Phosphatidylinositol Phosphatidylserine

Figure 11.21 Outline of synthesis of phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine. Note in the synthesis of phosphatidylinositol, the free base, inositol, is used directly. Inositol is produced in the phosphatase reactions that hydrolyse and inactivate the messenger molecule, inositol trisphosphate (IP3). This pathway recycles inositol, so that it is unlikely to be limiting for the formation of phosphatidylinositol bisphosphate (PIP )- This is important since inhibition of recycling is used to treat bipolar disease (mania) (Chapter 12, Figure 12.9). Full details of the pathway are presented in Appendix 11.5. Inositol, along with choline, is classified as a possible vitamin (Table 15.3).

Improvement of membrane separation technology has resulted in the isolation of MFGM-enriched material from commercially available products. A phospholipid-rich fraction can be extracted from whey (Boyd et al., 1999) and buttermilk (Sachedva and Buchheim, 1997) with a reported yield of 0.25 g of phospholipids/g of protein in buttermilk (Sachdeva and Buchheim, 1997). Microfiltration of whey derived from the Cheddar cheese process, using 0.2 pm ceramic filters results in a fraction containing two major phospholipids, phosphatidylcholine and phosphatidylethanolamine, and lesser amounts of phosphatidylinositol, phosphatidylserine, sphingomyelin and cerebrosides (Boyd et al., 1999). The phospholipid fraction separated from the total lipids contains a larger proportion of mono- and polyunsaturated fatty acids (mainly oleic, Cig i and linoleic, C ) compared to the total lipid and the neutral lipid fraction (Boyd et al., 1999). 

Triacylglycerols Phosphatidylethanolamine Phosphatidylinositol Phosphatidylserine Phosphatidylcholine Sphingomyelin Other... 

The inclusion of phosphoric acid in the mobile phase increases the potential for error in subsequent quantitation by lipid phosphorus determinations and has been replaced with a mobile phase containing acetonitrile-methanol-sulphuric acid (100 3 0.05) to provide resolution of phosphatidylinositol, phosphatidylserine, phosphatidyl-ethanolamine, phosphatidylcholine, lysophosphatidylcholine and sphingomyelin (Kaduce et al., 1983). The authors reported that a reduction in the sulphuric acid content of the mobile phase caused a broadening of the eluted peaks and an increase in the retention of phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine, while if omitted, these components did not elute. It was also noted that if the methanol content of the mobile phase was increased then the retention times of all the phospholipids were decreased. Samples were therefore injected in chloroform-diethyl ether (1 1) to avoid altering the concentration of methanol in the mobile phase. 

Fig. 5. Docosahexaenoic acid (DHA) in forebrain phosphatidylinositol/phosphatidylserine (PI/PS) of 13-wk-old spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats fed diets supplemented with arachidonic acid (AA) (0.5%) and DHA (1%) from weaning. Effect of diet in SHR, P< 0.05. Source adapted from Reference 37.

Data obtained by a variety of methods show that in eukaryotic cells the choline containing phospholipids are preferentially localized in the outer leaflet of the bi-layer (707o and 807o for phosphatidylcholine and sphingomyelin, respectively) while phosphatidylinositol, phosphatidylserine and phosphatidic acid are concentrated in the inner leaflet (VERKLEIJ, 1973 RENOOJ... 

ELISA) in which the antibodies bind to a target lipid, which is usually cardiolipin. Platelet antibodies directed to phosphatidylserine and phosphatidylinositol are referred to as anticardiolipin antibodies and their presence is characterized as anti-cardiolipin antibody-thrombosis syndrome (96). 

Two routes to phospholipid biosynthesis are known in either, the participation of CTP is necessary. The first route involves phosphatidic acid in phosphoglyceride biosynthesis. Phosphatidic acid reacts with CTP to yield CDP-diglyceride which, as a coenzyme, can participate in the transfer of diglyceride onto serine (or inositol) to produce phosphatidylserine (or phosphatidylinositol). Serine phosphatides are liable to decarboxylation (pyridoxal phosphate acting... 

Adapted from [3]. Ptdlns, phosphatidylinositol PtdIns(4,5)P2, phosphatidylinositol-4,5-insphosphate PtdCho, phosphatidylcholine PtdEtn, phosphatidylethanolamine PtdSer, phosphatidylserine. 

