Abstract
Phosphatidylethanolamine N-methyltransferase, which catalyzes all the three-step methylation from Phosphatidylethanolamine to phosphatidylcholine, was purified to homogeneity from the membrane fraction of Zymomonas mobilis. The purified enzyme exhibited a single band on SDS-polyacrylamide gel electrophoresis and its molecular weight was estimated to be 42, 000 on comparison with those of marker proteins. The three activities dependent on phosphatidylethanolamine, phosphatidyl-N-monomethylethanolamine and phosphatidyl-N, N'-dimethylethanolamine of the purified enzyme showed similar pH profiles with an optimum of pH 8.5, and were enhanced in the same manner by Triton X-100 and L-cysteine. The maximal velocities of the three reactions for S-adenosyl-L-methionine were 0.04, 1.36 and 0.69nmol/mg protein/min with apparent Michaelis constant values of 3.6, 1.9 and 3.9μM, respectively, indicating that the first-step methylation is rate-limiting for the pathway in the organism.
