Chemoselective synthesis of functional homocysteine residues in polypeptides and peptides.

A methodology was developed for efficient, chemoselective transformation of methionine residues into stable, functional homocysteine derivatives. Methionine residues can undergo highly chemoselective alkylation reactions at low pH to yield stable sulfonium ions, which could then be selectively demethylated to give stable alkyl homocysteine residues. This mild, two-step process is chemoselective, efficient, tolerates many functional groups, and provides a means for creation of new functional biopolymers, site-specific peptide tagging, and synthesis of biomimetic and structural analogs of peptides.

Genetic encoding of caged cysteine and caged homocysteine in bacterial and mammalian cells.

We report the genetic incorporation of caged cysteine and caged homocysteine into proteins in bacterial and mammalian cells. The genetic code of these cells was expanded with an engineered pyrrolysine tRNA/tRNA synthetase pair that accepts both light-activatable amino acids as substrates. Incorporation was validated by reporter assays, western blots, and mass spectrometry, and differences in incorporation efficiency were explained by molecular modeling of synthetase-amino acid interactions. As a proof-of-principle application, the genetic replacement of an active-site cysteine residue with a caged cysteine residue in Renilla luciferase led to a complete loss of enzyme activity; however, upon brief exposure to UV light, a >150-fold increase in enzymatic activity was observed, thus showcasing the applicability of the caged cysteine in live human cells. A simultaneously conducted genetic replacement with homocysteine yielded an enzyme with greatly reduced activity, thereby demonstrating the precise probing of a protein active site. These discoveries provide a new tool for the optochemical control of protein function in mammalian cells and expand the set of genetically encoded unnatural amino acids.

Novel thiol- and thioether-containing amino acids: cystathionine and homocysteine families.

Natural L-homocysteine and L,L-cystathionine, along with a series of unnatural analogues, have been prepared from L-aspartic and L-glutamic acid. Manipulation of the protected derivatives provided ω-iodoamino acids, which were used in thioalkylation reactions of sulfur nucleophiles, such as the ester of L-cysteine and potassium thioacetate.

[L-[methyl-(11C)]-methionine of high enantiomeric purity production via on-line 11C-methylation of L-homocysteine thiolactone hydrochloride].

L-[methyl-(11C)]-methionine ([11C]MET), labelled with carbon-11 (T1/2 = 20 min), is the most commonly used amino acid radiotracer for PET diagnostics of brain tumors. The production of [11C]MET via on-line 11C-methylation of L-homocysteine thiolactone hydrochloride (lactone) on C18 solid-phase extraction cartridge creates a problem of insufficient enantiomeric purity (content of L-isomer) of the product. The results of a systematic study of the influence of reaction parameters (lactone/base and the EtOH/H20 ratios, time of 11C-methylation) on the content of L-isomer in the preparation are presented. The developed method of on-line [11C]MET synthesis allows to obtain a product with a sufficiently high radiochemical yield (75 +/- 3%, n = 100, based on [11C]CH3I) and reliably high content of L-isomer (93.7 +/- 0.5%) satisfying the requirements of clinical applications. [11C] MET synthesis was performed on a fully automated module designed by the Institute of Human Brain (IHB RAS).

Radiosynthesis and biological evaluation of L- and D-S-(3-[18F]fluoropropyl)homocysteine for tumor imaging using positron emission tomography.

Interest in radiolabeled amino acids for metabolic imaging of cancer and limitations with [(11)C]methionine has prompted the development of a new (18)F-labeled methionine derivative S-(3-[(18)F]fluoropropyl)homocysteine ([(18)F]FPHCys). The L and D enantiomers of [(18)F]FPHCys were prepared from their respective protected S-(3-tosyloxypropyl)homocysteine precursors 1 by [(18)F]fluoride substitution using K(2.2.2) and potassium oxalate, followed by acid hydrolysis on a Tracerlab FX(FN) synthesis module. [(18)F]-L-FPHCys and [(18)F]-D-FPHCys were isolated in 20 ± 5% radiochemical yield and >98% radiochemical and enantiomeric purity in 65 min. Competitive uptake studies in A375 and HT29 tumor cells suggest that L- and D-[(18)F]FPHCys are taken up by the L-transporter system. [(18)F]-L-FPHCys and [(18)F]-D-FPHCys displayed good stability In Vivo without incorporation into protein at least 2 h postinjection. Biodistribution studies demonstrate good uptake in A375 tumor-bearing rodents with tumor to blood ratios of 3.5 and 5.0 for [(18)F]-L-FPHCys and [(18)F]-D-FPHCys, respectively, at 2 h postinjection.

