the azomethine ylide to the decarboxylated Amadori compound is the key step, which is favored if the carbonyl moiety contains a hydroxyl group in beta-position to the nitrogen atom. The beta-elimination step in the amino acid moiety was demonstrated by reacting under low moisture conditions decarboxylated model Amadori compounds obtained by synthesis. The corresponding vinylogous compounds were only generated if a beta-proton was available, for example, styrene from the decarboxylated Amadori compound of phenylalanine. Therefore, it is suggested that this thermal pathway may be common to other amino acids, resulting under certain conditions in their respective vinylogous reaction products.

Physiological and pathological implications of semicarbazide-sensitive amine oxidase

Semicarbazide-sensitive amine oxidase (SSAO) catalyzes the deamination of primary amines. Such deamination has been shown capable of regulating glucose transport in adipose cells. It has been independently discovered that the primary structure of vascular adhesion protein-1 (VAP-1) is identical to SSAO. VAP-1 regulates leukocyte migration and is related to inflammation. Increased serum SSAO activities have been found in patients with diabetic mellitus, vascular disorders and Alzheimer's disease. The SSAO-catalyzed deamination of endogenous substrates, that is, methylamine and aminoacetone, led to production of toxic formaldehyde and methylglyoxal, hydrogen peroxide and ammonia, respectively. These highly reactive aldehydes have been shown to initiate protein cross-linkage, exacerbate advanced glycation of proteins and cause endothelial injury. Hydrogen peroxide contributes to oxidative stress. 14C-methylamine is converted to 14C-formaldehyde, which then forms labeled long-lasting protein adduct in rodents. Chronic methylamine treatment increased the excretion of malondialdehyde and microalbuminuria, and enhanced the formation of fatty streaks in C57BL/6 mice fed with an atherogenic diet. Treatment with selective SSAO inhibitor reduces atherogenesis in KKAy diabetic mice fed with high-cholesterol diet. Aminoguanidine, which blocks advanced glycation and reduces nephropathy in animals, is in fact more potent at inhibiting SSAO than its effect on glycation. It suggests that SSAO is involved in vascular disorders under certain pathological conditions. Although SSAO has been known for several decades, its physiological and pathological implications are just beginning to be recognized.

Inactivation of cellular enzymes by carbonyls and protein-bound glycation/glycoxidation products

Diabetic plasma contains elevated levels of glucose and various low-molecular-weight carbonyl compounds derived from the metabolism of glucose and related materials. These compounds react with protein side chains (Arg, Lys, Cys, and His) to give glycated materials and advanced glycation end products. In this study, we have examined the effect of glucose and carbonyl compounds (methylglyoxal, glyoxal, glycolaldehyde, and hydroxyacetone), and glycation products arising from reaction of these materials with model proteins, on the activity of three key cellular enzymes: glyceraldehyde-3-phosphate dehydrogenase (GAPDH), glutathione reductase, and lactate dehydrogenase, both in isolation and in cell lysates. In contrast to glucose (1M, both fresh and aged for 8 weeks), which had no effect, marked inhibition of all three enzymes was observed with methylglyoxal and glyoxal. GAPDH was also inhibited by glycolaldehyde and hydroxyacetone. Incubation of these enzymes with proteins that had been preglycated with methylglyoxal, but not glucose, also resulted in significant time- and concentration-dependent inhibition with both isolated enzymes and cell lysates. This inhibition was not metal ion, oxygen, superoxide dismutase, or catalase dependent, suggesting that inhibition is not radical mediated. These effects are suggested to be due to direct adduction of the free- or protein-bound carbonyls with the target enzyme. Such an interpretation is supported by the detection of the loss of thiol groups on GAPDH and the detection of cross-linked materials on protein gels. Though direct comparison of the extent of inhibition induced by free versus protein-bound carbonyls was not possible, the significantly higher concentrations of the latter materials over the former in diabetic plasma and cells lead us to suggest that alterations in the activity of key cellular enzymes induced by glycated proteins may play a significant role in the development of diabetic complications.

