An Investigation into the In Vitro Metabolic Stability of Aryl Sulfonyl Fluorides for their Application in Medicinal Chemistry and Radiochemistry

Molecules that feature a sulfonyl fluoride (SO₂F) moiety have been gaining increasing interest due to their unique reactivity and potential applications in synthetic chemistry, medicinal chemistry, and other biological uses. A particular interest is towards ¹⁸F-radiochemistry where sulfonyl fluorides can be used as a method to radiolabel biomolecules or can be used as radiofluoride relay reagents that facilitate radiolabeling of other molecules. The low metabolic stability of sulfonyl fluoride S-F bonds, however, presents an issue and limits the applicability of sulfonyl fluorides. The aim of this work was to increase understanding of what features contribute to the metabolic instability of the S-F bond in model aryl sulfonyl fluorides and identify approaches to increasing sulfonyl fluoride stability for ¹⁸F-radiochemistry and other medicinal, synthetic chemistry and biological applications. To undertake this, 14 model aryl sulfonyl fluorides compounds with varying functional groups and substitution patterns were investigated, and their stabilities were examined in various media, including phosphate-buffered saline and rat serum as a model for biological conditions. The results indicate that both electronic and steric factors affect the stability of the S-F bond, with the 2,4,6-trisubstituted model aryl sulfonyl fluorides examined displaying the highest in vitro metabolic stability.

Electrophysiological Responses of Bactrocera kraussi (Hardy) (Tephritidae) to Rectal Gland Secretions and Headspace Volatiles Emitted by Conspecific Males and Females

Pheromones are biologically important in fruit fly mating systems, and also have potential applications as attractants or mating disrupters for pest management. Bactrocera kraussi (Hardy) (Diptera: Tephritidae) is a polyphagous pest fruit fly for which the chemical profile of rectal glands is available for males but not for females. There have been no studies of the volatile emissions of either sex or of electrophysiological responses to these compounds. The present study (i) establishes the chemical profiles of rectal gland contents and volatiles emitted by both sexes of B. kraussi by gas chromatography–mass spectrometry (GC–MS) and (ii) evaluates the detection of the identified compounds by gas chromatography–electroantennogram detection (GC–EAD) and –electropalpogram detection (GC–EPD). Sixteen compounds are identified in the rectal glands of male B. kraussi and 29 compounds are identified in the rectal glands of females. Of these compounds, 5 were detected in the headspace of males and 13 were detected in the headspace of females. GC–EPD assays recorded strong signals in both sexes against (E,E)-2,8-dimethyl-1,7-dioxaspiro[5.5]undecane, 2-ethyl-7-mehtyl-1,6-dioxaspiro[4.5]decane isomer 2, (E,Z)/(Z,E)-2,8-dimethyl-1,7-dioxaspiro[5.5]undecane, and (Z,Z)-2,8-dimethyl-1,7-dioxaspiro[5.5]undecane. Male antennae responded to (E,E)-2,8-dimethyl-1,7-dioxaspiro[5.5]undecane, 2-methyl-6-pentyl-3,4-dihydro-2H-pyran, 6-hexyl-2-methyl-3,4-dihydro-2H-pyran, 6-oxononan-1-ol, ethyl dodecanoate, ethyl tetradecanoate and ethyl (Z)-hexadec-9-enoate, whereas female antennae responded to (E,E)-2,8-dimethyl-1,7-dioxaspiro[5.5]undecane and 2-methyl-6-pentyl-3,4-dihydro-2H-pyran only. These compounds are candidates as pheromones mediating sexual interactions in B. kraussi.

Macquarie–Yaegl Partnership: Community Capability Strengthening Through Western and Indigenous Science

The rich customary knowledge possessed by Indigenous people from around the world has provided intellectually stimulating academic research opportunities and has been a successful avenue for healthcare and drug discovery as well as commercial native foods, flavours, fragrances, nutraceuticals, cosmetics, and agricultural products. When conducted with benefit sharing and reciprocity as core agenda, such research can provide community capability strengthening and immense rewards for both the Indigenous people and the academic research team involved, as well as benefiting potentially many others. This account shares my experiences as a natural products and medicinal chemistry academic, of working with Australian Aboriginal Elders, most notably from Yaegl Country of northern New South Wales, on investigating their bush medicines. Together we have facilitated the recognition and preservation of Yaegl Country customary knowledge and through initiation of a science leadership program, the National Indigenous Science Education Program, we have promoted educational attainment and STEM engagement in Australian Aboriginal youth. While this account is authored as my own personal statement of the Macquarie–Yaegl partnership, I am indebted to the Yaegl Aboriginal Elders and other Australian Aboriginal people I have worked with, and my university, school and community collaborators, my research team and student volunteers, who have all enabled the outcomes described in this account to be realised, and have made the experience so rewarding. I am also thankful to the Royal Australian Chemical Institute for the recognition of the value of this work through the award of a 2019 Royal Australian Chemical Institute Citation.

Methods to Enhance the Metabolic Stability of Peptide-Based PET Radiopharmaceuticals

The high affinity and specificity of peptides towards biological targets, in addition to their favorable pharmacological properties, has encouraged the development of many peptide-based pharmaceuticals, including peptide-based positron emission tomography (PET) radiopharmaceuticals. However, the poor in vivo stability of unmodified peptides against proteolysis is a major challenge that must be overcome, as it can result in an impractically short in vivo biological half-life and a subsequently poor bioavailability when used in imaging and therapeutic applications. Consequently, many biologically and pharmacologically interesting peptide-based drugs may never see application. A potential way to overcome this is using peptide analogues designed to mimic the pharmacophore of a native peptide while also containing unnatural modifications that act to maintain or improve the pharmacological properties. This review explores strategies that have been developed to increase the metabolic stability of peptide-based pharmaceuticals. It includes modifications of the C- and/or N-termini, introduction of d- or other unnatural amino acids, backbone modification, PEGylation and alkyl chain incorporation, cyclization and peptide bond substitution, and where those strategies have been, or could be, applied to PET peptide-based radiopharmaceuticals.