Prebiotic synthesis of methionine and other sulfur-containing organic compounds on the primitive Earth: a contemporary reassessment based on an unpublished 1958 Stanley Miller experiment.

Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH(4)), hydrogen sulfide (H(2)S), ammonia (NH(3)), and carbon dioxide (CO(2)). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H(2)S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery.

Constrained (l-)-S-adenosyl-l-homocysteine (SAH) analogues as DNA methyltransferase inhibitors.

Potent SAH analogues with constrained homocysteine units have been designed and synthesized as inhibitors of human DNMT enzymes. The five membered (2S,4S)-4-mercaptopyrrolidine-2-carboxylic acid, in 1a, was a good replacement for homocysteine, while the corresponding six-member counterpart was less active. Further optimization of 1a, changed the selectivity profile of these inhibitors. A Chloro substituent at the 2-position of 1a, compound 1d, retained potency against DNMT1, while N(6) alkylation, compound 7a, conserved DNMT3b2 activity. The concomitant substitutions of 1a at both 2- and N(6) positions reduced activity against both enzymes.

A simple, convenient method to synthesize cobalamins: synthesis of homocysteinylcobalamin, N-acetylcysteinylcobalamin, 2-N-acetylamino-2-carbomethoxyethanethiolatocobalamin, sulfitocobalamin and nitrocobalamin.

Glutathionylcobalamin, nitrocobalamin and sulfitocobalamin are important cobalamin metabolites isolable from human tissues. Herein we demonstrate that a procedure used to synthesize and isolate gamma-glutamylcysteinylcobalamin and glutathionylcobalamin in aqueous solution in high yield and purity can be used to synthesize other novel, biologically relevant thiolatocobalamins, including d,l-homocysteinylcobalamin, N-acetyl-l-cysteinylcobalamin (Na(+) salt) and 2-N-acetylamino-2-carbomethoxy-l-ethanethiolatocobalamin, as well as other non-alkylcobalamins, such as sulfitocobalamin (Na(+) salt) and nitrocobalamin. This uncomplicated, general procedure will assist researchers in identifying unknown cobalamin metabolites isolated from biological samples, and researchers interested in studying the uptake and intracellular cobalamin processing mechanisms utilizing non-alkylcobalamin derivatives that are not yet commercially available. The X-ray structure and XAS spectrum of N-acetyl-l-cysteinylcobalamin are also presented.

Synthesis of LuxS inhibitors targeting bacterial cell-cell communication.

[reaction: see text] Quorum sensing is a process by which bacteria sense cell density. This cell-cell communication process is mediated by autoinducers. A cross-species messenger, autoinducer-2 (AI-2) is produced from S-ribosyl-L-homocysteine by the LuxS enzyme. A proposed mechanism for LuxS is an aldose-ketose isomerization of S-ribosylhomocysteine followed by a beta-elimination. We report here the synthesis of two substrate analogues, S-anhydroribosyl-L-homocysteine and S-homoribosyl-L-cysteine, which prevent the initial and final step of the mechanism, respectively.

Quantitative assay of plasma homocysteine thiolactone by gas chromatography/mass spectrometry.

Enzymatic cyclization of homocysteine forms a reactive thiolactone that may play an important role in its cardiovascular toxicity, but reliable quantitation of the free thiolactone metabolite in physiological fluids has not been reported. We have therefore used a highly selective gas chromatography/mass spectrometry (GC/MS) technique combined with the sensitivity of negative chemical ionization (NCI) to develop a quantitative method for the detection of homocysteine thiolactone (HcyTL) in plasma. To improve accuracy the deuterated isomer d(4)-HcyTL was synthesized and added to plasma as internal standard. The plasma was then treated with silica solid-phase extraction and derivatized with heptafluorobutyric anhydride. The derivative was analyzed by GC/MS in NCI mode with methane as the reagent gas and quantified by analyzing for the HcyTL ion [M(-)[bond]HF] and its d(4)-HcyTL counterpart in single-ion monitoring mode. The calibration curve showed a dynamic linear range up to 40 nmol/L. Within-day precision (n = 20, nominal concentration 5.2 nmol/L) was 0.96% and between-day precision was 3.9%, with a detection limit of 1.7 nmol/L and quantification limit of 5.2 nmol/L. Two human plasma samples had HcyTL concentrations of 18 and 25 nmol/L. This facile method for quantitation of homocysteine thiolactone opens the way for more detailed clinical studies of its potential role in homocysteine-induced arteriosclerosis and vaso-occlusive disease.

