ISOLATION AND CHARACTERIZATION OF LIPID N-METHYLTRANSFERASE FROM DOG LUNG

A soluble protein with lipid N-methyltransferase activity has been isolated from microsomes of dog lung. This protein catalyzes the transfer of methyl groups from S-adenosylmethionine to phosphatidyl ethanolamine with the formation of phosphatidyl choline. The partially purified protein was stable at pH greater than 8.2 in the presence of cysteine. It contained 2-5% lipid and had optimal activity at pH 8.0-9.0. Reaction velocities were markedly increased by exclusion of O2 from the reaction system. When saturating levels of S-adenosylmethionine were used, highest reaction velocities were obtained with disaturated phosphatidyl ethanolamine and lower rates with more unsaturated substrates. Reaction rate in the liver particulate-bound system are higher than lung but are the same with saturated or unsaturated phosphatidyl ethanolamine as substrate. In sucrose density-gradient centrifugation experiments the transferase protein was shown to be associated with lamellated structures peculiar to lung pneumocytes. These results suggest that the N-methyltransferase of lung has special properties and may be important in the synthesis of disaturated surface-active phosphatidyl choline. © 1969.