Systematic Modification of Zingerone Reveals Structural Requirements for Attraction of Jarvis's Fruit Fly

Tephritid fruit flies are amongst the most significant horticultural pests globally and male chemical lures are important for monitoring and control. Zingerone has emerged as a unique male fruit fly lure that can attract dacine fruit flies that are weakly or non-responsive to methyl eugenol and cuelure. However, the key features of zingerone that mediate this attraction are unknown. As Jarvis's fruit fly, Bactrocera jarvisi (Tryon), is strongly attracted to zingerone, we evaluated the response of B. jarvisi to 37 zingerone analogues in a series of field trials to elucidate the functional groups involved in attraction. The most attractive analogues were alkoxy derivatives, with isopropoxy being the most attractive, followed by ethoxy and trifluoromethoxy analogues. All of the phenolic esters tested were also attractive with the response typically decreasing with increasing size of the ester. Results indicate that the carbonyl group, methoxy group, and phenol of zingerone are key sites for the attraction of B. jarvisi and identify some constraints on the range of structural modifications that can be made to zingerone without compromising attraction. These findings are important for future work in developing and optimising novel male chemical lures for fruit flies.

Antimicrobial and antioxidant activity and chemical characterisation of Erythrina stricta Roxb. (Fabaceae)

ETHNOPHARMACOLOGICAL RELEVANCE

The bark of Erythrina stricta Roxb. (Fabaceae) has been used in Indian indigenous systems as a remedy for rheumatism, stomach-ache, asthma, dysentery, contact dermatitis, eczema and skin infections. However, there have been limited phytochemical or biological studies on the bark of E. stricta and there are no studies that align with its traditional medicinal uses.

AIM OF THE STUDY

The aim of this study was to assess the antimicrobial and antioxidant activity of the stem bark of E. stricta to support its topical use in the treatment of contact dermatitis, eczema and skin infections and to isolate and identify any bioactive compounds.

MATERIALS AND METHODS

MTT microdilution and disc diffusion assays were used to determine the antimicrobial activities of n-hexane, dichloromethane, ethyl acetate, methanol and water extracts of the bark of E. stricta. Column and preparative thin layer chromatography were used for the purification of the dichloromethane extract. The structures of the compounds isolated were elucidated by extensive 1D and 2D NMR spectroscopic techniques and comparison with published data. The antioxidant activities of the extracts were determined by DPPH free radical scavenging and FRAP assays and the antioxidant activity of the pure compounds by dot-blot and DPPH staining methods.

RESULTS

The dichloromethane, ethyl acetate, and n-hexane extracts showed the most significant activity with MIC values of 7.8µg/mL, 125µg/mL, and 125µg/mL against a sensitive strain of Staphylococcus aureus. The dichloromethane and ethyl acetate extracts also showed significant activity against Candida albicans with MIC values of 125µg/mL and 1mg/mL respectively. GC-MS analysis of the n-hexane extract showed the presence of the antibacterial and antifungal compounds β-caryophyllene, caryophyllene oxide, α-selinene, β-selinene, selin-11-en-4-α-ol, α-copaene and δ-cadenine. Phytochemical studies of the dichloromethane extract led to the isolation of the novel compound erynone (1), together with six known compounds; wighteone (2), alpinum isoflavone (3), luteone (4), obovatin (5), erythrinassinate B (6) and isovanillin (7). Luteone (4) exhibited the most significant antibacterial activity with minimum inhibitory quantity (MIQ) values of 1.88µg, 1.88µg and 3.75µg, respectively, against sensitive (MSSA) and resistant strains (MRSA and MDRSA) of S. aureus using a TLC bioautography assay. Erynone (1) exhibited the greatest DPPH free radical scavenging activity.

CONCLUSIONS

Seven compounds, including a new chromanone, were isolated from the antimicrobial dichloromethane extract of the stem bark of E. stricta. Six of the seven compounds showed antibacterial and/or antioxidant activities. These findings provide support for the customary (traditional and contemporary) use of E. stricta bark for the treatment of skin and wound infections.

Raspberry Ketone Analogs: Vapour Pressure Measurements and Attractiveness to Queensland Fruit Fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae)

The Queensland fruit fly, Bactrocera tryoni (Froggatt) (Q-fly), is a major horticultural pest in Eastern Australia. Effective monitoring, male annihilation technique (MAT) and mass trapping (MT) are all important for control and require strong lures to attract flies to traps or toxicants. Lure strength is thought to be related in part to volatility, but little vapour pressure data are available for most Q-fly lures. Raspberry ketone (4-(4-hydroxyphenyl)-2-butanone) and analogs that had esters (acetyl, difluoroacetyl, trifluoroacetyl, formyl, propionyl) and ethers (methyl ether, trimethylsilyl ether) in replacement of the phenolic group, and in one case also had modification of the 2-butanone side chain, were measured for their vapour pressures by differential scanning calorimetry (DSC), and their attractiveness to Q-fly was assessed in small cage environmentally controlled laboratory bioassays. Maximum response of one category of compounds, containing both 2-butanone side chain and ester group was found to be higher than that of the other group of compounds, of which either of 2-butanone or ester functionality was modified. However, linear relationship between vapour pressure and maximum response was not significant. The results of this study indicate that, while volatility may be a factor in lure effectiveness, molecular structure is the dominating factor for the series of molecules investigated.

Ketimine reductase/CRYM catalyzes reductive alkylamination of α-keto acids, confirming its function as an imine reductase

Recently, crystalized mouse ketimine reductase/CRYM complexed with NADPH was found to have pyruvate bound in its active site. We demonstrate that the enzyme binds α-keto acids, such as pyruvate, in solution, and catalyzes the formation of N-alkyl-amino acids from alkylamines and α-keto acids (via reduction of imine intermediates), but at concentrations of these compounds not expected to be encountered in vivo. These findings confirm that, mechanistically, ketimine reductase/CRYM acts as a classical imine reductase and may explain the finding of bound pyruvate in the crystallized protein.