S-homoadenosyl-L-cysteine and S-homoadenosyl-L-homocysteine. Synthesis and binding studies of hon-hydrolyzed substrate analogues with S-adenosyl-L-homocysteine hydrolase.

Treatment of homoadenosine [9-(5-deoxy-beta-D-ribo-hexofuranosyl)adenine] with thionyl chloride and pyridine in acetonitrile gave 6'-chloro-6'-deoxyhomoadenosine, which underwent nucleophilic displacement with L-cysteine or L-homocysteine to give homologated analogues of S-adenosyl-L-homocysteine. Each amino acid in aqueous sodium hydroxide at 60 degrees C gave excellent conversion from the chloronucleoside, and adsorption on Amberlite XAD-4 resin provided more convenient isolation than prior methods. Weak binding of these non-hydrolyzed analogues to S-adenosyl-L-homocysteine hydrolase was observed.

Synthesis of optically active homocysteine from methionine and its use in preparing four stereoisomers of cystathionine.

In order to synthesize four stereoisomers of cystathionine (CYT), D- and L-homocysteines (D- and L-Hcy) were synthesized from methionine (Met) by a facile procedure. L-Met was reacted with dichloroacetic acid in concentrated hydrochloric acid under reflux to give (4S)-1,3-thiazane-2,4-dicarboxylic acid hydrochloride [(4S)-TDC.HCl]. L-Hcy was obtained by treatment of (4S)-TDC.HCl with hydroxylamine. D-Hcy was also synthesized from D-Met via (4R)-TDC.HCl intermediate. The obtained D- and L-Hcy were condensed with (R)- and (S)-2-amino-3-chloropropanoic acid hydrochlorides under alkaline conditions to give four stereoisomers of CYT.

Bioactivation mechanism of cytotoxic homocysteine S-conjugates.

S-(1,2-Dichlorovinyl)-L-homocysteine is a much more potent nephrotoxin than the corresponding cysteine S-conjugate S-(1,2-dichlorovinyl)-L-cysteine (A. A. Elfarra, L. H. Lash, and M. W. Anders (1986) Proc. Natl. Acad. Sci. USA 83, 2667-2671). The objective of the present experiments was to test the hypothesis that the increased toxicity of homocysteine S-conjugates may be associated with the formation of the reactive metabolite 2-oxo-3-butenoic acid, which may arise via a nonenzymatic retro-Michael elimination reaction from the 2-oxo acid metabolites of homocysteine S-conjugates. S-(2-Benzothiazolyl)-L-homocysteine, which was a substrate for purified bovine kidney cysteine conjugate beta-lyase (glutamine transaminase K) and whose metabolism was dependent on the presence of a 2-oxo acid, was cytotoxic in isolated rat kidney cells and was toxic to rat renal mitochondria, whereas the cysteine S-conjugate S-(2-benzothiazolyl)-L-cysteine had little effect. L-Methionine sulfoximine, L-canavanine, and the Michael acceptor methyl vinyl ketone were cytotoxic. The 2-hydroxy acid analogs of S-(1,2-dichlorovinyl)-L-homocysteine and 2-oxo-3-butenoic acid, S-(1,2-dichlorovinyl)-2-hydroxy-4-mercaptobutanoic acid and 2-hydroxy-3-butenoic acid, respectively, which are expected to be metabolized by rat renal L-2-hydroxy (L-amino) acid oxidase to yield 2-oxo-3-butenoic acid, were also cytotoxic. To obtain evidence for the formation of 2-oxo-3-butenoic acid as a product of the metabolism of L-homocysteine S-conjugates and analogs, trapping experiments were conducted. S-(2-Benzothiazolyl)-L-homocysteine, S-(1,2-dichlorovinyl)-L-homocysteine, L-methionine sulfoximine, and L-canavanine were converted by snake venom L-amino acid oxidase to 2-oxo-3-butenoic acid, which was trapped by the nucleophile methanethiol to yield 4-methylthio-2-oxobutanoic acid; the trapped product was derivatized with 2,4-dinitrophenylhydrazine and was identified by its electronic absorption spectrum and by high-performance liquid chromatography. Similar trapping experiments conducted with kidney homogenates and purified beta-lyase were not successful. The data indicate that the bioactivation of homocysteine S-conjugates and analogs involves the enzymatic formation of the corresponding 2-oxo acids followed by a nonenzymatic retro-Michael elimination reaction to yield the Michael acceptor 2-oxo-3-butenoic acid, which may contribute to the observed cytotoxicity of homocysteine S-conjugates.

