The kinetic analysis of the N -methylation of 4-phenylpyridine by nicotinamide N -methyltransferase: Evidence for a novel mechanism of substrate inhibition

Matthijs J. van Haren, Martin G. Thomas, Davide Sartini, David J. Barlow, David B. Ramsden, Monica Emanuelli, Fábio Klamt, Nathaniel I. Martin, Richard B. Parsons

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The N-methylation of 4-phenylpyridine produces the neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+). We investigated the kinetics of 4-phenylpyridine N-methylation by nicotinamide N-methyltransferase (NNMT) and its effect upon 4-phenylpyridine toxicity in vitro. Human recombinant NNMT possessed 4-phenylpyridine N-methyltransferase activity, with a specific activity of 1.7 ± 0.03 nmol MPP+ produced/h/mg NNMT. Although the Km for 4-phenylpyridine was similar to that reported for nicotinamide, its kcat of 9.3 × 10−5 ± 2 × 10−5 s−1 and specificity constant, kcat/Km, of 0.8 ± 0.8 s−1 M−1 were less than 0.15% of the respective values for nicotinamide, demonstrating that 4-phenylpyridine is a poor substrate for NNMT. At low (<2.5 mM) substrate concentration, 4-phenylpyridine N-methylation was competitively inhibited by dimethylsulphoxide, with a Ki of 34 ± 8 mM. At high (>2.5 mM) substrate concentration, enzyme activity followed substrate inhibition kinetics, with a Ki of 4 ± 1 mM. In silico molecular docking suggested that 4-phenylpyridine binds to the active site of NNMT in two non-redundant poses, one a substrate binding mode and the other an inhibitory mode. Finally, the expression of NNMT in the SH-SY5Y cell-line had no effect cell death, viability, ATP content or mitochondrial membrane potential. These data demonstrate that 4-phenylpyridine N-methylation by NNMT is unlikely to serve as a source of MPP+. The possibility for competitive inhibition by dimethylsulphoxide should be considered in NNMT-based drug discovery studies. The potential for 4-phenylpyridine to bind to the active site in two binding orientations using the same active site residues is a novel mechanism of substrate inhibition.
Original languageEnglish
Pages (from-to)127-136
JournalThe International Journal of Biochemistry & Cell Biology
Early online date13 Mar 2018
Publication statusPublished - 1 May 2018


  • enzyme kinetics
  • neurotoxicity
  • N-Methylation
  • substrate inhibition
  • substrate specificity


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