Methylating agents: their target amino acids in nuclear proteins.

We have tried to establish a correlation between the carcinogenic potency of four methylating compounds and their specific target sites in chromatin. We have therefore compared the nuclear metabolism of two relatively weak carcinogens radioactively labelled: dimethyl sulphate (DMS) and methyl methanesulphonate (MMS), and two potent carcinogens: N-methyl-N-nitrosourea (MNU) and N-methyl-N'-nitro-N-nitrosoguanosine (MNNG) in cultured primary hepatocytes and the V79 Chinese hamster cell line. Cysteine (when present), and to a lesser extent histidine, were methylated by MMS and DMS not only in the total acid-soluble nuclear protein (H) but also in purified histones H1 and H3. These compounds had the same effect not only on total non-histone nuclear protein (NH) but also on purified HMG1 and HMG2 (nuclear non-histone proteins with high electrophoretic mobility). Traces of methylarginine and methylated lysine could be detected in all samples. MNU and MNNG predominantly methylated lysine and arginine residues, the former being found mostly in acid soluble, the latter in non-histone nuclear protein. Methylated cysteine and histidine were present in trace amounts. Our preliminary data suggest specific amino acid methylation at the nuclear protein level for carcinogens with different potencies, similar to what has been found for DNA bases.