5-Amino-1MQ: Research Chemistry and Literature

Origin and identification

The enzyme came first. Aksoy, Szumlanski and Weinshilboum cloned the human liver complementary DNA in 1994 and characterised the recombinant protein, which established NNMT as a discrete cytosolic methyltransferase rather than an activity attributed to something else. Structural work followed much later. Peng and co-workers solved a ternary complex of the human enzyme with S-adenosyl-L-homocysteine and nicotinamide bound together in 2011, resolving the residues that line the acceptor site and giving synthetic chemists something concrete to design against.

The quinolinium series grew out of that structure. Neelakantan and colleagues screened N-methylated quinolinium, isoquinolinium, pyridinium and benzimidazolium analogues against the recombinant enzyme and published the resulting structure-activity relationship in 2017. Potencies spanned more than three orders of magnitude across the set, and the quinolinium core proved the productive one. Substitution at the 5-position with a primary amine combined useful inhibition with membrane permeability, and the compound, written as 5MQ through much of the literature, became the reference tool for the series in follow-up work the year after.

Preclinical research context

Most published characterisation is enzymology. Assays use recombinant human NNMT with either tritiated S-adenosyl-L-methionine and radiochemical detection, or direct quantification of the two products, 1-methylnicotinamide and S-adenosyl-L-homocysteine, by liquid chromatography with tandem mass spectrometry. Half-maximal inhibitory values are determined against a graded series of the nicotinamide substrate, which is what establishes the competitive mode. Counter-screens against other S-adenosyl-L-methionine dependent methyltransferases are routine in this field and appear in the primary papers.

In cell culture the recurring systems are murine 3T3-L1 adipocytes and primary adipocyte preparations. What gets measured is intracellular metabolite pools rather than anything phenotypic, principally 1-methylnicotinamide, oxidised nicotinamide adenine dinucleotide and S-adenosyl-L-methionine. The reason is biochemical. Nicotinamide serves as a methyl acceptor for this enzyme and as the precursor for NAD salvage, so NNMT sits at a branch point between methyl group disposal and NAD regeneration, and inhibiting it shifts flux between the two routes. Human tumour lines, HeLa among them, have carried proliferation studies with the same compound.

Rodent work in this area traces to Kraus and colleagues, who reported in Nature in 2014 that antisense knockdown of the enzyme in adipose tissue and liver of diet-induced obese mice altered S-adenosylmethionine and NAD availability in those tissues. The small-molecule literature adopted the same model systems. Reported readouts are tissue metabolite pools, transcript abundance in liver and adipose tissue, and plasma chemistry panels, presented as research observations in defined animal models.

Analytical characterisation

Purity is measured by reverse-phase HPLC with ultraviolet detection. The aminoquinolinium chromophore absorbs strongly in the near ultraviolet, so 254 and 275 nanometres both serve. Retention takes some thought. A permanently charged cation is poorly held by plain C18, so methods either add an ion-pairing reagent or move to a mixed-mode or hydrophilic interaction column. Area percent across the gradient gives the purity figure, and the counterion is quantified separately by ion chromatography, since the mass of a salt is not the mass of the cation inside it.

Identity rests on two independent methods. Electrospray mass spectrometry in positive mode returns the intact cation near m/z 159 with no protonation step, because the analyte already carries its charge, and that behaviour is itself diagnostic. Proton and carbon NMR then fixes the substitution pattern. The N-methyl singlet sits far downfield of an ordinary methyl resonance, near 4.5 parts per million, and the aromatic couplings separate the 5-amino isomer from the 4- and 8-substituted analogues that share its exact mass. Mass spectrometry alone cannot make that call, which is why NMR belongs on the record for a small molecule where it would be redundant for a peptide. Certificates of Analysis are issued by Janoshik Analytical and cover identity and chromatographic purity per lot, alongside water content and residual solvent.

Handling and storage

The material is a solid salt and belongs in a sealed container. Cool and dry covers the whole of the requirement. Ambient storage is tolerated over short periods, but a refrigerated desiccator is the better default for anything held beyond a few weeks, and the container should be amber or otherwise opaque, since aminoquinolinium compounds darken slowly under light.

Moisture is the practical problem. Quaternary nitrogen salts are hygroscopic, and a solid that has taken up water reads heavy on a balance and produces misleading gravimetric figures. Let a chilled container warm to room temperature before opening it, otherwise condensation settles straight onto the solid. Reseal promptly and keep the desiccant fresh.

In solution the compound is soluble in water and in dimethyl sulfoxide. Aqueous solutions are best made up fresh, because the aryl amine oxidises slowly in air and faster at alkaline pH, and a drift toward yellow or brown is the visible sign of it. Stocks in anhydrous dimethyl sulfoxide, split into single-use portions and frozen, hold their titre better than aqueous ones and sidestep the degradation that repeated thawing brings.

References

1. Human liver nicotinamide N-methyltransferase. cDNA cloning, expression, and biochemical characterization
2. Structural basis of substrate recognition in human nicotinamide N-methyltransferase
3. Structure-Activity Relationship for Small Molecule Inhibitors of Nicotinamide N-Methyltransferase
4. Selective and membrane-permeable small molecule inhibitors of nicotinamide N-methyltransferase reverse high fat diet-induced obesity in mice
5. Nicotinamide N-methyltransferase knockdown protects against diet-induced obesity
6. Small molecule inhibitor of nicotinamide N-methyltransferase shows anti-proliferative activity in HeLa cells
7. 5-Amino-1-methylquinolinium, PubChem Compound Summary CID 950107

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