Antimicrobial activity of customary medicinal plants of the Yaegl Aboriginal community of northern New South Wales, Australia: a preliminary study

BACKGROUND

This study is a collaboration between Macquarie University researchers and the Yaegl Aboriginal Community of northern NSW, Australia to investigate the antimicrobial potential of plants used in the topical treatment of wounds, sores and skin infections. Based on previously documented medicinal applications, aqueous and aqueous ethanolic extracts of Alocasia brisbanensis, Canavalia rosea, Corymbia intermedia, Hibbertia scandens, Ipomoea brasiliensis, Lophostemon suaveolens and Syncarpia glomulifera and the aqueous extracts of Smilax australis and Smilax glyciphylla were tested against common wound pathogens, including antibiotic resistant bacterial strains.

METHODS

Plant material was prepared as aqueous extractions modelled on customary preparations and using 80% aqueous ethanol. Extracts were assayed against a selection of clinically relevant Gram positive (Streptococcus pyogenes and sensitive and resistant strains of Staphylococcus aureus) and Gram negative (Pseudomonas aeruginosa, Escherichia coli and Salmonella typhimurium) bacteria and a fungus (Candida albicans) using disc diffusion and MTT microdilution methods. Viability of treated microorganisms was determined by subculturing from microdilution assays.

RESULTS

The extracts of Corymbia intermedia, Lophostemon suaveolens and Syncarpia glomulifera had promising levels of antimicrobial activity (MIC 31-1,000 µg/mL) against both antibiotic sensitive and resistant Staphylococcus aureus as well as the fungus Candida albicans (clinical isolate).

CONCLUSION

Aqueous and 80% aqueous ethanolic extracts of Lophostemon suaveolens, Corymbia intermedia and Syncarpia glomulifera exhibited promising levels of antimicrobial activity against a range of both antibiotic sensitive and resistant strains of microorganisms. This is the first report of antimicrobial activities for C. intermedia and L. suaveolens and the leaves of S. glomulifera. This study demonstrates the value of customary knowledge in the identification of new sources of antimicrobial treatments.

An ethnobotanical study of medicinal plants of Chungtia village, Nagaland, India

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional medicinal plant knowledge is an integral and very important part of Indigenous cultures worldwide. For many communities there is a great urgency in recording this knowledge in written form. This is the first ethnobotanical report of medicinal plant knowledge of the Nagaland Ao tribe of Chungtia village and is an important step in the preservation of this culturally and medicinally significant knowledge.

AIM OF THE STUDY

The aim of the presented work was to perform an ethnobotanical study on plants of medicinal and other significance to the Chungtia villagers of Nagaland, North East India.

MATERIALS AND METHODS

Ethnobotanical data were collected from traditional practitioners and Elders of Chungtia village by means of open group discussions and semi-structured interviews of groups and individuals using questionnaires. The interviews were also recorded in an audio format in the local Mongsen language. The gathered ethnobotanical knowledge was compared with reported ethnobotanical usages worldwide and reported biological properties and phytochemical studies relevant to the Chungtia villagers׳ applications.

RESULTS

A total of 135 plant species of 69 families and 123 genera were recorded for medicinal and household maintenance applications. Those applications were grouped into 13 categories based on Chungtia villagers׳ classification system. The families most represented were Asteraceae, Euphorbiaceae and Solanaceae. The most reported uses were for gastrointestinal problems, followed by dermatological problems. The most commonly used plant parts were leaves, followed by fruits and stems and they were most commonly administered as a paste, decoction, infusion, juice or poultice, or taken orally with no preparation. There was strong agreement among the informants as to the usages of the plants (informant consensus factor 0.80-0.91). The use value of 6 for Cassia floribunda, Dolichos lablab, Hedyotis scandens, Phyllanthus urinaria and Rhus javanica indicated these are the most important species. Forty four of the 135 plants had a fidelity level of 100%.

CONCLUSION

This study has helped to document and preserve in written format important traditional plant knowledge of 135 plants of the Chungtia villagers, assisting them in the continued preservation of their cultural values.

Carborane-Containing Hydroxyamidine Scaffolds as Novel Inhibitors of Indoleamine 2,3-Dioxygenase 1 (IDO1)

Two new carborane-containing hydroxyamidines were prepared as potential inhibitors of the indoleamine 2,3-dioxygenase 1 (IDO1) enzyme. One compound (3) displayed low micromolar (1.90 μM) inhibition of IDO1, with the related compound (4) displaying >5-fold lower inhibitory activity, i.e. subtle differences in structure between the two carborane compounds led to dramatic changes in inhibitor binding. In silico docking experiments unravel a possible molecular mechanism that is consistent with the observed difference in IDO1 binding for 3 and 4 and also for the phenyl bioisosteres 1 and 2.

The first indoleamine-2,3-dioxygenase-1 (IDO1) inhibitors containing carborane

The first examples of indoleamine-2,3-dioxygenase-1 (IDO1) inhibitors containing a carborane cage are reported.

Separate mechanisms for age-related truncation and racemisation of peptide-bound serine

Some amino acids are particularly susceptible to degradation in long-lived proteins. Foremost among these are asparagine, aspartic acid and serine. In the case of serine residues, cleavage of the peptide bond on the N-terminal side, as well as racemisation, has been observed. To investigate the role of the hydroxyl group, and whether cleavage and racemisation are linked by a common mechanism, serine peptides with a free hydroxyl group were compared to analogous peptides where the serine hydroxyl group was methylated. Peptide bond cleavage adjacent to serine was increased when the hydroxyl group was present, and this was particularly noticeable when it was present as the hydroxide ion. Adjacent amino acid residues also had a pronounced affect on cleavage at basic pH, with the SerPro motif being especially susceptible to scission. Methylation of the serine hydroxyl group abolished truncation, as did insertion of a bulky amino acid on the N-terminal side of serine. By contrast, racemisation of serine occurred to a similar extent in both O-methylated and unmodified peptides. On the basis of these data, it appears that racemisation of Ser, and cleavage adjacent to serine, occur via separate mechanisms. Addition of water across the double bond of dehydroalanine was not detected, suggesting that this mechanism was unlikely to be responsible for conversion of L-serine to D-serine. Abstraction of the alpha proton may account for the majority of racemisation of serine in proteins.

