THE ANIMAL PHOSPHOLIPIDS: THEIR STRUCTURE, METABOLISM and BIOLOGICAL SIGNIFICANCE
Authors: DAWSON R. M. C. 
Published: January 2008

Phospholipids are universally present in all mammalian tissues and in all parts of the cell, including the mitochondria, microsomes and nucleus. They appear often to be attached to the tissue proteins by some unknown type of bonding.

The use of physical methods of fractionation to separate crude lipids extracted from tissues has resulted in the discovery of a number of new phospholipids apart from lecithin, cephalin and sphingomyelin, notably phosphatidylserine, phosphatidylinositol and diphosphoinositide.

As isolated, the phosphoglycerides all have the a structure and the L configuration.

Evidence is accumulating that individual animal tissues can synthesize their complete requirements of new phospholipids without resource to other organs. In all tissues, even in the adult animal, the phospholipids are continually being broken down and resynthesized at the same rate, so that under normal physiological conditions a constant concentration is maintained.

The mechanism of phospholipid biosynthesis has only been ascertained up to now for the phosphoglycerides: lecithin and phosphatidylethanolamine. With these compounds a phosphorylated base molecule (phosphorylcholine or phosphoryl‐ethanolamine) first reacts with cytidine triphosphate, forming an activated complex, cytidine diphosphate ‘base’. This then combines with a diglyceride molecule, forming the phosphoglyceride, and splitting off cytidine monophosphate. It is possible that the diglyceride needed by the system is formed by the dephosphoryla‐tion of phosphatidic acid, a lipid which is enzymically synthesized in tissues from α‐glycerophosphoric acid and two coenzyme A‐activated fatty acids.

One route for the catabolism of lecithin and phosphatidylethanolamine appears to be through the lysophosphoglycerides and glycerophosphorylated bases. The glycerophosphate finally formed can either be used for phosphoglyceride resynthesis or it can enter the glycolytic system of the tissue.

Certain isotopic evidence suggests that, as well as this catabolic process, the phosphorylated base ‘units’ may turnover independently and more rapidly than the glyceride part of the phospholipid molecule. Possibly the recent and unconfirmed demonstration of a phospholipase C, which forms phosphorylcholine from lecithin, may be connected with this process.

The rate at which individual phospholipids turnover in various organs has still to be ascertained. Many previous estimates are unsatisfactory because inadequate experimental results have been applied to theoretically derived formulae expressing turnover rates. For similar reasons it is often extremely difficult to interpret reported changes in the incorporation rate of isotopes into phospholipids during physiological activity.

No definite metabolic role can yet be ascribed to phospholipids which would account for their rapid turnover in tissues. The evidence is against their being obligatory intermediates, either in the resynthesis of triglycerides during fat digestion or in the oxidation of neutral fat. Neither do they appear to play a direct chemical role in the transportation of fatty acids between tissues.

There are indications that the structural integrity of the cell is dependent upon phospholipids, and it seems likely that in the complex enzyme systems present in mitochondria they can act as structural units which orientate the active centres of the enzymes for maximal activity of the complex. It is also possible that they play an essential part in the transportation of ions through the cell membranes.