| Summary: | Phosphatidylethanolamine (PE) N-methyltransferase catalyzes the stepwise transfer of methyl groups from S-adenosyl-L-methionine (AdoMet) to the amino headgroup of PE. Successive methylation results in the formation of the two intermediates, phosphatidyl-N-monomethylethanolamine (PMME) and phosphatidyl-N, N-dimethylethanolamine (PDME), and the final product phosphatidylcholine (PC). PE N-methyltransferase is an integral membrane protein localized primarily in the endoplasmic reticulum (microsomal fraction) of liver.
PE-, PMME- and PDME-dependent PE N-methyltransferase activities were purified from Triton X-100 solubilized microsomes 429-, 1542- and 832-fold, respectively. The purified enzyme was composed of a single 18.3 kDal protein as determined by SDS-PAGE. Molecular mass analysis of purified PE N-methyltransferase (in Triton X-100 micelles) by gel filtration on Sephacryl S-300 indicated the enzyme existed as a 24.7 kDal monomer. PE N-methyltransferase catalyzed the complete conversion of PE to PC and had a pH optimum of 10 for all three steps. A Triton X-100 mixed micelle assay was developed to assay PE-, PMME- and PDME-dependent activities of both pure and microsomal PE N-methyltransferase. The AT-terminal amino acid sequence of rat liver PE N-methyltransferase and the recently cloned 23.1 kDal S. cerevisiae PEM 2 were found to be 35% homologous.
Double reciprocal plots for PE N-methyltransferase at fixed Triton X-100 concentrations and increasing PE, PMME or PDME were highly cooperative. Similar cooperative effects were noted when phospholipid was fixed and Triton X-100 increased. The cooperativity could be partially abolished if a fixed mol% of nonsubstrate phospholipid such as PC was included in the assay. This would indicate that PE N-methyltransferase has specific binding requirements for a site(s) in contact with the micellar substrate. The occupation of this boundary layer by phospholipid is essential for full expression of enzyme activity. Kinetic analysis revealed that PMME and PDME methylation followed an ordered Bi-Bi mechanism. The overall mechanism involves initial binding of PE to a common site and successive methylation steps involving the binding and release of AdoMet and S-adenosyl-L-homocysteine, respectively. Cysteine residue(s) (which are rapidly oxidized in the absence of reduced thiols) are involved in the catalytic mechanism.
Reverse-phase HPLC was used to fractionate the phospholipid products of PE N-methyltransferase into individual molecular species. Substrate specificity experiments on PE N-methyltransferase in vitro and in vivo revealed no selectivity for any molecular species of diacyl PE, PMME or PDME. The PE-derived PC, which is rich in 16:0-22:6, is rapidly remodeled to conform to the molecular species compositon of total hepatocyte PC in vivo .
The 18.3 kDal PE N-methyltransferase was found to be a substrate for cAMP-dependent protein kinase in vitro. However, only 0.25 mol phosphorus/mol of PE Af-methyltransferase was incorporated, with no observed effect on activity. Studies on PE N-methyltransferase regulation in choline-deficient rat liver indicated that activity changes were due to elevated levels of cellular PE. Immunoblotting of choline-deficient liver microsomes or hepatocyte membranes with a anti-PE N-methyltransferase antibody revealed no alteration in enzyme mass. While more work is needed, initial indications are that hepatic PE N-methyltransferase is a constitutive enzyme regulated primarily by substrate and product levels. === Medicine, Faculty of === Biochemistry and Molecular Biology, Department of === Graduate
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