Milk, phosphatides

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 knowledge of exact turnover figures is of interest when comparing, for example, the amount of fat metabolized with the amount of phospha-tides turned over. In many cases however we can draw conclusions merely by comparing the specific activity of the phosphatides present in an organ under varying conditions. Such a procedure is applied in the study of the effect of ingested fat on the rate of renewal of phosphatides, of the effect of choline, cholesterol, and other substances on the turnover of liver phosphatides, of the site of fonnation of the yolk, embryo, and milk phosphatides, and so on. 

In many in vitro studies the acylation of the sn-3 position appears to be the rate-limiting step in TG synthesis. It has been suggested that the intracellular concentration of medium chain fatty acids may limit the final acylation reaction in TG synthesis (Dimmena and Emery 1981). Another theory is that the concentration of phosphatidate phosphatase, the enzyme that hydrolyzes the phosphate bond in phospha-tidic acid, yielding DG, may be the limiting factor (Moore and Christie 1978). The DG acyltransferase responsible for the final acylation of milk TG has been studied in mammary tissue from lactating rats (Lin et al. 1976). It was observed to be specific for the sn-1,2 DG, with very little activity observed with the sn-1,3 or sn-2,3 DG. It exhibited a broad specificity for acyl donors. The acyl-CoA specificity was not affected by the type of 1,2 DG acceptor offered, which implies that the type of fatty acid introduced into the glycerol backbone was not influenced by the specificity of subsequent acylation steps. However, the concentration of acyl donors will affect the final acylation. It was ob-... 

Duin, H. van. 1958. Investigation into the carbonyl compounds in butter. III. Phosphatide-bound aldehydes. Neth. Milk Dairy J. 12, 90-95. 

Synthesis of phosphatidylinositol follows a slightly different pathway from the other phosphoglycerides. Phosphatidate is activated by CTP-phos-phatidate cytidyl transferase to form CDP-diacylglycerol. Free inositol is then incorporated by CDP-diacylglycerol inositol transferase, with the release of CMP. Phosphatidylinositol usually constitutes less than 5% of total milk phospholipids (Bitman and Wood, 1990). 

Koops, J., Tarassuk, N.P. 1959. The effect of various processing treatments on the partition of phosphatides between the fat phase and the milk plasma. Neth. Milk Dairy J. 13, 180-189. 

Chylomicrons, the diameters of which range from 1000-10,000 A, are small droplets of tria-cylglycerol stabilized in the aqueous medium by a membrane-like structure composed of protein, phosphatides and cholesterol. The role of chylomicrons in blood is to transport triacylglycerols to various organs, but preferentially from the intestines to adipose tissue and the liver. The milk fat globules (cf. 10.1.2.3) have a structure... 

Figure 44 shows the flow curve for various levels of a typical combination of emulsifiers in milk chocolate [108], The synergy between PGPR and lecithin or ammonium phosphatide is clearly shown. There is no difference in the yield obtained whether lecithin or ammonium phosphatide is used. The slope of the curve, however, shows that ammonium phosphatide is somewhat superior with respect to the reduction of viscosity. A dose of 0.3% PGPR lowers the yield value from 26 Pa to 2 Pa. An addition of 0.9% lecithin lowers the yield only to 24 Pa. 

The origin of phosphatides of goat milk (15,16) was investigated along similar lines to studies of the origin of yolk phosphatides. Phosphatides extracted from milk a few hours after subcutaneous injection of labeled phosphate were found to be much more active than those present in the plasma. Thus, phosphatide molecules of the milk cannot have originated in the plasma, but must have been built up mainly in the mammary gland. The specific activity of the phosphatide phosphorus extracted from the... 

Activity of Phosphatide Phosphorus op Milk and Organs of a Goat (15) ... 

---