S-adenosylmethionine metabolism and liver disease.

Methionine is an essential amino acid that is metabolized mainly by the liver where it is converted to S-adenosylmethionine (SAMe) by the enzyme methionine adenosyltransferase. Although all mammalian cells synthesize SAMe, the liver is where the bulk of SAMe is generated as it is the organ where about 50% of all dietary methionine is metabolized. SAMe is mainly needed for methylation of a large variety of substrates (DNA, proteins, lipids and many other small molecules) and polyamine synthesis, so if the concentration of SAMe falls below a certain level or rises too much the normal function of the liver will be also affected. There are physiological conditions that can affect the hepatic content of SAMe. Consequently, to control these fluctuations, the rate at which the liver both synthesizes and catabolizes SAMe is tightly regulated. In mice, failure to do this can lead to fatty liver disease and to the development of hepatocellular carcinoma (HCC). Therefore, maintaining SAMe homeostasis may be a therapeutic target in nonalcoholic steatohepatitis, alcoholic- and non-alcoholic liver cirrhosis, and for the chemoprevention of HCC formation.