Lysine metabolism in mammalian brain: an update on the importance of recent discoveries

The lysine catabolism pathway differs in adult mammalian brain from that in extracerebral tissues. The saccharopine pathway is the predominant lysine degradative pathway in extracerebral tissues, whereas the pipecolate pathway predominates in adult brain. The two pathways converge at the level of ∆(1)-piperideine-6-carboxylate (P6C), which is in equilibrium with its open-chain aldehyde form, namely, α-aminoadipate δ-semialdehyde (AAS). A unique feature of the pipecolate pathway is the formation of the cyclic ketimine intermediate ∆(1)-piperideine-2-carboxylate (P2C) and its reduced metabolite L-pipecolate. A cerebral ketimine reductase (KR) has recently been identified that catalyzes the reduction of P2C to L-pipecolate. The discovery that this KR, which is capable of reducing not only P2C but also other cyclic imines, is identical to a previously well-described thyroid hormone-binding protein [μ-crystallin (CRYM)], may hold the key to understanding the biological relevance of the pipecolate pathway and its importance in the brain. The finding that the KR activity of CRYM is strongly inhibited by the thyroid hormone 3,5,3'-triiodothyronine (T3) has far-reaching biomedical and clinical implications. The inter-relationship between tryptophan and lysine catabolic pathways is discussed in the context of shared degradative enzymes and also potential regulation by thyroid hormones. This review traces the discoveries of enzymes involved in lysine metabolism in mammalian brain. However, there still remain unanswered questions as regards the importance of the pipecolate pathway in normal or diseased brain, including the nature of the first step in the pathway and the relationship of the pipecolate pathway to the tryptophan degradation pathway.

Discovery and characterisation of hydrazines as inhibitors of the immune suppressive enzyme, indoleamine 2,3-dioxygenase 1 (IDO1)

Screening of a fragment library identified 2-hydrazinobenzothiazole as a potent inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1), an enzyme expressed by tumours that suppresses the immune system. Spectroscopic studies indicated that 2-hydrazinobenzothiazole interacted with the IDO1 haem and in silico docking predicted that the interaction was through hydrazine. Subsequent studies of hydrazine derivatives identified phenylhydrazine (IC50=0.25 ± 0.07 μM) to be 32-fold more potent than 2-hydrazinobenzothiazole (IC50=8.0 ± 2.3 μM) in inhibiting rhIDO1 and that it inhibited cellular IDO1 at concentrations that were noncytotoxic to cells. Here, phenylhydrazine is shown to inhibit IDO1 through binding to haem.

Lysine metabolism in mammalian brain: an update on the importance of recent discoveries

The lysine catabolism pathway differs in adult mammalian brain from that in extracerebral tissues. The saccharopine pathway is the predominant lysine degradative pathway in extracerebral tissues, whereas the pipecolate pathway predominates in adult brain. The two pathways converge at the level of ∆(1)-piperideine-6-carboxylate (P6C), which is in equilibrium with its open-chain aldehyde form, namely, α-aminoadipate δ-semialdehyde (AAS). A unique feature of the pipecolate pathway is the formation of the cyclic ketimine intermediate ∆(1)-piperideine-2-carboxylate (P2C) and its reduced metabolite L-pipecolate. A cerebral ketimine reductase (KR) has recently been identified that catalyzes the reduction of P2C to L-pipecolate. The discovery that this KR, which is capable of reducing not only P2C but also other cyclic imines, is identical to a previously well-described thyroid hormone-binding protein [μ-crystallin (CRYM)], may hold the key to understanding the biological relevance of the pipecolate pathway and its importance in the brain. The finding that the KR activity of CRYM is strongly inhibited by the thyroid hormone 3,5,3'-triiodothyronine (T3) has far-reaching biomedical and clinical implications. The inter-relationship between tryptophan and lysine catabolic pathways is discussed in the context of shared degradative enzymes and also potential regulation by thyroid hormones. This review traces the discoveries of enzymes involved in lysine metabolism in mammalian brain. However, there still remain unanswered questions as regards the importance of the pipecolate pathway in normal or diseased brain, including the nature of the first step in the pathway and the relationship of the pipecolate pathway to the tryptophan degradation pathway.

Imine reductases: a comparison of glutamate dehydrogenase to ketimine reductases in the brain

A key intermediate in the glutamate dehydrogenase (GDH)-catalyzed reaction is an imine. Mechanistically, therefore, GDH exhibits similarities to the ketimine reductases. In the current review, we briefly discuss (a) the metabolic importance of the GDH reaction in liver and brain, (b) the mechanistic similarities between GDH and the ketimine reductases, (c) the metabolic importance of the brain ketimine reductases, and (d) the neurochemical consequences of defective ketimine reductases. Our review contains many historical references to the early work on amino acid metabolism. This work tends to be overlooked nowadays, but is crucial for a contemporary understanding of the central importance of ketimines in nitrogen and intermediary metabolism. The ketimine reductases are important enzymes linking nitrogen flow among several key amino acids, yet have been little studied. The cerebral importance of the ketimine reductases is an area of biomedical research that deserves far more attention.

Natural Product-Inspired Pyranonaphthoquinone Inhibitors of Indoleamine 2,3-Dioxygenase-1 (IDO-1)

A series of pyranonaphthoquinone derivatives possessing structural features present in both natural products annulin B and exiguamine A have been shown to exhibit low micromolar inhibition of indoleamine 2,3-dioxygenase-1 (IDO-1). These inhibitors retain activity against the enzyme in a cellular context with an approximate one-log loss of dose potency against IDO-1 in cells. One particular analogue, triazole 8 shows good inhibition of IDO-1 along with little loss of cell viability at low drug concentrations. These results have extended the naphthoquinone series of novel IDO-1 inhibitors based on lead compounds from nature.

Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?

Since crystallins in the human lens do not turnover, they are susceptible to modification by reactive molecules over time. Methylation is a major post-translational lens modification, however the source of the methyl group is not known and the extent of modification across all crystallins has yet to be determined. Sites of methylation in human lens proteins were determined using HPLC/mass spectrometry following digestion with trypsin. The overall extent of protein methylation increased with age, and there was little difference in the extent of modification between soluble and insoluble crystallins. Several different cysteine and histidine residues in crystallins from adult lenses were found to be methylated with one cysteine (Cys 110 in γD crystallin) at a level approaching 70%, however, methylation of crystallins was not detected in fetal or newborn lenses. S-adenosylmethionine (SAM) was quantified at significant (10-50 μM) levels in lenses, and in model experiments SAM reacted readily with N-α-tBoc-cysteine and N-α-tBoc-histidine, as well as βA3-crystallin. The pattern of lens protein methylation seen in the human lens was consistent with non-enzymatic alkylation. The in vitro data shows that SAM can act directly to methylate lens proteins and SAM was present in significant concentrations in human lens. Thus, non-enzymatic methylation of crystallins by SAM offers a possible explanation for this major human lens modification.

