Synthesis pathways plasmalogens

Although the mitochondria are the primary site of oxidation for dietary and storage fats, the peroxisomal oxidation pathway is responsible for the oxidation of very long-chain fatty acids, jS-methyl branched fatty acids, and bile acid precursors. The peroxisomal pathway also plays a role in the oxidation of dicarboxylic acids. In addition, it plays a role in isoprenoid biosynthesis and amino acid metabolism. Peroxisomes are also involved in bile acid biosynthesis, a part of plasmalogen synthesis and glyoxylate transamination. Furthermore, the literature indicates that peroxisomes participate in cholesterol biosynthesis, hydrogen peroxide-based cellular respiration, purine, fatty acid, long-chain... 

The results of Fig,6 are of particular interest, since they might suggest the existence of a new and important pathway for choline plasmalogen synthesis in brain. The mechanisms for the formation of this lipid in brain are in fact stiU unknown, in contrast to v4iat is known about ethanolamine plasmalogen synthesis. 

Figure 24-4. Biosynthesis of ether lipids, including plasmalogens, and platelet-activating factor (PAF). In the de novo pathway for PAF synthesis, acetyl-CoA is incorporated at stage, avoiding the last two steps in the pathway shown here.

As already noted, the final steps In the synthesis of cholesterol and phospholipids take place primarily In the ER, although some of these membrane lipids are produced In mitochondria and peroxisomes (plasmalogens). Thus the plasma membrane and the membranes bounding other organelles (e.g., Golgi, lysosomes) must obtain these lipids by means of one or more Intracellular transport processes. For example. In one Important pathway, phosphatidylserine made In the ER Is transported to the Inner mitochondrial membrane where It Is decarboxylated to phosphatidylethanolamlne, some of which either returns to the ER for conversion Into phosphatidylcholine or moves to other organelles. 

A genus of bacteria, termed the Sphingobacterium, produces sphingolipids by a pathway similar to that in mammals. Clostridia produce plasmalogens (l-alk-l -enyl lipids) by an anaerobic pathway clearly different from the Oj-dependent pathway in mammals (Chapter 9). Branched-chain fatty acids are also found in which the methyl group is inserted post-synthetically into the middle of the chain, in a manner analogous to cyclopropane fatty acid synthesis (Section 5.5). S-adenosylmethionine is also the methyl donor for these reactions. The biochemistry surrounding the formation of these and many other bacterial phospholipids remains to be elucidated. 

The data of the present work indicate that rat brain microsomes are able to methylate stepwise phospholipids to form phosphatidylcholine (Ptd-choline) and in addition to produce choline plasmalogen by similar pathway. The rate of synthesis of Ptd-choline by the... 

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