Cells were grown in 13CD3-methionine for over a week to allow for near fully labeled generation of 13CD3-S-adenosyl methionine, which were incorporated into histones as methyl groups. Greater than 98% labeling efficiency of most histone methylation sites was generally detected using this approach. Using these 312636-16-1 nuclei as the reaction template, unlabeled light SAM was added along with either a scrambled sequence control or an inhibitor peptide and the nuclei were incubated in the buffer for 2 hours. Previously methylated histone sites would all be heavy labeled, while newly methylated sites would all be light labeled. This in nucleo assay monitored the Ribociclib hydrochloride effect that the control or inhibitor peptides exhibited on newly methylated histone sites and hence how they affected HMT activity. If the peptide had an inhibitory effect on the function of a particular histone methyltransferase, then the addition of new methyl groups to the histone sites would be reduced in comparison to a control peptide with no inhibitory effect. As a result, the ratio of old to new methylated histone sites produced with the addition of an inhibitory peptide would be reduced in comparison to the ratio produced with the addition of a control peptide with no inhibitory effect. A snapshot of these experiments is shown in Figure 7.A-F. Mass spectra are shown of the 27�C40 residue peptide from canonical histone H3 containing the K27 trimethylation site from the in nucleo reactions with SAM only, SAM plus the scrambled sequence control peptide and SAM plus the SQ037 inhibitor peptide. The peak at 552.680 m/z corresponds to the Mz3H_3z ion that is the heavy-labeled old H3K27me3 peptide species. The old H3K27me3 is separated by from the light-labeled new H3K27me3 species that represents the newly synthesized methylation mark. As this in nucleo assay is only performed for a relatively short time, only a small amount of new H3K27 trimethylation was generated when only light SAM i
