Microsomal phosphatidylethanolamine methyltransferase: some physical and kinetic properties.

Some physical and kinetic properties of the microsomal enzyme(s) that convert phosphatidyl-ethanolamine to phosphatidylcholine in rat and guinea pig livers have been investigated. The pH optima, of the reactions were 9.8, 9.3 and 9.5 for the first, second and third methylation reactions, respectively. Incomplete heat denaturation of the protein catalyzing the first reaction contrasts with inactivation at 60 C of the enzymes catalyzing the second and third methylations. The maximal velocity of the first reaction of the guinea pig liver enzyme is 48 p mol/min/mg protein, substantially less than exhibited rate-limiting reaction of the three step methylation sequence in rat liver, 114 pmol/min/mg. The affinity of the microsomal enzyme for S-adenosylmethionine is greater in rat liver (Km=18.2 μM) than in guinea pig liver (Km=302 μM).