Formation of CO2 from the carboxyl group of S-adenosylmethionine by liver membrane-associated enzymes involves the demethylation-transsulphuration pathway

The activity released from membrane fragments into the supernatant fraction of rat liver homogenate by Triton X-100 and forming ¹⁴CO₂ from carboxyl-labeled S-adenosylmethionine (1) is not a true S-adenosylmethionine decarboxylase. It did not produce decarboxylated S-adenosylmethionine but was also able to use S-adenosylhomocysteine as a substrate. The formation of CO₂ from these two substrates was absolutely dependent on the presence of cytosol proteins and low-molecular weight compounds and it accounted for 5 to 10% of the total S-adenosylmethionine degrading activity of the supernatant fraction. The reaction showed abn initial lag period and was inhibited by every intermediate of the transsulphuration pathway. It is concluded that the formation of CO₂ from S-adenosylmethionine involves the demethylation-transsulphuration route from S-adenosylmethionine to α-ketobutyric acid which is finally decarboxylated.