Enzymatic aziridine synthesis from beta amino-alcohols--a new example of endogenous carcinogen formation

Recent Developments in the Biosynthesis of Aziridines

Only 0.016 % of all known natural products contain an aziridine ring, but this unique structural feature imparts high reactivity and cytotoxicity to the compounds in which it is found. Until 2021, no naturally occurring aziridine-forming enzymes had been identified. Since 2021, the biosynthetic enzymes for ~10 % of known aziridine containing natural products have been identified and characterized. This article describes the recent advances in our understanding of enzyme-catalyzed aziridine formation in the context of historical methods for aziridine formation through synthetic chemistry.

Insight into enzyme-catalyzed aziridine formation mechanism in ficellomycin biosynthesis

Ficellomycin is an aziridine-containing antibiotic, produced by Streptomyces ficellus. Based on the newly identified ficellomycin gene cluster and the assigned functions of its genes, a possible pathway for aziridine ring formation in ficellomycin was proposed, which is a complex process involving at least 3 enzymatic steps. To obtain support for the proposed mechanism, the targeted genes encoding sulfate adenylyltransferase, adenylsulfate kinase, and a putative sulfotransferase were respectively disrupted and the subsequent analysis of their fermentation products revealed that all the three genes were involved in aziridine formation. To further confirm the mechanism, the key gene encoding a putative sulfotransferase was over expressed in Escherichia coli Rosseta (DE3). Enzyme assays indicated that the expressed sulfotransferase could specifically transfer a sulfo group from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) onto the hydroxyl group of (R)-(-)-2-pyrrolidinemethanol. This introduces a good leaving group in the form of the sulfated hydroxyl moiety, which is then converted into an aziridine ring through an intramolecular nucleophilic attack by the adjacent secondary amine. The sulfation/intramolecular cyclization reaction sequence maybe a general strategy for aziridine biosynthesis in microorganisms. Discovery of this mechanism revealed an enzyme-catalyzed route for the synthesis of aziridine-containing reagents and provided an important insight into the functional diversity of sulfotransferases.

The Fate of Molecular Oxygen in Azinomycin Biosynthesis

The azinomycins are a family of aziridine-containing antitumor antibiotics and represent a treasure trove of biosynthetic reactions. The formation of the azabicyclo[3.1.0]hexane ring and functionalization of this ring system remain the least understood aspects of the pathway. This study reports the incorporation of ¹⁸O-labeled molecular oxygen in azinomycin biosynthesis including both oxygens of the diol that ultimately adorn the aziridino[1,2- a]pyrrolidine moiety. Likewise, two other sites of heavy atom incorporation are observed.

Priming of Azabicycle Biosynthesis in the Azinomycin Class of Antitumor Agents

The biosynthesis of the azabicyclic ring system of the azinomycin family of antitumor agents represents the "crown jewel" of the pathway and is a complex process involving at least 14 enzymatic steps. This study reports on the first biosynthetic step, the inroads, in the construction of the novel aziridino [1,2-a]pyrrolidine, azabicyclic core, allowing us to support a new mechanism for azabicycle formation.

Theoretical study concerning the reactivity of imine derivatives of polycyclic aromatic hydrocarbons

The opening reaction of N-protonated polycyclic aromatic hydrocarbon imines has been computed by means of ab initio, density functional, and semiempirical methods of calculation. Imines are predicted to be more stable than the corresponding O-protonated derivatives, epoxides and diol epoxides. On the other hand, the activated N-methanesulfonylbenzene imine presented more favorable DeltaE( not equal ) and DeltaE(r) for ring opening due to the effect of hydrogen-bond interactions. Anti and syn trans-diol benzene imines did not show a different behavior from the unsubstituted imine. According to these calculations, bay-region, fjord-region, and bay-region methyl-substituted compounds opened more easily among the imine derivatives, following the same reactivity pattern as the oxygen analogs. The exothermicity of the opening process correlated with the charge delocalization in the resulting carbocation.