Mammalian forebrain ketimine reductase identified as μ-crystallin; potential regulation by thyroid hormones

Ketimine reductase (E.C. 1.5.1.25) was purified to apparent homogeneity from lamb forebrain by means of a rapid multi-step chromatography protocol. The purified enzyme was identified by MS/MS (mass spectrometry) as μ-crystallin. The identity was confirmed by heterologously expressing human μ-crystallin in Escherichia coli and subsequent chromatographic purification of the protein. The purified human μ-crystallin was confirmed to have ketimine reductase activity with a maximum specific activity similar to that of native ovine ketimine reductase, and was found to catalyse a sequential reaction. The enzyme substrates are putative neuromodulator/transmitters. The thyroid hormone 3,5,3'-l-triiodothyronine (T3) was found to be a strong reversible competitive inhibitor, and may have a novel role in regulating their concentrations. μ-Crystallin is also involved in intracellular T3 storage and transport. This research is the first to demonstrate an enzyme function for μ-crystallin. This newly demonstrated enzymatic activity identifies a new role for thyroid hormones in regulating mammalian amino acid metabolism, and a possible reciprocal role of enzyme activity regulating bioavailability of intracellular T3.

Tryptophan-derived ultraviolet filter compounds covalently bound to lens proteins are photosensitizers of oxidative damage

The human eye is chronically exposed to light of wavelengths >300 nm. In the young human lens, light of wavelength 300-400 nm is predominantly absorbed by the free Trp derivatives kynurenine (Kyn), 3-hydroxykynurenine (3OHKyn), and 3-hydroxykynurenine-O-beta-D-glucoside (3OHKynG). These ultraviolet (UV) filter compounds are poor photosensitizers. With age, the levels of the free UV filters in the lens decreases and those of protein-bound UV filters increases. The photochemical behavior of these protein-bound UV filters and their role in UV damage are poorly elucidated and are examined here. UVA illumination of protein-bound UV filters generated peroxides (principally H2O2) in a metabolite-, photolysis-time-, and wavelength-dependent manner. Unmodified proteins, free Trp metabolites, and Trp metabolites that do not bind to lens proteins gave low peroxide yields. Protein-bound 3OHKyn (principally at Cys residues) yielded more peroxide than comparable Kyn and 3OHKynG adducts. Studies using D2O and sodium azide implicated 1O2 as a key intermediate. Illumination of the protein-bound adducts also yielded protein-bound Tyr oxidation products (DOPA, di-tyrosine) and protein cross-links via alternative mechanisms. These data indicate that the covalent modification of lens proteins by Kyn derivatives yields photosensitizers that may enhance oxidation in older lenses and contribute to age-related nuclear cataract.

Mouse and human indoleamine 2,3-dioxygenase display some distinct biochemical and structural properties

The hemoprotein indoleamine 2,3-dioxygenase (IDO) is the first and rate-limiting enzyme in the most significant pathway for mammalian tryptophan metabolism. It has received considerable attention in recent years, particularly due to its dual role in immunity and the pathogenesis of many diseases. Reported here are differences and similarities between biochemical behaviour and structural features of recombinant human IDO and recombinant mouse IDO. Significant differences were observed in the conversion of substrates and pH stability. Differences in inhibitor potency and thermal stability were also noted. Secondary structural features were broadly similar but variation between species was apparent, particularly in the alpha-helix portion of the enzymes. With mouse models substituting for human diseases, the differences between mouse and human IDO must be recognised before applying experimental findings from one system to the next.

Reversible Binding of Kynurenine to Lens Proteins: Potential Protection by Glutathione in Young Lenses

PURPOSE

Human ultraviolet light (UV) filters, such as kynurenine (Kyn), readily deaminate to reactive unsaturated ketones that covalently modify proteins in older human lenses. The aim of this study was to examine in vitro rates of formation and decomposition of the three major Kyn-amino acid adducts and possible consequences for the lens.

METHODS

The t-Boc-protected Kyn-His, Kyn-Lys, and Kyn-Cys adducts and Kyn-Cys were synthesized from the corresponding amino acids and Kyn. Calf lens proteins were modified with Kyn by incubation at pH 7. Stability and competition studies of the adducts were conducted under physiological conditions. Kyn-amino acids and their decomposition products were quantified using HPLC.

RESULTS

At physiological pH, Kyn-Cys adducts formed more rapidly than either Lys or His adducts, but they also decomposed readily. By contrast, His adducts were stable. Cysteine (Cys) residues in beta-crystallins were major sites of modification. The Kyn moiety, initially bound to Cys residues, was found to transfer to other amino acids. Glutathione promoted the breakdown of Kyn-Cys.

CONCLUSIONS

These data may help explain why proteins in young lenses are not modified by UV filters in situ. The initial phase of the modification of proteins in the human lens by UV filters may be a dynamic process. In lenses, Cys residues of crystallins modify preferentially, but these adducts also decompose to release deaminated Kyn. This can then potentially react with other amino acids. Glutathione, which is present in high concentrations in the lenses of young people, may play a vital role in keeping proteins free from modification by intercepting reactive deaminated kynurenines formed by the spontaneous breakdown of free UV filters, promoting the decomposition of Kyn-Cys residues, and sequestering the unsaturated ketones once they are released from modified proteins.

Identification of the new UV filter compound cysteine-l-3-hydroxykynurenineO-β-d-glucoside in human lenses

UV filters protect the human lens and retina from UV light-induced damage. Here, we report the identification of a new UV filter, cysteine-l-3-hydroxykynurenine O-beta-d-glucoside, which is present in older normal human lenses. Its structure was confirmed by independent synthesis. It is likely this novel UV filter is formed in the lens by nucleophilic attack of cysteine on the unsaturated ketone derived from deamination of 3-hydroxykynurenine O-beta-d-glucoside. Quantitation studies revealed considerable variation in normal lens levels that may be traced to the marked instability of the cysteine adduct. The novel UV filter was not detected in advanced nuclear cataract lenses.

