Surface membrane-associated phosphatidylethanolamine N-methyltransferase activity in L-929 cells

Area of Science:

• Biochemistry
• Cell Biology
• Membrane Biology

Background:

• Cell surface membranes play vital roles in cellular processes.
• Phospholipid methylation is a key modification in membrane dynamics.
• Understanding methyltransferase activity is essential for cell membrane research.

Purpose of the Study:

• To investigate the presence and characteristics of methyltransferase activity in L-929 cell surface membranes.
• To identify the specific phospholipids methylated by the enzyme.
• To determine the contribution of this activity to phosphatidylcholine synthesis.

Main Methods:

• Incubation of L-929 cell surface membranes with S-adenosyl-L-[methyl-3H]-methionine.
• Assay of N-methyltransferase activity at varying pH and in the presence/absence of divalent cations and exogenous phospholipids.
• Inhibition studies using S-adenosyl-L-homocysteine and local anesthetics.
• Comparison of specific methyltransferase activities in surface membranes, whole cell homogenates, and microsomes.

Main Results:

• Identified phosphatidylethanolamine: S-adenosylmethionine N-methyltransferase activity in L-929 cell surface membranes.
• The enzyme methylated endogenous phosphatidylethanolamine and its derivatives to phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine, and phosphatidylcholine.
• Optimal activity at pH 6.9, independent of divalent cations and exogenous phospholipids.
• Activity was inhibited by S-adenosyl-L-homocysteine and local anesthetics.
• Surface membranes exhibited higher specific methyltransferase activity than whole cell homogenates or microsomes.
• In vitro activity rate was low (approx. 1 pmol/min·mg protein).

Conclusions:

• Phosphatidylethanolamine N-methyltransferase is present in L-929 cell surface membranes.
• This enzyme contributes to the synthesis of methylated phospholipids, including phosphatidylcholine.
• The low in vitro activity suggests phospholipid methylation is not a major source of surface membrane phosphatidylcholine.
• The enzyme's localization and characteristics provide insights into membrane lipid metabolism.