Sulfotransferases in the bioactivation of xenobiotics

Conjugation of xenobiotics is often associated with detoxification. However, this traditional view is one-sided. In particular, numerous compounds are known that are metabolized to chemically reactive metabolites via sulfation (O-sulfonation). This can be rationalized by the fact that the sulfate group is electron-withdrawing and may be cleaved off heterolytically in appropriate molecules, thus leading to the formation of a strongly electrophilic cation. The heterologous expression of sulfotransferases in indicator cells of standard mutagenicity tests has substantially improved the accessibility of this activation pathway. The use of this technology is important, since many reactive sulfate conjugates only show strong toxicological effects if they are generated directly within the indicator cell, due to their insufficient penetration of cell membranes. Xenobiotic-metabolizing sulfotransferases are cytosolic enzymes, which form a superfamily (SULT). Eleven distinct human SULT forms are known, which strongly differ in their tissue distribution and their substrate specificity. Common functionally relevant genetic polymorphisms of the transcribed region are known for two of the forms, SULT1A1 and 1A2. Studies using recombinant test systems demonstrate that many promutagens are activated with high selectivity by an individual SULT form. Pronounced differences in promutagen activation were detected between the different human forms, including their allelic variants, and also between orthologous SULTs from different species. Therefore, SULTs may be involved in the individual genetic disposition, species differences, and organotropisms for toxicological effects of chemicals. Activation by SULTs differs from other activation pathway in its cyclic nature: reaction of a sulfuric acid ester with water usually regenerates the hydroxylated compound, which becomes available for a new cycle of activation. SULT-mediated reactivation may even occur if another initial reactive species, e.g. an epoxide, has reacted with water.

Bioactivation of benzylic and allylic alcohols via sulfo-conjugation

Although sulfo-conjugation, in general, has been regarded as a detoxification process in the xenobiotic metabolism, there is a substantial body of data supporting that the same reaction can also lead to activation of certain types of chemical carcinogens and mutagens. Examples include some aromatic amines and amides, alkenylbenzenes, methyl-substituted polyaromatic hydrocarbons, nitrotoluenes and nitrosamines. The N- or O-hydroxy derivatives of these compounds undergo sulfonation to form extremely reactive sulfuric acid esters that can play a role as ultimate carcinogenic/mutagenic metabolites. Previous studies from several laboratories have shown that hydroxymethyl polyarenes, such as hydroxymethylbenz[a]anthracenes, 6-hydroxymethylbenzo[a]pyrene, and 1-hydroxymethylpyrene, are activated to reactive benzylic sulfuric acid esters, preferentially by rat hepatic hydroxysteroid sulfotransferase. Some aromatic hydrocarbons bearing the secondary benzylic hydroxy functionality can also yield electrophilic sulfate esters in the presence of hepatic sulfotransferase activity. Thus, benzylic mono- and dihydroxy derivatives of cyclopenta[cd]pyrene form mutagenic and DNA binding species when incubated with rat liver cytosol and the sulfo-group donor, 3'-phosphoadenosine-5'-phosphosulfate. 1-Hydroxy-3-methylcholanthrene that also possesses the cyclopenta-fused ring system appears to be metabolically activated through sulfo-conjugation. Likewise, benzo[a]pyrene tetraol might be activated through sulfuric acid esterification at one of two benzylic hydroxyl groups. Methylene-bridged polyarenols represent another potential group of cyclic secondary benzylic alcohols that can be activated by sulfotransferases. Certain non-polycyclic aromatic type benzylic alcohols have also been proposed to undergo sulfotransferase-mediated activation. Besides benzylic sulfonation, sulfuric acid esterification of certain allylic alcohols can produce reactive species.

Mutagenicity of K-region oxides and imines of chrysene, benzo[c]phenanthrene and benzo[g]chrysene in Salmonella typhimurium

The K-region oxides and imines of chrysene, benzo[c]phenanthrene and benzo[g]chrysene were investigated for mutagenicity in Salmonella typhimurium TA98 and TA100, using two different exposure media. All six compounds were mutagenic under all four experimental conditions. The imines were 17-3800 times more potent than the corresponding oxides. Omission of KCl (125 mM) from the exposure medium resulted in enhanced mutagenic effects. The enhancement was stronger in strain TA98 (3.1-5.2-fold) than in strain TA100 (1.3-2.5-fold), suggesting an influence on the bacteria rather than on the chemicals.

Carcinogen activation by sulfate conjugate formation

The foregoing pages presented a substantial body of data that established that sulfotransferase conjugation can transform many xenobiotics into agents that can modify cellular macromolecules. However, activation by sulfation is rarely the only metabolic pathway that is open to these compounds; other pathways can become more important in response to a variety of factors. This metabolic switching can be produced by substrate concentration, cofactor availability, kinetic factors that dictate the velocity of the various possible conjugation reactions, and, in some cases, competition between Phase-I and Phase-II metabolism. Also, it is important to realize that demonstration of activation by sulfate ester formation in vitro does not necessarily mean that a similar activation process will occur in vivo. Experience also teaches that argument by analogy can be very misleading in the case of sulfate activation. Small structural differences can upset the delicate balance between sulfate activation and the various other competing pathways. Nevertheless, sulfation is an important mechanism by which a number of chemicals are transformed to their activated forms.