The metabolism of aminoacetone to methylglyoxal by semicarbazide-sensitive amine oxidase in human umbilical artery

The aliphatic amine aminoacetone has been described previously as a product of mitochondrial metabolism of threonine and glycine. Here, aminoacetone is shown to be deaminated to methylglyoxal by supernatants obtained by low speed centrifugation (600 g/10 min) of human umbilical artery homogenates, and also by membrane fractions isolated by high speed centrifugation (105,000 g/60 min) of these supernatants. Metabolism of 100 microM aminoacetone was completely inhibited by 1 mM propargylamine and MDL 72145, drugs which are capable of inhibiting the membrane-bound semicarbazide-sensitive amine oxidase (SSAO) activity found in vascular smooth muscle cells, whereas 1 mM pargyline and deprenyl which are inhibitors of monoamine oxidase, were without inhibitory effect. Estimated kinetic constants (at pH 7.8) for aminoacetone metabolism were Km = 92 microM; Vmax = 270 nmol/hr/mg protein. In addition, aminoacetone was a competitive inhibitor (Ki = 83 microM and 128 microM in low speed supernatants and high speed membrane fractions, respectively) of [14C]benzylamine metabolism by SSAO in this tissue. Aminoacetone would appear to be an endogenously occurring amine with a Km for metabolism by SSAO far lower than other aliphatic and aromatic biogenic amines examined previously as potential physiological substrates for the human vascular enzyme and possible implications of this are discussed.

The Prodrug Activator EtaA from Mycobacterium tuberculosis Is a Baeyer-Villiger Monooxygenase

EtaA is a newly identified FAD-containing monooxygenase that is responsible for activation of several thioamide prodrugs in Mycobacterium tuberculosis. It was found that purified EtaA displays a remarkably low activity with the antitubercular prodrug ethionamide. Hinted by the presence of a Baeyer-Villiger monooxygenase sequence motif in the EtaA sequence, we have been able to identify a large number of novel EtaA substrates. It was discovered that the enzyme converts a wide range of ketones to the corresponding esters or lactones via a Baeyer-Villiger reaction, indicating that EtaA represents a Baeyer-Villiger monooxygenase. With the exception of aromatic ketones (phenylacetone and benzylacetone), long-chain ketones (e.g. 2-hexanone and 2-dodecanone) also are converted. EtaA is also able to catalyze enantioselective sulfoxidation of methyl-p-tolylsulfide. Conversion of all of the identified substrates is relatively slow with typical k(cat) values of around 0.02 s(-1). The best substrate identified so far is phenylacetone (K(m) = 61 microM, k(cat) = 0.017 s(-1)). Redox monitoring of the flavin cofactor during turnover of phenylacetone indicates that a step in the reductive half-reaction is limiting the rate of catalysis. Intriguingly, EtaA activity could be increased by one order of magnitude by adding bovine serum albumin. This reactivity and substrate acceptance-profiling study provides valuable information concerning this newly identified prodrug activator from M. tuberculosis.

Acetone metabolism in humans during diabetic ketoacidosis

Plasma acetone turnover rates were measured with the primed continuous infusion of 2-[14C]acetone in patients with moderate to severe diabetic ketoacidosis. Plasma acetone turnover rates ranged from 1.52 to 15.9 mumol X kg-1 X min-1 (108-1038 mumol X 1.73 m-2 X min-1) and were directly related to the plasma acetone concentrations that ranged from 0.47 to 7.61 mM. The average acetone turnover rate was 6.45 mumol X kg-1 X min-1 (533 mumol X 1.73 m-2 X min-1), a value twice that obtained in a similar group of diabetic ketoacidotic patients via the single-injection technique of 2-[14C]acetone administration. Degradation of urine glucose revealed that 14C from administered 2-[14C )acetone was principally located in carbons 1, 2, 5, and 6 of the glucose molecule in five of six patients. This distribution is similar to that expected from 2-[14C]pyruvate, suggesting that acetone was converted to glucose through pyruvate. In one patient, label was located predominantly in glucose carbons 3 and 4, indicating that acetone metabolism may be different in some patients. Acetol (1-hydroxyacetone) and 1,2-propanediol (PPD), two possible metabolites of acetone, were detected in plasma of the patients. The concentrations of Acetol ranged from 0 to 0.48 mM and of PPD ranged from 0 to 0.53 mM. The concentrations of each metabolite were directly related to the plasma acetone concentrations. During the continuous infusion of 2-[14C]acetone, the specific activities of plasma glucose and PPD rose continuously but did not reach constant values. Estimates of the minimal percent plasma glucose and PPD derived from plasma acetone averaged 2.1 and 74%, respectively.