Bactericidal and cyclooxygenase inhibitory diterpenes from Eremophila sturtii

Two serrulatane diterpenes, 3,8-dihydroxyserrulatic acid (1) and serrulatic acid (2), have been isolated from Eremophila sturtii through bioassay-guided fractionation. These compounds inhibit the inflammation pathway enzymes cyclooxygenase 1 and 2, and exhibit bactericidal activity against Staphylococcus aureus.

Inhibition of indoleamine 2,3 dioxygenase activity by H2O2

Indoleamine 2,3-dioxygenase is the first and rate limiting enzyme of the kynurenine pathway of tryptophan metabolism, has potent effects on cell proliferation and mediates antimicrobial, antitumorogenic, and immunosuppressive effects. As a potent cytotoxic effector, the mechanisms of indoleamine 2,3-dioxygenase inhibition deserve greater attention. The work presented here represents the first systematic study exploring the mechanisms by which low levels of hydrogen peroxide (10-100 microM) inhibit indoleamine 2,3-dioxygenase in vitro. Following brief peroxide exposure both enzyme inhibition and structural changes were observed. Loss of catalysis was accompanied by oxidation of several cysteine residues to sulfinic and sulfonic acids, observed by electrospray and MALDI mass spectrometry. Enzyme activity could in part be preserved in the presence of sulfhydryl containing compounds, particularly DTT and methionine. However, these structural alterations did not prevent substrate (l-tryptophan) binding. Some enzyme activity could be recovered in the presence of thioredoxin, indicating that the inhibitory effect of H(2)O(2) is at least partially reversible in vitro. We present evidence that cysteine oxidation represents one mechanism of indoleamine 2,3-dioxygenase inhibition.

A fluorescence-based assay for indoleamine 2,3-dioxygenase

A rapid and sensitive fluorescence-based bioassay for determination of indoleamine 2,3-dioxygenase (IDO) activity has been developed. This assay relies on the quantification of the amount of kynurenine produced in the assay medium by fluorescence and complements the standard absorbance and high-performance liquid chromatography (HPLC) assay methods. The fluorescence method has limits of detection similar to those of the standard assay methods. Measured activities of IDO, including in the presence of tryptophan-based inhibitors, were in statistical agreement with the absorbance and HPLC assay methods. The fluorescence-based assay was also suitable for assessment of IDO inhibition by compounds that are incompatible with the absorbance method.

Protein-bound kynurenine is a photosensitizer of oxidative damage

Human lens proteins become progressively modified by tryptophan-derived UV filter compounds in an age-dependent manner. One of these compounds, kynurenine, undergoes deamination at physiological pH, and the product binds covalently to nucleophilic residues in proteins via a Michael addition. Here we demonstrate that after covalent attachment of kynurenine, lens proteins become susceptible to photo-oxidation by wavelengths of light that penetrate the cornea. H2O2 and protein-bound peroxides were found to accumulate in a time-dependent manner after exposure to UV light (lambda > 305-385 nm), with shorter-wavelength light giving more peroxides. Peroxide formation was accompanied by increases in the levels of the protein-bound tyrosine oxidation products dityrosine and 3,4-dihydroxyphenylalanine, species known to be elevated in human cataract lens proteins. Experiments using D2O, which enhances the lifetime of singlet oxygen, and azide, a potent scavenger of this species, are consistent with oxidation being mediated by singlet oxygen. These findings provide a mechanistic explanation for UV light-mediated protein oxidation in cataract lenses, and also rationalize the occurrence of age-related cataract in the nuclear region of the lens, as modification of lens proteins by UV filters occurs primarily in this region.

Lenticular Levels of Amino Acids and Free UV Filters Differ Significantly between Normals and Cataract Patients

PURPOSE

To determine the levels of free UV filters and selected amino acids in cataract lenses compared with normal lenses.

METHODS

Indian cataract lenses (n=39) and normal lenses (n=6) were examined by HPLC to quantify levels of UV filter compounds, the UV filter precursor amino acid tryptophan (Trp), as well as tyrosine (Tyr) and uric acid.

RESULTS

The levels of the two major primate UV filters, 3-hydroxykynurenine glucoside (3OHKG) and 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid glucoside (AHBG), in cataract lenses were markedly decreased compared with levels in normal lenses. By contrast, the levels of Trp were greatly increased. Mean Trp concentrations were an order of magnitude higher than in normal lenses, with 86% of dark-colored cataract lens nuclei having Trp concentrations greater than the mean level in the normal lenses. The concentrations of Tyr were also higher in cataract lenses. The levels of Kyn, however, were unchanged, and the uric acid levels were substantially lower.

CONCLUSIONS

The levels of the free UV filter compounds 3OHKG and AHBG, and also of Trp, Tyr, and uric acid were different in cataract lenses compared to normal lenses. These data suggest that the metabolism of a large proportion of patients with cataract may be substantially different than in persons with normal lenses. Although the mechanism of such metabolic defects are unknown, the authors speculate that an amino acid transporter system may be upregulated in patients with cataract. Because kynurenine levels in cataract were not significantly different from those of normal lenses, there may be a defect in the lenticular UV filter pathway at one, or both, of the steps that convert kynurenine to 3OHKG.

Optimised expression and purification of recombinant human indoleamine 2,3-dioxygenase

The hemoprotein indoleamine 2,3-dioxygenase (IDO) is the first and rate-limiting enzyme in mammalian tryptophan metabolism. It has received considerable attention in recent years, particularly due to its role in the pathogenesis of many diseases. Here, we report attempts to improve soluble expression and purification of hexahistidyl-tagged recombinant human IDO from Escherichia coli (EC538, pREP4, and pQE9-IDO). Significant formation of inclusion bodies was noted at the growth temperature of 37 degrees C, with reduced formation at 30 degrees C. The addition of the natural biosynthetic precursor of protoporphrin IX, delta-aminolevulinic acid (ALA), coupled with optimisation of IPTG induction levels during expression at 30 degrees C and purification by nickel-agarose and size exclusion chromatography, resulted in protein with 1 mol of heme/mol of protein and a specific activity of 160 micromol of kynurenine/h/mg of protein (both identical to native human IDO). The protein was homogeneous in terms of electrophoretic analysis. Yields of soluble protein (3-5 mg/L of bacterial culture) and heme content are greater than previously reported.

