What is the function of phospholipid N-methylation?

Most if not all membrane functions are made possible by the unique composition of proteins and lipids in biological membranes. Although most biological membranes carry out similar functions, their composition varies largely (reviewed by Quinn, 1977). Thus, the protein/lipid ratio of rat plasma membranes can vary from 0.25 in myelin to 1.5 in the liver. The ratio cholesterol/phospholipid can also vary from 1.0 in rat erythrocytes to 0.5 in rat liver plasma membranes. The phospholipid composition is also very variable; rat erythrocytes contain about 30% phosphatidylcholine but sheep erythrocytes contain only 5%. Variations in the phospholipid content are also found within a cell. Thus, the content of phosphatidylcholine of liver cell membranes varies from 61% in rough endoplasmic reticulum to 35% in plasma membranes, while the content of phosphatidylethanolamine is about 20% in all liver membrane fractions (McMurray & McGee, 1972). It is important to remember these data when making generalizations. Another important feature of membranes is the rapid turnover of their phospholipids. The half-life of phosphatidylcholine in rat liver varies from 1 h to 1Oh depending on the type of precursor used (Tolbert & Okey, 1952; Arvidson, 1968). Different cell fractions of rat liver have also different phospholipid turnover. Thus, the half-life of phosphatidylcholine in rat liver mitochondria and microsomes is 16 h and 8 h respectively (Bygrave, 1969). The significance of this rapid turnover is not well understood, but may result from membrane repair mechanisms as well as from structural changes which may render cell membranes more sensitive to activation. In this Review we will deal with the control mechanisms of phosphatidylcholine synthesis by the CDP-choline and transmethylation pathways. The catabolism of phosphatidylcholine has been recently reviewed (Van den Bosch, 1980).