Methionine adenosyltransferase (adenosine 5'-triphosphate-L-methionine S-adenosyltransferase, EC 2.5.1.6) was found to occur only as the gamma isozyme in Friend erythroleukemic cells. Enzyme activity in a dialyzed 105,000 X g supernatant was linear for at least 30 min and dependent on the amount of protein added. The Km for methionine was 10.6 microM. In the presence of 10% dimethyl sulfoxide, the enzyme activity was slightly inhibited and dithiothreitol was not required for maximum activity. These properties identify the methionine adenosyltransferase in Friend erythroleukemic cells as the gamma isozyme. L-Ethionine served as a substrate with a Km of 30 microM, and competitively inhibited the enzyme activity with a Ki of 150 microM. Friend erythroleukemic cells grown in the presence of ethionine accumulated S-adenosylethionine, which was dose and time dependent. Therefore, the gamma isozyme of methionine adenosyltransferase from Friend erythroleukemic cells utilize L-ethionine as substrate, resulting in an accumulation of S-adenosylethionine. These studies provide a mechanism by which ethionine through S-adenosylethionine may alter the methylation of DNA in Friend erythroleukemic cells.

The use of a 32P-labeling procedure has allowed us to monitor the stability of 5-methylcytosine (5-mC) in the existing fraction of DNA in differentiating Friend erythroleukemia cells (FELCs). The levels of 5-mC in existing DNA of control cells remained constant for 20 h, at nearly 3.95% of total labeled cytosine. Exposure of pre-labeled cells to 5 mM N'-methylnicotinamide (N'-MN), 4 mM hexamethylene bisacetamide (HMBA), or 270 mM dimethylsulfoxide (DMSO) over the same time-period resulted in the loss of 6.1, 4.8 and 1.8%, respectively, of the 5-mC from the pool of labeled cytosine present in the existing fraction of the DNA.