UV filter instability: consequences for the human lens

Human lenses appear to become coloured with age primarily due to the covalent binding of UV filter compounds to lens proteins. These crystallin modifications result from the inherent instability of the kynurenine UV filters. Here we investigate this decomposition, the role this may have in the formation of other primate UV filters, and the interaction of the intermediates (alpha,beta-ketoalkenes) with lens components. The UV filters kynurenine, 3-hydroxykynurenine and 3-hydroxykynurenine glucoside were incubated at neutral pH in the presence or absence of NADH or NADPH. The three UV filters underwent spontaneous deamination, such that at pH 7 less than half of the starting materials (kynurenine (42%), 3-hydroxykynurenine glucoside (30%) and 3-hydroxykynurenine (21%)) remained after 7 days. In the presence of NAD(P)H, the double bond of the UV filter-derived deamination compounds, were reduced. Deamination of 3-hydroxykynurenine glucoside, followed by reduction with NAD(P)H, could thus account for the formation of the major lens UV filter 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid glucoside. beta-Benzoylacrylic acid, which possesses the same alpha,beta-ketoalkene sidechain as the deaminated kynurenine UV filters, underwent Michael addition with glutathione, was reduced (hydrogenated) by NAD(P)H, however was unreactive with ascorbate. Surprisingly, at pH 7 the UV filter-derived alpha,beta-ketoalkene intermediates, do not readily undergo intramolecular cyclization. This feature makes the double bond more available for reaction with protein nucleophilic residues and other lens components such as glutathione. On the basis of these data it is likely that glutathione and NAD(P)H, but not ascorbate, protect proteins in the lens from modification by UV filters.

Glutathione and NADH, but not ascorbate, protect lens proteins from modification by UV filters

Age-dependent human lens colouration and fluorescence may stem primarily from the covalent binding of UV filters to crystallins. The tendency of the kynurenine (Kyn) UV filters to deaminate at neutral pH, with the generation of reactive alpha,beta-ketoalkenes, underlies this phenomenon. In this study the authors examined the ability of small molecular weight antioxidants, which are known to be present in the lens, to inhibit this process. Crystallins were incubated with Kyn at pH 7 in the presence of glutathione (GSH), ascorbate or NADH. Ascorbate, even at high (15 m M) levels, was not found to significantly retard the time-dependent covalent binding of Kyn to the proteins. GSH, and to a lesser extent NADH, however, had a major impact in preventing this modification. The increase in protein UV absorbance and fluorescence was inhibited by GSH intercepting the reactive ketone intermediate, to form a GSH-Kyn adduct. NADH seemed to protect by both reduction of the reactive ketone intermediate and by competing with Kyn for presumably hydrophobic sites on the crystallins. This may indicate that the covalent attachment of aromatic Kyn molecules could be facilitated by initial hydrophobic interactions. Since GSH is present at far greater concentrations than NADH, these results show that in primate lenses, GSH is the key agent responsible for protecting the crystallins from covalent modification.

A new oleanane triterpenoid from Gordonia ceylanica

The chemical investigation of the hexane extract of the stem bark of Gordonia ceylanica afforded 3beta-acetoxy-11alpha(2',3'-epoxyferulyloxy)-olean-13(18)-ene as a new natural product and alpha-spinasterol for the first time from Gordonia.

Novel protein modification by kynurenine in human lenses

It is known that human lenses increase in color and fluorescence with age, but the molecular basis for this is not well understood. We demonstrate here that proteins isolated from human lenses contain significant levels of the UV filter kynurenine covalently bound to histidine and lysine residues. Identification was confirmed by synthesis of the kynurenine amino acid adducts and comparison of the chromatographic retention times and mass spectra of these authentic standards with those of corresponding adducts isolated from human lenses following acid hydrolysis. Using calf lens proteins as a model, covalent binding of kynurenine to lens proteins has been shown to proceed via side chain deamination in a manner analogous to that observed for the related UV filter, 3-hydroxykynurenine O-beta-D-glucoside. Levels of histidylkynurenine and lysylkynurenine were low in human lenses in subjects younger than 30, but thereafter increased in concentration with the age of the individual. Post-translational modification of lens proteins by tryptophan metabolites therefore appears to be responsible, at least in part, for the age-dependent increase in coloration and fluorescence of the human lens, and this process may also be important in other tissues in which up-regulation of tryptophan catabolism occurs.

Identification of a new human lens UV filter compound

A new UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-diglucoside, has been identified in human lenses. The structure suggests that it is a further metabolic product of the second most abundant UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-glucoside. Quantification studies on the new compound show that it decreases towards zero in both the nucleus and cortex as a function of age. The discovery of this novel disaccharide completes the identification of the major UV filter compounds present in the human lens.

Major changes in human ocular UV protection with age

PURPOSE

Age-dependent human lens coloration may be explained by the binding of UV filters to crystallins. It has been proposed that glutathione may compete for reaction with UV filter degradation products and therefore protect crystallins from modification. To understand this process, UV filters were quantified together with oxidized and reduced glutathione in human lenses of varying age.

METHODS

Lens tissues were homogenized in ethanol to extract the UV filters. Metabolites were quantified by HPLC and correlations between them in the nuclear and cortical regions of the lens were examined.

RESULTS

The concentrations of the UV filters 3-hydroxykynurenine, kynurenine, and 3-hydroxykynurenine glucoside decreased linearly with age, with slightly lower levels in the nucleus than the cortex. 4-(2-Amino-3-hydroxyphenyl)-4-oxobutanoic acid glucoside was found in higher levels in the nucleus than the cortex and decreased slowly in both regions with age. Glutathionyl-3-hydroxykynurenine glucoside was present in higher concentrations in the nucleus, barely detectable in young lenses, but increased significantly after age 50. Reduced glutathione levels were lower in the nucleus and decreased in both regions with age, yet oxidized glutathione increased in the nucleus but remained constant in the cortex.