Characterization of the explosive triacetone triperoxide and detection by ion mobility spectrometry

The improvised explosive triacetone triperoxide (TATP) was synthesized and characterized by 1H-nuclear magnetic resonance (NMR), 13C-NMR, Raman and infrared (IR) spectroscopy. Triacetone triperoxide was subsequently analyzed by ion mobility spectrometry (IMS) in positive ion mode, and detected as a cluster of three peaks with a drift time of the most intense peak at 13.06 ms. Triacetone triperoxide was then analyzed after dissolution in toluene, where a dramatic increase in peak intensity was observed, at a flight time of 12.56 ms (K0=2.71 cm2V(-1)s(-1)). Triacetone triperoxide was subsequently analyzed by coupling the ion mobility spectrometer to a triple quadrupole mass spectrometer, where a single peak at m/z of 223 atomic mass units identified the species present in the ion mobility spectra as being triacetone triperoxide.

Disinfection byproduct formation and fractionation behavior of natural organic matter surrogates

While natural organic matter (NOM) surrogates are established in disinfection byproduct (DBP) research, their use in fractionation studies is rare. To understand how surrogates relate to drinking waters, a range of NOM surrogates were fractionated with XAD resins. Their trihalomethane (THM), haloacetic acid (HAA), haloacetaldehyde, haloacetonitrile, and haloketone formations after chlorination were recorded. While compounds with higher log K(ow) values behaved as hydrophobic acids, fractionation of the more hydrophilic compounds did not clearly correlate to the log K(ow). High HAA formation from ferulic and aspartic acids and 1,1,1-trichloropropanone (1,1,1-TCP) formation from 3-oxopropanoic acid were notable. Three amino acids, asparagine, aspartic acid, and tryptophan, formed significant levels of dichloroacetonitrile (DCAN) and trichloroacetaldehyde (TCA). Formation of DBPs did not correlate to any compound physical property; however, there were several correlations between DBP groups. The most significant were between dichloroacetic acid (DCAA) and dichloroacetonitrile (DCAN), DCAN and TCA, and dichloroacetaldehyde (DCA) and trichloroacetaldehyde, indicating the possibility of similar relationships in natural waters.

Transition state analogs as ligands for affinity purification of juvenile hormone esterase

Insect juvenile hormones are metabolized in numerous species of caterpillars by low abundance, highly specific esterases. Because of their role in regulating and possibly disrupting juvenile hormone titer and thus insect metamorphosis, they are of interest to developmental biologists as well as scientists interested in selective insect control. However, the enzymes have defied attempts to purify and characterize them. Juvenile hormone esterase activity can be inhibited by a variety of 3-substituted 1,1,1-trifluoropropanone sulfides. These apparent transition state analogs were used as ligands and eluting agents to purify juvenile hormone esterase from four insect species from 500-fold to over 1000-fold in high yield. After elution from the affinity column, the enzymes were radiolabeled with paraoxon and analyzed by electrophoresis, and the results demonstrate a high degree of purity. Transition state analogs may be useful for the affinity purification of other enzymes.

Where does plasma methylglyoxal originate from?

The elevation of plasma methylglyoxal levels in diabetic humans is widely observed, but it is unknown to what extent different sources of methylglyoxal contribute to its plasma concentration. A retrospective analysis of clinical findings has been undertaken. There is controversy about the correlation of plasma methylglyoxal concentrations with fasting or postprandial glucose levels, and the relationship with HbA1c. There is only one study in which plasma ketone body levels have been monitored in parallel with methylglyoxal and a positive correlation between plasma methylglyoxal and β-hydroxybutyrate was observed. There are no reports on plasma aminoacetone levels and methylglyoxal in diabetic humans. This paper suggests that although there is a close association between methylglyoxal and carbohydrate metabolism, the presence of this 1,2-dicarbonyl in the plasma is mainly due to other mechanisms. Protein glycation and aminoacetone degradation are proposed to be the major and the minor sources of plasma methylglyoxal under normal conditions.

The pancreatic beta-cell recognition of insulin secretagogues: does cyclic AMP mediate the effect of glucose?

