Most investigators now prefer non-radioactive assays for DNMT. Further, many assay kits to assess the activity are now available. Therefore, if possible, try these kits. Find some details about the assay using radioactive material in the link.
Lalita Why you want to perform radioactive based assay for DNMTs, try from the following 1. DNMT1 Assay Kit (ab113469), DNMT Activity / Inhibition Assay rxns 55006 by active motif.EpiQuik DNMT1 Activity/Inhibitor Screening Assay Core Kit.