Frosolono and Currie (1985) investigated the effect of phosgene on the pulmonary surfactant-system (PSS) in groups of six to 14 rats exposed to phosgene at 1 ppm for 4 h. The exposure system and parameters were similar to those described in Section 3.2.1 (Hatch et al. 1986). The actual chamber concentration was 1.0 0.06 ppm. Animals were sacrificed immediately after exposure, or on postexposure days 1, 2, or 3. Pulmonary edema was present immediately after exposure and persisted through day 3. Phosphatidylinositol levels were significantly (p<0.05) decreased compared with controls immediately after exposure only. Phosphatidylserine and phosphatidylethanolamine levels were significantly increased compared with controls on days 1, 2, and 3 postexposure. Phosphatidylcholine levels were increased at all time points compared with controls. 

Today s mitochondria lack most of the genes involved in phosphohpid metabolism. Therefore, mitochondria have to import most of their hpids. Phospholipids such as phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, and phosphatidylinositol must be synthesized in the endoplasmatic reticulum under the control of nuclear genes and then transferred to mitochondria (Voelker, 2000) (Figure 1). Mitochondria use both nuclear and mitochondrial encoded proteins to further diversify phospholipids (Dowhan, 1997 Kent, 1995 Daum, 1985). Thus, a nuclear phosphatidylserine decarboxylase converts phosphatidylserine into phosphatidylethanolamine, or mitochondrial encoded cardiolipin synthase converts phosphatidylglycerol into cardiolipin which is incorporated exclusively into the inner mitochondrial membrane. 

Figure 1. Control of mitochondrial biogenesis by the nuclear genome. Most mitochondrial proteins, including cytochrome c, are nuclear gene products which are subsequently imported into mitochondria. Similarly, most enzymes involved in synthesis of mitochondrial phosphoplipids are encoded in the nuclear genome. Being located in the endoplasmatic reticulum, they synthesize phosphatidylcholine (PtdCho), phosphatidylserine (PtdSer), phosphatidylglycerol (PG) and phosphatidylinositol (Ptdins). The phospholipids are transferred to the outer membrane. The imported lipids then move into the inner membrane at contact sites. Mitochondria then diversify phospholipids. They decarboxylate phosphatidylserine to phosphatidylethanolamine (PtdEtN), but the main reaction is the conversion of imported phosphatidylglycerol to cardiolipin (CL). Cardiolipins localize mainly in the outer leaflet of the inner membrane.

It can be seen from Figure 1 that the choline-containing phospholipids, phosphatidylcholine and sphingomyelin are localized predominantly in the outer monolayer of the plasma membrane. The aminophospholipids, conprising phosphatidylethanolamine and phosphatidylserine, by contrast, are enriched in the cytoplasmic leaflet of the membrane (Bretcher, 1972b Rothman and Lenard, 1977 Op den Kamp, 1979). The transmembrane distribution of the minor membrane lipid components has been determined by reaction with lipid-specific antibodies (Gascard et al, 1991) and lipid hydrolases (Biitikofer et al, 1990). Such studies have shown that phosphatidic acid, phosphatidylinositol and phosphatidylinositol-4,5-fc -phosphate all resemble phosphatidylethanolamine in that about 80% of the phospholipids are localized in the cytoplasmic leaflet of the membrane. 

Phosphatidic acid Phosphatidylethanolamine Phosphatidylcholine Occithin) Phosphatidylserine Phosphatidylinositol... 

Phosphatidylcholine (lecithin) is the most abundant phospholipid in membranes. Phosphatidylethanolamine (cephalin) has an ethanolamine residue instead of choline, and phosphatidylserine has a serine residue. In phosphatidylinositol, phosphatidate is esterified with the sugarlike cyclic polyalcohol myo-inositol. A doubly phosphorylated derivative of this phospholipid, phosphatidylinositol 4,5-bisphosphate, is a special component of membranes, which, by enzymatic cleavage, can give rise to two second messengers, diacylglycerol (DAG) and inositol l,4,5trisphosphate (InsPsi see p.386). 

Our present ideas about the nature of biological membranes, which are so fundamental to all biochemical processes, are based on the Singer-Nicholson mosaic model. This model of the membrane is based on a phospholipid bilayer that is, however, asymmetrical. In the outside monolayer, phosphatidylcholine (lecithin) predominates, whereas the inner monolayer on the cytoplasmic side is rich in a mixture of phos-phatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Cholesterol molecules are also inserted into the bilayer, with their 3-hydroxyl group pointed toward the aqueous side. The hydrophobic fatty acid tails and the steran skeleton of cholesterol... 

Additional information <8> (<8> introduction of phosphatidylserine, produces an 8fold decrease in Vmax for ATP using phosphatidylinositol. Without phosphatidylserine, phosphatidylinositol 4-phosphate produces non-linear enzyme kinetics [8]) [8]... 

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