Insulin release and the content of cAMP were studied in microdissected pancreatic islets of noninbred ob/ob (obese) mice. In the absence of 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, 20 mM glucose had no effect on cAMP save a very small initial rise detectable by a freeze-stop perifusion technique only. However, combined with this methylxanthine, 20 mM glucose produced significant increases of cAMP both in perifused islets and in islets conventionally incubated in closed vials. Glucose shared this capacity to raise the cAMP level with D-glyceraldehyde and 1,3-dihydroxyacetone. Isobutylmethylxanthine (0.05-1.0 mM) or 5 mug/ml of cholera toxin, an activator of adenylate cyclase, also increased the islet cAMP level; the effects of the methylxanthine, whether or not combined with cholera toxin, were potentiated by glucose. Isobutylmethylxanthine (0.05-1.0 mM) or 5 mug/ml of cholera toxin potentiated insulin release in response to 20 mM glucose. However, only 0.5-1.0 mM isobutylmethylxanthine stimulated insulin release in the presence of 3 mM glucose, whereas 0.05-0.1 mM isobutylmethylxanthine or 5 mug/ml of cholera toxin had no effect on secretion at the low glucose concentration. These discrepancies between cAMP-promoting and insulin-releasing activities suggest that glucose does not initiate insulin release by activating the beta-cell adenylate cyclase. By being metabolized in the beta-cells, glucose may both create a release-initiating signal not identical with cAMP and enhance cAMP formation, leading to potentiation of the effect of the initiator signal.

Microbial synthesis of chiral amines by (R)-specific transamination with Arthrobacter sp. KNK168

Arthrobacter sp. KNK168 shows (R)-enantioselective transaminase [(R)-transaminase] activity, which converts prochiral ketones into the corresponding chiral (R)-amines in the presence of an amino donor. The cultural conditions and reaction conditions for asymmetric synthesis of chiral amines with this microorganism were examined. The transaminase was inducible, and its production was enhanced by the addition of sec-butylamine and 3-amino-2,2-dimethylbutane to the culture medium. (R)-1-Phenylethylamine was a good amino donor for amination of 3,4-dimethoxyphenylacetone with Arthrobacter sp. KNK168. Under the optimum conditions, 126 mM (R)-3,4-dimethoxyamphetamine (DMA) [>99% enantiomeric excess (ee)] was synthesized from 154 mM 3,4-dimethoxyphenylacetone and 154 mM (R)-1-phenylethylamine through the whole cell reaction with an 82% conversion yield. (R)-Enantiomers of other amines, such as (R)-4-methoxyamphetamine, (R)-1-(3-hydroxyphenyl)ethylamine and (R)-1-(3-hydroxyphenyl)ethylamine, were also synthesized from the corresponding carbonyl compounds through asymmetric amination with Arthrobacter sp. KNK168.

A convenient ultrasound-promoted synthesis of some new thiazole derivatives bearing a coumarin nucleus and their cytotoxic activity

Successful implementation of ultrasound irradiation for the rapid synthesis of a novel series of 3-[1-(4-substituted-5-(aryldiazenyl)thiazol-2-yl)hydrazono)ethyl]-2H-chromen-2-ones 5a–h, via reactions of 2-(1-(2-oxo-2H-chromen-3-yl)ethylidene) thiosemicarbazide (2) and the hydrazonoyl halides 3(4), was demonstrated. Also, a new series of 5-arylidene-2-(2-(1-(2-oxo-2H-chromen-3-yl)ethylidene)hydrazinyl)thiazol-4(5H)-ones 10a–d were synthesized from reaction of 2 with chloroacetic acid and different aldehydes. Moreover, reaction of 2-cyano-N'-(1-(2-oxo-2H-chromen-3-yl)ethylidene)-acetohydrazide (12) with substituted benzaldehydes gave the respective arylidene derivatives 13a–c under the conditions employed. The structures of the synthesized compounds were assigned based on elemental analyses and spectral data. Also, the cytototoxic activities of the thiazole derivative 5a was evaluated against HaCaT cells (human keratinocytes). It was found that compound 5a possess potent cytotoxic activity.