Synthesis and radiobiological applications of [35S]L-homocysteine thiolactone.

[35S]L-Homocysteine thiolactone ([35S]L-HCTL) was synthesized and its biodistribution evaluated as a potential brain radioprotective agent and as a tissue hypoxia marker. Drug uptake in mouse brain exceeded that in s.c. tumor 3 h post injection only. Multiple indicator dilution experiments in the rabbit heart indicate that membrane permeability of [35S]L-HCTL does not limit its usefulness as a hypoxia marker. In addition, a positive correlation was observed between regional coronary blood flow and myocardial content of [35S]adenosylhomocysteine formed from [35S]homocysteine and adenosine.

Chemopreventive effect of N-homocysteine thiolactonyl retinamido cobalamin on carcinogenesis by ethyl carbamate in mice.

Because of abnormalities of metabolism of homocysteine thiolactone and methionine in malignant cells, and because of the chemopreventive activity of N-homocysteine thiolactonyl retinamide against chemical carcinogenesis by ethyl carbamate in mice, the cobalamin derivative of this retinamide was prepared and tested for chemopreventive activity. The substance, N-homocysteine thiolactonyl retinamido cobalamin, was found to have a different UV-visible absorption spectrum from that of 5'-deoxyadenosyl cobalamin or N-homocysteine thiolactonyl retinamide. Spectral analysis suggests a ratio of 2 mol of retinamide/mol of cobalamin within the molecule. To demonstrate chemopreventive activity, ethyl carbamate was given in a dose of 2 mg/animal to A/J mice (15-18 g) weekly over a period of 10 weeks to induce pulmonary tumors. A total dose of N-homocysteine thiolactonyl retinamido cobalamin of 60 mg/kg, given for a total of 16 weeks, decreased by one fourth (P less than 0.05) the number of pulmonary tumors induced by ethyl carbamate. An equimolar dose of 5'-deoxyadenosyl cobalamin (40 mg/kg) increased the number of tumors by one third (P less than 0.001), and an equimolar dose of N-homocysteine thiolactonyl retinamide (20 mg/kg) had no effect on the number of pulmonary tumors. No mortality was observed in the experiment. When the ethyl carbamate was given in a single dose of 20 mg/animal, all three substances produced significant mortality in doses of 0.75-30 mg/kg. In the survivors of this experiment, doses of 0.75-30 mg/kg of N-homocysteine thiolactonyl retinamido cobalamin decreased the number of pulmonary tumors induced by ethyl carbamate to 52-82% of controls (P less than 0.01). The results show that N-homocysteine thiolactonyl retinamido cobalamin has chemopreventive activity against chemical carcinogenesis by ethyl carbamate in mice.

Isozyme-specific enzyme inhibitors. 13. S-[5'(R)-[(N-triphosphoamino)methyl]adenosyl]-L-homocysteine, a potent inhibitor of rat methionine adenosyltransferases.

A synthesis is described of the title compound and its 5'S epimer, which are two-substrate adducts of adenosine 5'-triphosphate (ATP) and L-methionine (Met) in which the C(5')H2OP system in ATP is replaced by CH(R)CH2NHP [R = L-S(CH2)2CH(NH2)CO2H]. The 5'R epimer was a potent nonselective competitive inhibitor [averaged Ki = 0.32 microM; KM(ATP)/Ki = 440] vs. ATP of the rat M-2 (normal tissue) and M-T (Novikoff ascitic hepatoma) variants of methionine adenosyltransferase. It produced simple noncompetitive inhibition (averaged Ki = 2.7 microM) vs. Met with both variants. The 5'S epimer inhibited M-T competitively vs. ATP, but was 74-fold less effective than the 5'R epimer. Replacement of the homocysteine moiety in the 5'R epimer by hydrogen markedly reduced inhibitory potency, as indicated by Ki values of 14 microM for competitive inhibition vs. ATP and 580 microM for noncompetitive inhibition vs. Met with M-2. The data suggest that the 5'R epimer can interact simultaneously with two enzymic sites. Information on the kinetic mechanism of a human counterpart of M-2 and inhibitor properties of a previously studied Met-ATP adduct are consistent with the view that the two sites might resemble those that interact with the initial products of the reaction, S-adenosylmethionine and triphosphate.

