Description:
<jats:title>Abstract</jats:title><jats:p>Natural product methyltransferases (NPMTs) represent an emerging class of enzymes that can be of great use for the structural and functional diversification of bioactive compounds, such as the strategic modification of <jats:italic>C</jats:italic>‐, <jats:italic>N</jats:italic>‐, <jats:italic>O</jats:italic>‐ and <jats:italic>S</jats:italic>‐moieties. To assess the activity and the substrate scope of the ever‐expanding repertoire of NPMTs, a simple, fast, and robust assay is needed. Here, we report a continuous spectroscopic assay, in which <jats:italic>S</jats:italic>‐adenosyl‐L‐methionine‐dependent methylation is linked to NADH oxidation through the coupled activities of <jats:italic>S</jats:italic>‐adenosyl‐L‐homocysteine (SAH) deaminase and glutamate dehydrogenase. The assay is highly suitable for a high‐throughput evaluation of small molecule methylation and for determining the catalytic parameters of NPMTs under conditions that remove the potent inhibition by SAH. Through the modular design, the assay can be extended to match the needs of different aspects of methyltransferase cascade reactions and respective applications.</jats:p>