Reactive intermediates and their toxicological significance

Many chemicals that cause toxicity do so via metabolism to biologically reactive metabolites. However, the nature of the interaction between such reactive metabolites and various cellular components, and the mechanism(s) by which these interactions eventually lead to cell death are poorly understood. The relative importance of macromolecular alkylation (covalent binding), lipid peroxidation, alterations in thiol, calcium and energy homeostasis are discussed with reference to specific toxicants. It is concluded that the cytotoxic effects of reactive metabolites are a consequence of simultaneous and/or sequential alterations in several cellular processes. Further studies are required to determine the relationship between these alterations and cell death.

Mutagenicity of N-substituted phenanthrene 9,10-imines in Salmonella typhimurium and Chinese hamster V79 cells

We previously showed that some (nonsubstituted) aziridines derived from polycyclic aromatic hydrocarbons (arene imines) elicit various mutagenic and genotoxic effects in bacteria and mammalian cells and that these arene imines are active at much lower concentrations than the corresponding epoxide analogues. In the present study, N-substituted derivatives of phenanthrene 9,10-imine were investigated. All 10 derivatives studied showed direct mutagenicity in Salmonella typhimurium TA100. Some of the compounds additionally exhibited weak effects in the strains TA98 and TA1537. Most N-substituted derivatives were weaker mutagens than unsubstituted phenanthrene 9,10-imine but stronger mutagens than phenanthrene 9,10-oxide. Bulky substituents reduced the mutagenicity more than did small substituents. In addition, the derivatives with electron-withdrawing substituents (with the exception of N-chlorophenanthrene 9,10-imine) were weaker mutagens than those with electron-donating substituents. Phenanthrene 9,10-imine and five N-substituted derivatives were investigated to determine whether they induce gene mutations at the hgprt locus in V79 cells. Four compounds, including the parent aziridine, were positive in the V79 test. The other two compounds were negative. The mutagenic potencies in the V79 cell system did not correlate well with those obtained with the Salmonella system. Overall, the study shows that in addition to unsubstituted arene imines, N-substituted derivatives are mutagenic. This finding is of interest, as metabolic pathways leading from aromatic compounds to N-substituted arene imines are conceivable.

Species differences in the metabolism of pamatolol, a cardioselective beta--adrenoceptor antagonist

The metabolism of pamatolol was studied in man, dogs, rats and mice after oral administration of a single dose. The drug was well absorbed in the gastro-intestinal tract and excreted in the urine, mainly in unchanged form, within 24 hrs. Four urinary metabolites were identified by gas chromatographic-mass spectrometric techniques. The metabolic data, in man, dog and mouse was found to be similar, both qualitatively and quantitatively. One metabolism route, involving aliphatic hydroxylation and subsequent oxidation, was found, to a significant extent only in the rat. The species variation between the mouse and the rat with regard to long-term toxicity of pamatolol is discussed. Artefact formation during trace analysis was observed.

Genetic activity spectra of some antischistosomal compounds, with particular emphasis on thioxanthenones and benzothiopyranoindazoles

In this review we note that hycanthone (Etrenol) is mutagenic for bacteriophage, bacteria, yeast, Neurospora, Drosophila, and for mammalian tissue culture cells, and we point out other genetic activities of this thioxanthenone and of related compounds. One alarming genetic activity is the ability of hycanthone to cause transformation of tissue culture cells in vitro in a test designed to detect carcinogens, results that parallel the direct demonstration of carcinogenic activity of hycanthone in the mouse in vivo. These and other results are compatible with the somatic mutation theory of cancer induction. Factors likely to affect the quantitative genetic activity of hycanthone and its congeners are summarized. Attempts are made to weave the more critical experimental evidence into a molecular model that accounts for the genetic activities of this series of compounds. We conclude that hycanthone is a directly acting mutagen that intercalates into DNA and preferentially alkylates deoxyguanosine residues via formation of a strongly electrophilic molecular species, the carbonium ion. Finally, we show that genetic activity can be dissociated from schistosomicidal activity by appropriate modifications in the thioxanthenone molecule. Preliminary experiments on a newly synthesized piperazinyl N-oxide derivative demonstrate no detectable mutagenic activity; yet considerable schistosomicidal activity is retained.