Phospha tidylserine

An, X., Guo, X., Sum, H., Morrow, J., Gratzer, W., and Mohandas, N. (2004). Phospha-tidylserine binding sites in erythroid spectrin Location and implications for membrane stability. Biochemistry 43, 310-315. 

This enzyme is considered to be important to maintenance of the proper level of calcium ion in the erythrocyte. As such it is also considered to be typical of a membrane-bound enzyme. Upon purification of this enzyme from human erythrocytes, it was found that the purified enzyme exhibited no activity until a mixture of a nonionic detergent and a charged phospholipid (e.g., phospha-tidylserine or phosphatidylinositol), were included in the assay system (Nelson and Hanahan, 1985). Ten- to twelvefold increases in activity could be achieved. If a neutral phospholipid such as phosphatidylcholine were substituted for the phosphatidylserine (or phosphatidylinositol), essentially there was little or no enzymatic activity. 

Phosphodiesterase (Hydrolysis) Activity. A rather broad substrate specificity is exhibited by the purified phospholipase D (phosphodiesterase activity). It can attack phosphatidylcholine, phosphatidylethanolamine, phospha-tidylserine, and phosphatidylglycerol. In most cases, Ca2+ was an activator, but variable results were obtained on the positive influence of diethyl ether on the catalytic activity of different sources of this enzyme. Usually the optimum pH was in the range from 5.0 to 7.0. Mammalian phospholipase D, containing both the phosphodiesterase and transphosphatidylase activities, exhibited a broad-range substrate specificity similar to that of the plant enzyme. However, the mammalian enzyme showed a dependency for the presence of oleic acid in the reaction system (Kobayashi and Kanfer, 1991). 

Synthesis of phosphatidylcholine and phosphatidylethanolamine begins with activation of choline (or ethanolamine) with ATP via choline kinase to yield phosphocholine (phosphoethanolamine) + ADP the activated base is transferred via CTP and phosphocholine cytidyl transferase to form CDP-choline (CDP-ethanolamine) and PPi. The base is then transferred to the sn-3 of diacylglycerol via phosphocholine diacylglycerol transferase to yield phosphatidylcholine (phosphatidylethanolamine) + CMP. The cytidyl transferase is believed to be the rate-limiting or regulatory step in the pathway. Phospha-tidylserine is formed by a direct transfer and substitution of serine for ethanolamine in phosphatidylethanolamine. Phosphatidyl serine can be dec-arboxylated to form phosphatidylethanolamine. 

FIGURE 6.5 Phosphatidylethanolflinjne (PE) synthesis by decarboxylation of phospha-tidylserine (i S). Vitamin is required for catalytic activity. 

The average human red cell plasma membrane is lipid-rich with 240 million PL molecules, 190 million cholesterol molecules, 12 million glyco-lipid molecules and only 4 million protein molecules (Lux and Glader, 1981). The PLs of the red cell plasma membrane are asymmetrically distributed in the bilayer. The choline-containing PLs, PC and sphingomyelin represent 40% and 15%, respectively, of the total PL, and are enriched in the outer leaflet. In contrast, the amino PLs, PE and phospha-tidylserine (PS) are exclusively confined to the inner leaflet and constitute 35% and 15%, respectively, of total PLs. 

Arroyo, A. et aL NADPH oxidase-dependent oxidation and extemahzation of phospha-tidylserine during apoptosis in MejSO-differentiated HL-60 cells. Role in phagocytic clearance. ioL Oiem. 277, 49965-75, 2002. 

Comparing the activity of the Mg2+-ATPase in the presence of water suspensions of gangliosides and phosphatidylserine it was found that only the gangliosides activated. However, when sodium-deoxycholate (0.06 mM) was added to the incubation medium phospha-tidylserine also activated the enzyme. 

---