Potential inhibitors of S-adenosylmethionine-dependent methyltransferases. 10. Base- and amino acid modified analogues of S-aristeromycinyl-L-homocysteine.

A series of base- and amino acid modified analogues of S-aristeromycinyl-L-homocysteine, a carbocyclic nucleoside, were synthesized and evaluated as inhibitors of S-adenosyl-L-methionine-dependent methyltransferases, including catechol O-methyltransferase, phenylethanolamine N-methyltransferase, and histamine N-methyltransferase. The base-modified analogues (8-azaadenine, 3-deazaadenine, and N6-methyladenine) were prepared by reaction of the corresponding carbocyclic 5'-chloro-5'-deoxynucleosides with the anion of homocysteine generated in situ either from L-homocystine or S-benzyl-L-homocysteine in Na/liquid NH3 or with DL-homocysteine thiolactone in alkaline solution. S-Aristeromycinyl-D-homocysteine was prepared with use of D-homocystine in the Na/liquid NH3 reaction. The sulfoxide and sulfone analogues were prepared by oxidation of S-aristeromycinyl-L-homocysteine. The various base- and amino acid modified analogues of S-aristeromycinyl-L-homocysteine were inactive as inhibitors of catechol O-methyltransferase. In contrast, the 3-deaza analogue was a good inhibitor (Ki = 20.5 +/- 1 microM) of phenylethanolamine N-methyltransferase whereas S-aristeromycinyl-D-homocysteine was an excellent inhibitor (Ki = 10.4 +/- 2.4 microM) of histamine N-methyltransferase. On the basis of these results, it would appear that the structural requirements for the binding S-aristeromycinyl-L-homocysteine are similar to those for binding S-adenosyl-L-homocysteine. Therefore, these carbocyclic analogues have the potential of being better inhibitors in vivo, because they should be more stable to metabolism than the ribosyl analogues.

Synthesis of bridged catechol-homocysteine derivatives as potential inhibitors of catechol O-methyltransferase.

Catechol derivatives, covalently joined to homocysteine by sulfide or sulfonium linkages, were synthesized as potential catechol O-methyltransferase multisubstrate inhibitors which might bridge the enzymatic binding sites for the catechol substrate and the amino acid portion of the methyl donor S-adenosylmethionine. These compounds were found to be less effective inhibitors than the product inhibitor S-adenosylhomocysteine.

Potential inhibitors of S-adenosylmethionine-dependent methyltransferases. 3. Modifications of the sugar portion of S-adenosylhomocysteine.

Structural analogs of S-ADENOSYL-L-HONOCYSTEINE (L-SAG), WITH MODIFICATION IN THE RIBCOSE PORTION OF THE MOLECULE, HAVE BEEN SYNTHESIZED AND THEIR ABILITIES TO INHIBIT CATECHOL O-METHYLTRANSFERECE(COMT), phenylethanolamine N-methltransferase (PNMT) histamine N-methyltransferase (HMT),and hydroxyindole o-methytransferase (HIOMT) have been investigated. From these studies it was concluded that, in general, the 2'-hydroxyl and 3'-hydroxyl groups of the ribcose moiety of SAH play crucial roles in the binding of this molecule to most methyltransferases. However several interesting exceptions to this strict structural specificity have been observed. While S-3'-DEOXY-ADENOSYL-L-HOMOCYSTEINE PRODUCED NO INHIBITION OF HMT and HIOMT, it produced strong inhibition of the transmethylation catalyzed by PNMT and COMT. Likewise, S-2'-DEOXYADENOSYL-L-HOMOCYSTEINE AND S-5'-(9-(arabinofuranosyl)adenyl)-l-homocysteine had little or no effect of COMT, HMT, and HIOMT but were potent inhibtors of PNMT. The significance of these data relative to the nature of the SAH binding sites and the potential inhibitors of PNMT. The significance of these data relative to the nature of the SAH binding sites and the potential for in vivo differential inhibition of methyltransferases will be discussed.