Structural Studies of Nicotinoids: Cotinine versus Nicotine

An update on the formation in tobacco, toxicity and carcinogenicity of N'-nitrosonornicotine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco-specific nitrosamines N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are considered 'carcinogenic to humans' by the International Agency for Research on Cancer (IARC) and are believed to be important in the carcinogenic effects of both smokeless tobacco and combusted tobacco products. This short review focuses on the results of recent studies on the formation of NNN and NNK in tobacco, and their carcinogenicity and toxicity in laboratory animals. New mechanistic insights are presented regarding the role of dissimilatory nitrate reductases in certain microorganisms involved in the conversion of nitrate to nitrite that leads to the formation of NNN and NNK during curing and processing of tobacco. Carcinogenicity studies of the enantiomers of the major NNK metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and the enantiomers of NNN are reviewed. Recent toxicity studies of inhaled NNK and co-administration studies of NNK with formaldehyde, acetaldehyde, acrolein and CO2, all of which occur in high concentrations in cigarette smoke, are discussed.

From cultivation to cancer: formation of N-nitrosamines and other carcinogens in smokeless tobacco and their mutagenic implications

Tobacco use is a major cause of preventable morbidity and mortality globally. Tobacco products, including smokeless tobacco (ST), generally contain tobacco-specific N-nitrosamines (TSNAs), such as N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-butanone (NNK), which are potent carcinogens that cause mutations in critical genes in human DNA. This review covers the series of biochemical and chemical transformations, related to TSNAs, leading from tobacco cultivation to cancer initiation. A key aim of this review is to provide a greater understanding of TSNAs: their precursors, the microbial and chemical mechanisms that contribute to their formation in ST, their mutagenicity leading to cancer due to ST use, and potential means of lowering TSNA levels in tobacco products. TSNAs are not present in harvested tobacco but can form due to nitrosating agents reacting with tobacco alkaloids present in tobacco during certain types of curing. TSNAs can also form during or following ST production when certain microorganisms perform nitrate metabolism, with dissimilatory nitrate reductases converting nitrate to nitrite that is then released into tobacco and reacts chemically with tobacco alkaloids. When ST usage occurs, TSNAs are absorbed and metabolized to reactive compounds that form DNA adducts leading to mutations in critical target genes, including the RAS oncogenes and the p53 tumor suppressor gene. DNA repair mechanisms remove most adducts induced by carcinogens, thus preventing many but not all mutations. Lastly, because TSNAs and other agents cause cancer, previously documented strategies for lowering their levels in ST products are discussed, including using tobacco with lower nornicotine levels, pasteurization and other means of eliminating microorganisms, omitting fermentation and fire-curing, refrigerating ST products, and including nitrite scavenging chemicals as ST ingredients.

Nicotine and its metabolite cotinine target MD2 and inhibit TLR4 signaling

Nicotine is the principal alkaloid of tobacco often manufactured into cigarettes and belongs to a highly addictive class of drugs. Nicotine attenuates the neuroinflammation induced by microglial activation. However, the molecular target(s) underlying anti-inflammatory action of nicotine has not been fully understood. Considering the psychoactive substances morphine, cocaine, and methamphetamine act as xenobiotic-associated molecular patterns and can be specifically sensed by the innate immune receptor Toll-like receptor 4 (TLR4), here we sought to delineate whether nicotine and/or its metabolite cotinine may be recognized by the innate immune system via myeloid differentiation protein 2 (MD2), an accessory protein of TLR4 that is responsible for ligand recognition. MD2-intrinsic fluorescence titrations, surface plasmon resonance, and competitive displacement binding assays with curcumin (MD2 probe) demonstrated that both nicotine and cotinine targeted the lipopolysaccharide (LPS; TLR4 agonist) binding pocket of MD2 with similar affinities. The cellular thermal shift assay indicated that nicotine binding increased, while cotinine binding decreased, MD2 stability. These biophysical binding results were further supported by in silico simulations. In keeping with targeting MD2, both nicotine and cotinine inhibited LPS-induced production of nitric oxide and tumor necrosis factor alpha (TNF-α) and blocked microglial activation. Neither a pan nicotinic acetylcholine receptor (nAChR) inhibitor nor RNAi for nAChRs abolished the suppressive effect of nicotine- and cotinine-induced neuroinflammation. These data indicate that TLR4 inhibition by nicotine and cotinine at the concentrations tested in BV-2 cells is independent of classic neuronal nAChRs and validate that MD2 is a direct target of nicotine and cotinine in the inhibition of innate immunity.

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Nicotine and cotinine bind to MD2 in microglia cell

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Nicotine and cotinine inhibit the expression of pro-inflammatory factors

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The activity of nicotine and cotinine in microglia is independent of nAChRs

Observation of the Unbiased Conformers of Putative DNA-Scaffold Ribosugars

The constitution, configuration, and flexibility of the core sugars of DNA molecules alter their function in diverse roles. Conformational itineraries of the ribofuranosides (fs) have long been known to finely determine rates of processing, yet we also know that, strikingly, semifunctional DNAs containing pyranosides (ps) or other configurations can be created, suggesting sufficient but incompletely understood plasticity. The multiple conformers involved in such processes are necessarily influenced by context and environment: solvent, hosts, ligands. Notably, however, to date the unbiased, "naked" conformers have not been experimentally determined. Here, the inherent conformational biases of DNA scaffold deoxyribosides in unsolvated and solvated forms have now been defined using gas-phase microwave and solution-phase NMR spectroscopies coupled with computational analyses and exploitation of critical differences between natural-abundance isotopologues. Serial determination of precise, individual spectra for conformers of these 25 isotopologues in alpha (α-d) and beta (β-d); pyrano (p) and furano (f) methyl 2-deoxy-d-ribosides gave not only unprecedented atomic-level resolution structures of associated conformers but also their quantitative populations. Together these experiments revealed that typical ²E and ³E conformations of the sugar found in complex DNA structures are not inherently populated. Moreover, while both OH-5' and OH-3' are constrained by intramolecular hydrogen bonding in the unnatural αf scaffold, OH-3' is "born free" in the "naked" lowest lying energy conformer of natural βf. Consequently, upon solvation, unnatural αf is strikingly less perturbable (retaining ²T₁ conformation in vacuo and water) than natural βf. Unnatural αp and βp ribosides also display low conformational perturbability. These first experimental data on inherent, unbiased conformers therefore suggest that it is the background of conformational flexibility of βf that may have led to its emergence out of multiple possibilities as the sugar scaffold for "life's code" and suggest a mechanism by which the resulting freedom of OH-3' (and hence accessibility as a nucleophile) in βf may drive preferential processing and complex structure formation, such as replicative propagation of DNA from 5'-to-3'.

Competing Dispersive Interactions: From Small Energy Differences to Large Structural Effects in Methyl Jasmonate and Zingerone

Modern structural studies of biologically relevant molecules require an exhaustive interplay between experiment and theory. In this work, we present two examples where a poor choice of the theoretical method led to a misinterpretation of experimental results. We do that by performing a rotational spectroscopy study on two large and flexible biomolecules: methyl jasmonate and zingerone. The results show the enormous potential of rotational spectroscopy as a benchmark to evaluate the performance of theoretical methods.