PubMed1972
The occurrence and subcellular distribution of enzymes of the cytidine diphosphate choline pathway of lecithin synthesis have been examined. Choline kinase (EC 2.7.1.32) was completely soluble, while phosphorylcholine-cytidyl transferase (EC 2.7.7.15) and phosphorylcholine-glyceride transferase (EC 2.7.8.2) were associated with particulate fractions. Although components sedimenting at 10,000 to 100,000 x g contained both enzymes, phosphorylcholine-cytidyl transferase and particularly phosphorylcholine-glyceride transferase were present in the 10,000 x g pellet, which contained the major organelles, mitochondria, and glyoxysomes. When the crude homogenate was centrifuged on a sucrose density gradient, four major bands of particulate protein were recovered. A band at density 1.24 g/cm(3) contained the glyoxysomes and was devoid of phosphorylcholine-cytidyl transferase and phosphorylcholine-glyceride transferase activity. Enzyme activity was barely detectable in the mitochondria, at density 1.18 g/cm(2). Phosphorylcholine-glyceride transferase was found almost exclusively in a sharp band at density 1.12 g/cm(3), and phosphorylcholinecytidyl transferase was found in the uppermost band at density 1.08 g/cm(3). Thus, for the synthesis of lecithin in their membranes, the glyoxysomes and mitochondria depend on enzymes elsewhere in the cell; the final two steps in lecithin formation occur, apparently exclusively, in separate particulate cell components.