Selective Reagent for Detection of N‐ε‐Monomethylation of a Peptide Lysine Residue through SNAr Reaction

Abstract

Methylations of specific lysine residues of histone proteins are catalyzed by histone methyltransferases (HMTs) and play key roles in the epigenetic control of gene expression. Several methods to detect N‐ε‐methylation of the lysine residue have been established in order to evaluate the activity of HMTs, to develop inhibitors, and to identify substrates. However, they mostly employ specific antibodies or enzymes such as peptidases, and their reliability and reproducibility often depend on the quality of the protein reagents and the reaction conditions. Here, we describe a convenient method to detect N‐ε‐monomethylation of the lysine residue through a simple chemical reaction. We focused on nucleophilic aromatic substitution reaction (SNAr reaction) between an aromatic electrophile and a primary or monomethylated amino group. Screening of various electrophiles indicated that 4‐fluoro‐2‐nitroacetophenone (1g) has high selectivity for the N‐ε‐monomethylated amino group of lysine. Furthermore, the reaction products of 1g with lysine and N‐ε‐monomethylated lysine, 5g and 6g, respectively, show different absorption spectra, that is, the absorbance at 350 nm of 6g is 13 times larger than that of 5g. We show that these characteristic properties of 1g can be utilized for the selective detection of the methylation state of lysine residues in HMT substrate peptides, and for an assay of HMT activity.