Separation of scrapie prion infectivity from PrP amyloid polymers

The prion protein (PrP) undergoes a profound conformational change when the cellular isoform (PrPC) is converted into the scrapie form (PrPSc). Limited proteolysis of PrPsc produces PrP 27-30 which readily polymerizes into amyloid. To study the structure of PrP amyloid, we employed organic solvents that perturb protein conformation. Hexafluoro-2-propanol (HFIP), which promotes alpha-helix formation, modified the ultrastructure of rod-shaped PrP amyloids; flattened ribbons with a more regular substructure were found. As the concentration of HFIP was increased, the beta-sheet content and proteinase K resistance of PrP 27-30 as well as prion infectivity diminished. HFIP reversibly decreased the binding of Congo red dye to the rods while inactivation of prion infectivity was irreversible. In contrast to 10% HFIP, 1,1,1-trifluoro-2-propanol (TFIP) did not inactivate prion infectivity but like HFIP, TFIP did alter the morphology of the rods and abolish Congo red binding. This study separates prion infectivity from the amyloid properties of PrP 27-30 and underscores the dependence of prion infectivity on PrPSc conformation. The results also demonstrate that the specific beta-sheet-rich structures required for prion infectivity can be differentiated from those needed for amyloid formation as determined by Congo red binding.

Conversion of methylglyoxal to acetol by Escherichia coli aldo-keto reductases

Methylglyoxal (MG) is a toxic metabolite known to accumulate in various cell types. We detected in vivo conversion of MG to acetol in MG-accumulating Escherichia coli cells by use of (1)H nuclear magnetic resonance ((1)H-NMR) spectroscopy. A search for homologs of the mammalian aldo-keto reductases (AKRs), which are known to exhibit activity to MG, revealed nine open reading frames from the E. coli genome. Based on both sequence similarities and preliminary characterization with (1)H-NMR for crude extracts of the corresponding mutant strains, we chose five genes, yafB, yqhE, yeaE, yghZ, and yajO, for further study. Quantitative assessment of the metabolites produced in vitro from the crude extracts of these mutants and biochemical study with purified AKRs indicated that the yafB, yqhE, yeaE, and yghZ genes are involved in the conversion of MG to acetol in the presence of NADPH. When we assessed their in vivo catalytic activities by creating double mutants, all of these genes except for yqhE exhibited further sensitivities to MG in a glyoxalase-deficient strain. The results imply that the glutathione-independent detoxification of MG can occur through multiple pathways, consisting of yafB, yqhE, yeaE, and yghZ genes, leading to the generation of acetol.

Catabolism of threonine in mammals by coupling of L-threonine 3-dehydrogenase with 2-amino-3-oxobutyrate-CoA ligase

There is doubt about the L-threonine 3-dehydrogenase (EC 1.1.1.103) and threonine aldolase (EC 2.1.2.1) catabolic pathways of L-threonine in mammals which are believed to produce aminoacetone and glycine plus acetaldehyde, respectively. L-Threonine 3-dehydrogenase in disrupted guinea-pig liver mitochondria was investigated in a reaction mixture containing L-threonine without and with CoA and oxaloacetate; L-[U-14C]threonine was included in four similar experiments for autoradiograms. Threonine aldolase was examined in similar mitochondria from liver and kidney. CoA reduced the aminoacetone formed from L-threonine to 10-14% and CoA plus oxaloacetate produced citrate (from CoASAc) in approximately equal amounts to the decrease in aminoacetone. Autoradiograms confirmed the decrease in aminoacetone with the simultaneous appearance of citrate and glycine. No evidence was obtained that threonine aldolase catabolised L-threonine at the concentration used to assay the dehydrogenase. It is concluded that 2-amino-3-oxobutyrate (precursor of aminoacetone), which is produced from L-threonine by L-threonine 3-dehydrogenase, undergoes CoA-dependent cleavage to glycine and CoASAc by 2-amino-3-oxobutyrate-CoA ligase. The results suggest that the coupling of these enzymes provides a new pathway for the catabolism of threonine in mammals.

A partition assay for the simultaneous determination of insect juvenile hormone esterase and epoxide hydrolase activity

A partition assay was developed to measure insect juvenile hormone (JH) I and III metabolism in biological samples containing both JH esterase and JH epoxide hydrolase activity. The assay utilizes commercially available radiochain 3H-labeled JH as substrate and the selective JH esterase inhibitor 3-octylthio-1,1,1-trifluoro-2-propanone. JH partitions into an isooctane phase and the metabolites JH acid, JH diol, and JH diol-acid into aqueous methanol after incubation of JH substrate with inhibited and uninhibited sample. The assay provides a time- and cost-efficient alternative to the currently available thin-layer chromatography method for the measurement of JH esterase and epoxide hydrolase activity.