Serine phosphoglyceride

A very low level of a vinyl ether component has been reported in the serine phosphoglycerides, but not in any other types. Over the next 20 years, this field of study developed in a significant manner the contributions of a number of capable investigators are summarized in a very clear manner in a review by Debuch and Seng (1972). 

Abbreviations PA, phosphatidic acid DG, diacylglycerol TG, triacylglycerols IPG, CPG, EPG and SPG, inositol-, choline-, ethanolamine- and serine - phosphoglyceride. 

Values are the average S.E. of determinations carried out on 4 samples. EPG = ethanolamine phosphoglyceride CPG = choline phosphoglyceride SPG = serine phosphoglyceride IPG = inositol phosphoglyceride. ... 

The fatty acid composition of the serine phosphoglycerides was relatively simple (Marcus et al., 1962). The main saturated fatty acid was stearic, which was present in amounts equal to the sum of the unsaturated fatty acids, oleic and arachidonic. There was very little plasmalogen in the serine phosphoglycerides, as was also noted by Speer and Ridgway (1962). The fatty acid and aldehyde composition of inositol phosphatide (PI) was quite similar to that of PS. There were trace amounts of serine phosphoglycerides in the PI fractions, but it was felt that they did not contribute to the overall pattern. In contrast to the cephalin fractions, the lecithins contained large quantities of palmitic and oleic acids. As with platelet PS, the plasmalogen component in the lecithin fractions was small. 

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... 

Of further interest, there are no detectable alkenylacyl or alkylacyl phos-phoglycerides found in the serine- or inositol-containing phosphoglycerides or in the sphingomyelin fraction. 

Notwithstanding its limited status in the biochemical behavior of a cell, phosphatidylserine is nevertheless an interesting compound. It is the only amino acid-containing phosphoglyceride found in mammalian cells. Furthermore, it exists only in the diacyl form, with no evidence to date for an alkylacyl or alkenylacyl form. Thus, it can be described as a 1,2-diacyl-, m-glycero-3-phospho-L-serine. 

Phospholipase A2 Action. Incubation of phosphatidylserine with phospholipase A2 obtained from Crotalus adamanteus or Naja Naja snake venom will show that the serine-containing phosphoglyceride was smoothly and completely converted to a lysophosphatidylserine with liberation of 1 mol of fatty acid per mole of lipid P. The experimental procedure was the same as the one described before in this and in the previous chapter. The products of the reaction can be recovered by thin-layer chromatography on Whatman K6 plates in a solvent system of chloroform-acetone-methanol-acetic acid-water (4.5 2 1 1.3 0.5, v/v). 

An alternative route to isolation of serine is to eliminate the above sulfonic acid reation and simply crystallize the serine from the hydrolysate (filtered) by the addition of ethanol. The recoveries are not as good as with the above derivative procedure, and the latter also provides additional proof for the presence of serine in the phosphoglyceride preparation. 

The complex lipids in milk fat are comprised of the phosphoglycerides, phosphatidyl choline, phosphatidyl serine, phosphatidyl ethanolamine, phosphatidylinositol and plasmalogens. Also, the non-glyceride phospholipid, sphingomyelin, occurs in important amounts (Jensen, 2002). Bitman and Wood (1990) described the distribution of phospholipid classes in bovine milk and their fatty acid composition. The phospholipids comprise about 1% and cholesterol 0.4—0.5% of the total milk fat. These occur almost completely in the milk fat globule membrane. 

The major phosphoglycerides are derived from phosphatidate by the formation of an ester bond between the phosphate group of phosphatidate and the hydroxyl group of one of several alcohols. The common alcohol moieties of phosphoglycerides are the amino acid serine, ethanolamine, choline, glycerol, and the inositol. 

The structural formulas of phosphatidyl choline and the other principal phosphoglycerides—namely, phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl inositol, and diphosphatidyl glycerol—are given in Figure 12.5. 

In bacteria, the decarboxylation of phosphatidyl serine by a pyridoxal phosphate-dependent enzyme yields phosphatidyl ethanolamine, another common phospholipid. The amino group of this phosphoglyceride is then methylated three times to form phosphatidyl choline. S-Adenosylmethionine is the methyl donor. 

Phosphatidate is formed by successive acylations of glycerol 3-phosphate by acyl CoA. Hydrolysis of its phosphoiyl group followed by acyl- ation yields a triacylglycerol. CDP-diacylglycerol, the activated intermediate in the de novo synthesis of several phosphoglycerides, is formed from phosphatidate and CTP. The activated phosphatidyl unit is then transferred to the hydroxyl group of a polar alcohol, such as serine, to form a phospholipid such as phosphatidyl serine. 

Phosphoglycerides (the major phospholipids) contain glycerol, fatty acids, and phosphate. The phosphate is esterified to choline, serine, ethanolamine, or inositol. 

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