A methyl donor and a cofactor for enzyme-catalyzed methylations, including catechol O-methyltransferase (COMT) and DNA methyltransferases (DNMT). Although present in all cells, it is concentrated in liver where 85% of all methylation reactions occur. SAM is anti-apoptotic in normal hepatocytes and normal colon epithelial cells but pro-apoptotic in liver human hepatocellular carcinoma (HCC), HepG2 cells and colon cancer cells.