Phosphatidylethanolamine production

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.

Lysophospholipids have been found in butter serum by Cho et al. (1977). They characterized the sn-1 and -2 lysophosphatidylcholines and phosphatidylethanolamines. It is not known if these compounds are products of degradation or remnants of biosynthesis. Cho et al. (1977) searched for, but did not find, another possible product of enzymatic degradation of milk, phosphatidic acid. Phosphatidic acid can be formed by the action of phospholipase D on phosphatidylcholine, for example, but this enzymatic activity was not detected. The compound is also an important intermediate in the biosynthesis of lipids, but the concentration in tissue is always very low. The amount is also low in milk. Cho et al. (1977) found 1.2 and 0.9 (percent of total lipid P) of the lyso compounds above. The quantities of the other phospholipids were phosphatidylethanolamine, 27.3 -choline, 29.1 -serine, 13.4 -inositol, 2.5 and sphingomyelin, 25.6. 

The remainder of the radioactivity is associated with three main phospholipid classes, namely, phosphatidylcholine, phosphatidylethanolamine, and the phosphoinositides, mainly PIP2. Nonetheless, this type of result is commonly attributed to the action of a phospholipase A2, which is activated upon agonist interaction with the platelet. The other presumed product, a lysolecithin, would not be labeled in the above experimental protocol and thus, due to the very small amount formed in the reaction, could not be detected. Though one could potentially label the polar head group of the parent phosphoglycerides, there is little need to do so since the arachidonic acid is associated almost exclusively with the sn-2 ester position on phosphoglycerides. Consequently, the release of free arachidonic acid can be safely attributed to phospholipase A2 activity. While the yield of arachidonic acid is very low, the activation of the cell occurs only over a short time span, anywhere from 5 sec to 1 min. Thus self-control of cell activation is evident. 

Phospholipase A2 Action. As in the case of phosphatidylcholine, the above-mentioned phospholipases will attack only the sn-3 form of naturally occurring (as well as synthetic) phosphatidylethanolamine. The products are, of course, lysophosphatidylethanolamine (1 -6>-acyl-2-lyso-.rn-glycero-3-phosphoethanolamine) and the fatty acids (liberated from the sn-2 position). The latter can be analyzed for composition and structure, as the methyl esters, by gas-liquid chromatography coupled with mass spectrometry. Usually these acyl groups are largely the unsaturated types. 

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). 

A number of the products of the decarboxylation of amino acids shown in Table 9.2 are important as neurotransmitters and hormones, such as dopamine, noradrenaline, adrenaline, serotonin (5-hydroxytryptamine), histamine, and Y - aminobutyric acid (GABA), and as the diamines agmatine andput-rescine and the polyamines spermidine and spermine, which are involved in the regulation of DNA metabolism. The decarboxylation of phosphatidylser-ine to phosphatidylethanolamine is important in phospholipid metabolism (Section 14.2.1). 

Cmde oils generally contain phospholipids that are removed during the degum-ming stage of refining as a crude mixture (lecithin). This valuable product is the basis of the phospholipid industry, and phospholipids are used extensively in food products, in animal feeds, and in industrial processes. The major members are phosphatidylcholines, phosphatidylethanolamines, and phosphatidylinositols and are accompanied by smaller proportions of other phospholipids. Soybean oil (3.2%), rapeseed oil (2.5%), and sunflower seed oil (1.5%) contain the proportions of total phospholipids indicated in parentheses and are the main sources of commercial lecithins, especially soya lecithin. Palm oil contains little or no phospholipids (7-9). 

Further fractionation can separate the alcohol-soluble phosphatidylcholine from the alcohol-insoluble phosphatidylinositol. Commercial products that are alcohol-soluble contain concentrated phosphatidylcholine (40-60%) and only low levels of phosphatidylinositol. The alcohol-insoluble products are enriched in phosphatidylinositol (40-60%), whereas their phosphatidylcholine content is greatly reduced. The phosphatidylethanolamine component is approximately equally partitioned between the two fractions. The alcohol-soluble grades tend to be more oil-in-water emulsifiers, whereas the alcohol-insoluble grades are more effective in water-in-oil systems (7). 

To obtain individual phospholipids of greater than 50-60% purity, some form of selective adsorption process is usually required. Adsorption and distribution chromatography present these options. Treatment of the alcohol-soluble lecithin with alumina yields a fraction very rich in phosphatidylcholine and free of phosphatidylethanolamine and phosphatidylinositol (167). Although these products are available only in very limited quantities for highly specialized markets, products such as a lecithin containing up to 95% PC can be obtained commercially. 

Lecithin, an edible by-product of oil processing possessing a variety of useful functionalities, is primarily a mixture of phospholipids such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and phospha-tidic acid and contains minor quantities of other water-soluble or hydratable components such as glycolipids and oligosaccharides (78). The degumming of oil with... 

Cmde fractions can be obtained by solvent fractionation treatments utilizing lower alcohols, such as ethanol, or alcohol-water mixtures. The product is a soluble fraction rich in phosphatidylcholine, whereas phosphatidic acid and phophatidyli-nositol predominate in the insoluble fraction. The shift in the ratio of phosphatidylcholine to phosphatidylethanolamine improves the emulsification and antispattering capabilities of the soluble fraction. The products of this process can be used as they are or can be further purified with adsorbents. The soluble fraction is an excellent oil-in-water emulsifier and is predominately used in margarine. The acidic phospholipids of the insoluble fraction are used in water-in-oil systems. The chocolate manufacturing industry uses this fraction to increase the viscosity of chocolate masses, thereby reducing the requirement for cocoa butter (83, 84). 

---