PHOSPHATIDYLETHANOLAMINE METHYLTRANSFERASE AND PHOSPHOLIPID METHYLTRANSFERASE ACTIVITIES FROM SACCHAROMYCES-CEREVISIAE - ENZYMOLOGICAL AND KINETIC-PROPERTIES

In the yeast Saccharomyces cerevisiae, two membrane-associated enzymes catalyze the three-step methylation of Phosphatidylethanolamine (PE) to phosphatidylcholine (PC). Phosphatidylethanolamine methyltransferase (PEMT) catalyzes the first methylation reactions (PE → phosphatidylmonomethylethanolamine (PMME)) and phospholipid methyltransferase (PLMT) catalyzes the second two methylation reactions (PMME → phosphatidyldimethylethanolamine (PDME) → PC). Using gene disruption mutants of the S. cerevisiae OPI3 and CHO2 genes, we independently studied the enzymological properties of microsome-associated PEMT and PLMT, respectively. The enzymological properties of the enzymes differed with respect to their pH optima, cofactor requirements and thermal lability. For the PEMT reactions, the apparent Km values for PE and S-Adenosylmethionine (AdoMet) were 57 μM and 110 μM, respectively. For the PLMT reactions, the apparent Km values for PMME and PDME were 380 μM and 180 μM, respectively. The apparent Km values for AdoMet were 54 μM and 59 μM with PMME and PDME as substrates, respectively. S-Adenosylhomocysteine (AdoHcy) was a competitive inhibitor of PEMT (Ki = 12 μM) and PLMT (K)i = 57 μM and Ki = 54 μM for PMME and PDME, respectively) with respect to AdoMet. AdoHcy was a noncompetitive inhibitor of PEMT (K)i, = 160 μM) and PLMT Ki = 320 μM and Ki = 120 μM) with respect to PE and PMME and PDME, respectively. © 1990.