Mammalian glycine N-methyltransferases. Comparative kinetic and structural properties of the enzymes from human, rat, rabbit and pig livers.

1. Human liver contains a rather high level of glycine N-methyltransferase. 2. The enzymes from human, rat, rabbit and pig livers are all tetramers and exhibit positive cooperativity toward S-adenosylmethionine and Michaelis-Menten kinetics toward glycine. The [S]0.5 values for S-adenosylmethionine and glycine of the rat enzyme are considerably lower than those of three other enzymes. 3. The subunit of rat glycine N-methyltransferase is shorter by two residues compared with the subunits of human, rabbit and pig glycine N-methyltransferases. Except for this difference, however, all enzymes show a high degree of sequence homology.