CONCLUSIONS

Results are consistent with a predominantly nuclear origin for both 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid glucoside and glutathionyl-3-hydroxykynurenine glucoside. This is in accord with their proposed mechanism of formation, which involves an initial deamination of 3-hydroxykynurenine glucoside. This process is more pronounced in older lenses, possibly because of the barrier to diffusion. The barrier may also explain the increase in nuclear oxidized glutathione that is observed with age.

Antibacterial compounds from Carissa lanceolata R.Br

The dichloromethane extract of the wood of Carissa lanceolata R.Br. (Apocynaceae) afforded the eudesmanes carissone, dehydrocarissone and carindone. This is the first account of carissone being isolated from the wood of C. lanceolata, and of carindone being isolated from this Carissa species. Dehydrocarissone has not been isolated previously from any Carissa species. The antibacterial activity of these natural products were examined against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. All three compounds showed activity, with dehydrocarissone and carindone having a minimum inhibitory concentration less than 0.5 mg/ml against S. aureus and E. coli.

Two oleanane triterpenoids from gordonia ceylanica and their conversions to taraxarane triterpenoids

Chemical investigation of the hot hexane extract of the stem bark of Gordonia ceylanica afforded two new oleanane triterpenoids, 3beta-acetoxy-11alpha, 13beta-dihydroxyolean-12-one and 3beta,11alpha-diacetoxy-13beta-hydroxyolean-12-one (2) The attempted acid hydrolysis of these two compounds resulted the dehydration and subsequent methyl group migration to afford the taraxarane triterpenoids 3beta,11alpha-dihydroxytaraxer-14-en-12-one (4) and 3beta-hydroxy-11alpha-acetoxytaraxer-14-en-12-one (5), respectively. These taraxaranes have not been previously reported.

Expression and purification of recombinant human indoleamine 2, 3-dioxygenase

Indoleamine 2,3-dioxygenase, the first and rate-limiting enzyme in human tryptophan metabolism, has been implicated in the pathogenesis of many diseases. The human enzyme was expressed in Escherichia coli EC538 (pREP4) as a fusion protein to a hexahistidyl tag and purified to homogeneity in terms of electrophoretic and mass spectroscopic analysis, by a combination of phosphocellulose and nickel-agarose affinity chromatography. The yield of the fusion protein was 1.4 mg per liter of bacterial culture with an overall recovery of 56% from the crude extract. When the culture medium was supplemented with 7 microM hemin, the purified protein contained 0.8 mol of heme per mole of enzyme and exhibited an absorption spectrum consistent with the ferric form of hemoprotein. The pI value of the recombinant enzyme was 7.09 compared with 6.9 for the native enzyme. This was as expected from the addition of the hexahistidyl tag. Similar to the native enzyme, the recombinant enzyme required methylene blue and ascorbic acid for enzyme activity and oxidized not only l-tryptophan but also d-tryptophan and 5-hydroxy-l-tryptophan. The molecular activities for these substrates and their K(m) values were similar to those of the native enzyme, indicating that the addition of the hexahistidyl tag did not significantly affect catalytic activity. The recombinant protein can therefore be used to investigate properties of the native enzyme. This will aid the development of specific inhibitors of indoleamine 2,3-dioxygenase, which may be effective in halting disease progression.

Regulation of indoleamine 2,3-dioxygenase, the first enzyme in UV filter biosynthesis in the human lens. Relevance for senile nuclear cataract

3-Hydroxykynurenine (3OHKyn), the precursor of UV filters in human lens, is highly autooxidizable, generates H2O2, and binds to lens proteins, yielding a tanned/yellow product resembling senile nuclear cataractous materials. Thus, if 3OHkyn can be shown to be the causative agent in cataract, it may be possible to prevent the disease by lowering the level of 3OHKyn. To this end, indoleamine 2,3-dioxygenase, the first enzyme in UV filter synthesis, was studied using lens epithelial cell lines. The results indicated that the IDO expression is mediated by IFN-gamma. Immuno-suppressants which inhibit production of IFN-gamma may act as anti-cataract agents. Another way to lower the level of 3OHKyn is to use specific inhibitors for IDO. A recombinant human IDO was expressed to develop the inhibitors.

UV filter compounds in human lenses: the origin of 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-beta-D-glucoside

PURPOSE

To investigate UV filter synthesis in the human lens, in particular the biosynthetic origin of the second most abundant UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-beta-D-glucoside.

METHODS

Human lenses were analyzed by high-performance liquid chromatography (HPLC) after separate incubation with 3H-tryptophan (3H-Trp), beta-benzoylacrylic acid, D,L-alpha-amino-beta-benzoylpropionic acid, or D,L-3-hydroxykynurenine O-beta-D-glucoside. The effect of pH on the model compound D,L-alpha-amino-beta-benzoylpropionic acid and D,L-3-hydroxykynurenine O-beta-D-glucoside was also investigated.

RESULTS

UV filters were not detected in fetal lenses, despite a 5-month postnatal lens displaying measurable levels of UV filters. In adults no radiolabel was incorporated into 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-beta-D-glucoside after 3H-Trp incubations. Beta-benzoylacrylic acid was readily reduced in lenses. D,L-alpha-amino-beta-benzoylpropionic acid and D,L-3-hydroxykynurenine O-beta-D-glucoside slowly deaminated at physiological pH and were converted to beta-benzoylpropionic acid and 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-beta-D-glucoside, respectively, after lens incubations.

CONCLUSIONS

UV filter biosynthesis appears to be activated at or near birth. Compounds containing the kynurenine side chain slowly deaminate, and in the lens, the newly formed double bond is rapidly reduced. These findings suggest that 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-beta-D-glucoside is derived from L-3-hydroxykynurenine O-beta-D-glucoside through this deamination-reduction process. The slowness of the deamination presumably accounts for the absence of incorporation of radiolabel from 3H-Trp into 4(2-amino-3-hydroxyphenyl)4-oxobutanoic acid O-beta-D-glucoside.