Turnover of Phosphatides

As was shown by early experiments, the presence of labeled phosphatides can be detected in the tissues shortly after administration of labeled 

The rapid increase in labeled phosphatide content of the liver indicates 

The above data indicate the formation of a labeled phosphatide mixture mainly composed of lecithins, cephalins, and sphingomyelins. The rates of formation of these constituents of the phosphatides vary, though the differences are, in most cases, as discussed on page 137, not very pronounced. The maximum activity of the total labeled phosphatide content of the rat is reached after the lapse of about 24 hours while in the liver the most active organ, a maximum content is obtained about 10 hours after administration of labeled phosphate. 

In the fasting mouse, Hodge and his associates (93) found the specific activity of all phosphatide fractions, with the exception of the a-cephalin fraction, to increase appreciably. A maximum specific activity for the a-cephalin was reached on the second fasting day. 

If we make the assumption that phosphate ions or phosphorus-containing precursors which attain rapid equilibrium with the phosphate ions are incorporated into the phosphatide molecule, we can determine the rate of renewal of phosphatide molecules by comparing the specific activity of the phosphatide phosphorus at the end of the experiment with the mean specific activity of the intracellular inorganic phosphorus prevailing during the experiment. This calculation involves the further assumption that it is the intracellular, inorganic phosphorus which is incorporated into the phosphatide molecule. The rate of renewal of phosphatides extracted 

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W. Turnover of Phosphatides in Cell Nuclei. Effect of Roentgen Rays... 

Some time ago Artom and colleagues (14) showed that feeding of oil promotes the turnover of phosphatides. Some of their results are recorded in Table XIV. 

Choline phosphate inhibits turnover of phosphatides in liver. The inhibition appears to be limited to the noncholine phosphatide fraction. Choline phosphate, as a unit, is probably not utilized in the synthesis of phosphatides (147). 

W. TURNOVER OF PHOSPHATIDES IN CELL NUCLEI. EFFECT OF ROENTGEN RAYS ON PHOSPHATIDE TURNOVER... 

A comparison of turnover rates of phosphatides in sarcoma (83) shows that no pronounced difference is found between rates of renewal of phosphatides in nuclei and in tissue, and correspondingly in cytoplasm. In nuclei of liver, however, rate of renewal of phosphatides clearly lags behind the rapid rate of turnover of these compounds in cytoplasm, as is seen in Table XXXI. To what extent failure to detect appreciable differences between turnover of phosphatides in nuclei and in tissue of sarcoma is due to a relatively rapid interchange between phosphatide molecules of cytoplasm and those of nuclei is not yet elucidated. 

The effect of Roentgen rays on turnover rate of phosphatides present both in tissue and in nuclei was investigated as well. Two groups of twelve rats, after irradiation with 1000 r, are given labeled phosphate, while nonirradiated, control groups are treated in a similar way. After the lapse of two hours the animals are sacrificed and the sarcoma and livers are pooled separately. An aliquot is used in the determination of specific activities of inoi anic and phosphatide phosphorus of the tissue, while from the bulk of the material cell nuclei are isolated by the method of Bounce (42). The specific activities of the corresponding phosphorus fractions of the nuclei are also determined, and furthermore the activity of the inorganic phosphorus of the pooled blood plasma is measured. As seen in Tables XXXI and XXXII, the rate of turnover of phosphatides in liver... 

The expected incorporation of mi/o-inositol into phospholipids has been observed in two31 126 and in kidney preparations in vitro,138 and some progress has been made toward understanding the enzymic mechanism involved.101 1 138,139 It appears that the inositol is neither phosphorylated nor converted to a coenzyme prior to incorporation. Hokin and Hokin140 have made the interesting observation that the turnover of myo-inositol in the monophosphoinositide of brain and pancreas slices is stimulated by acetylcholine. The effects of various drugs on this turnover have been studied by other workers. It has been postulated that the decomposition and resynthesis of inositol phosphatide is a part of the membrane-transport process.140... 

