Phosphatidyl choline biosynthesis

Fig. 6.11. Formation and function of diacylglycerol. The figure schematically shows two main pathways for formation of diacylglycerol (DAG). DAG can be formed from PtdInsP2 by the action of phospholipase C (PL-C). Another pathway starts from phosphatidyl chohne. Phospholipase D (PL-D) converts phosphatidyl choline to phosphatidic add (Ptd), and the action of phosphatases results in DAG. Arachidonic add, the starting point of biosynthesis of prostaglandins and other intracellular and extracellular messenger substances, can be cleaved from DAG. PKC protein kinase C Ptdins phosphatidyl inositol.

Holub, B.J., Piekarski, J. and Leatherland, J.F. (1977). Differential biosynthesis of molecular species of 1,2-diacyl-glycerols and phosphatidyl cholines in cold and warm acclimated goldfish. Lipids 12,316-318. 

Zwingelstein, G., Malak, A.N. and Brichon, G. (1978). Effect of environmental temperature on biosynthesis of liver phosphatidyl choline in the trout. Journal of Thermal Biology 3,229-233. 

Membrane proteins have a unique orientation because they are synthesized and inserted into the membrane in an asymmetric manner. This absolute asymmetry is preserved because membrane proteins do not rotate from one side of the membrane to the other and because membranes are always synthesized by the growth of preexisting membranes. Lipids, too, are asymmetrically distributed as a consequence of their mode of biosynthesis, but this asymmetry is usually not absolute, except for glycolipids. In the red-blood-cell membrane, sphingomyelin and phosphatidyl choline are preferentially located in the outer leaflet of the bilayer, whereas phosphatidyl ethanolamine and phosphatidyl serine are located mainly in the inner leaflet. Large amounts of cholesterol are present in both leaflets. 

In the present paper we describe the results of our studies on the biosynthesis and turnover of phosphatidyl choline (PC) in rat brain. We will discuss the apparent lack of phospholipid transport mechanisms in cell membranes of the CNS and will show that all brain subcellular fractions seem to have a certain degree of independence for the synthesis of this phospholipid. The presence of rapid and slow turnover pools for PC will be described and possible explanations for these findings will be presented. 

The D-a,/8-diglycerides are the naturally occurring diglycerides, found as hydrolysis products of phosphatidyl choline acted upon by phospholipase C (chapter IV, 2) or as intermediates in the biosynthesis of a number of glycero-phosphohpids and triglycerides. 

Our results show that the components of the phosphatidyl inositol cycle are present in suspension cultured tobacco cells, which is in agreement with previous report (5). The incorporation of [ P] into phosphoinositides was rather fast, preferentially into PIP which peaked at 5 min (70% of tot. inc.) and then levell (fig lb). The fast incorporation of [ P] into PIP, PIP2 and PA is considered as a the result of rapid phosphoinositide turnover (3). Moreover, changes in the [ P] PA to PI ratio are good indicators of the rapid biosynthesis of the major membrane phospholipids. In tobacco cells we noted a lack of response of PIP2 to cold stress or to added substances as choline chloride or ABA to the culture medium. Cold temperature resulted in an enhanced level of PA and PIP suggesting that Pl-kinase and the enzymes involved in PA formation (e.g. DAG-kinase and phospholipases C and D) remained active in cold. 

Recent work in the general field of lipid biochemistry has revealed that although the carbon skeleton of serine is extensively used in the biosynthesis of ethanolamine (and hence choline and acetylcholine), decarboxylation of free serine does not occurnor is the ethanolamine formed from serine found free. It appears that the hydroxyl group of serine is first esterified with phosphatidic acid by reaction with cytidine diphosphate diglyceride (Figure 21). Phosphatidyl... 

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