Munnik, T., Van Himbergen, J.A.J., Ter Riet, B., Braun, F.-J., Irvine, F.F., Van den Ende, H. and Musgrave, A., 1998b, Detailed analysis of the turnover of polyphosphoinositides and phosphatidic acid upon activation of phospholipases C and D in Chlamydomonas cells treated with non-permeabilizing concentrations of mastoparan. Planta 207 133-145. 

Raymond, V., Leung, P.C.K. and Labrie, F. (1983). Stimulation of PGp2a of phosphatidic acid-phosphatidylinositol turnover in rat luteal cells. Biochem. Biophys. Res. Commm., 116, 39-46... 

The hydrolysis of phosphatidylinositol may be of special biological interest [32,33]. This phospholipid consists largely of l-stearoyl-2-arachidonoyl-glycero-3-phosphoinositol in many tissues (e.g. platelets, brain, liver and mouse pancreas [34,35]). Stimulation of several endocrine or exocrine glands has been found to involve an increased turnover of this phospholipid and formation of phosphatidic acid (see ref. 33 for review). This phosphatidylinositol response might be important for hormone induced mobilization of Ca ", at least in some tissues (see refs. 33, 36, 37 and for critical comments refs. 38 and 39). 

Fig. 1. Stimulus-induced turnover of phosphatidylinositol 4,5-bisphosphate (PIPo) and the role of turnover products in signal transduction. PI, phosphatidylinositol PIP, phosphatidylinositol 4-phosphate PIPo, phosphatidylinositol 4,5 bisphosphate IP, inositol trisphosphate IP, inositol tetrakisphosphate IPo inositol bisphosphate IP, inositol monophosphate ER, endoplasmic reticulum DG, diacylglycerol MG, monoglyceride AA, arachidonic acid PA, phosphatidic acid. [Adapted from 38]...

Although there is an increased turnover of phosphatidylinositol in stimulated tissue, the primary action of acetylcholine is to cause a net breakdown of this phosphatide. The breakdown was first observed as a loss of radioactivity from prelabeled phosphatidylinositol in response to acetylcholine in the avian salt gland (Hokin, M.R. ... 

Formation and turnover of stearoyl,arachidonoyl phosphatidic acid... 

The I PI-phosphatidic acid formed in the stimulated state in pancreas is lost from the phosphatidic acid fraction, together with the specific disappearance of (18 0,20 4)phosphatidic acid. This indicates that the (18 0,20 4)phosphatidic acid which is formed from phosphatidylinositol in the stimulated state is the only species of phosphatidic acid which undergoes increased turnover of its phosphate group. 

To summarize these changes which I have discussed, stimulation of salt gland and pancreas with acetylcholine leads to a breakdown of phosphatidylinositol. Under some conditions, the (18 0,20 4)-diglyceride moiety of this phosphatidylinositol is converted almost quantitatively to (18 0,20 4)phosphatidic acid. During the stimulated state, this novel species of phosphatidic acid undergoes continuous turnover of its phosphate group, presumably with (18 0, 20 4)diglyceride as an intermediate. It is specifically used for the resynthesis of phosphatidylinositol when stimulated tissue reverts to the unstimulated state (Fig. 1). 

The hormone pancreozymin (cholecystokinin-pancreoz3miin) stimulates the protein secretory cycle in the exocrine pancreas (Harper Raper, 1943) it also elicits phosphatidylinositol breakdown, formation of phosphatidic acid, and increased turnover of the polar headgroups of these phosphatides. These effects are essentially the same as those observed in response to acetylcholine (Hokin, M.R.,... 

To obtain renewal figures that are certain to be correct, we should know the specific activity of the immediate phosphorus-containing precursor of the phosphatide molecule, and insofar as different paths of phosphatide formation exist, each involving a phosphorus-containing precursor, we should know the specific activity of all these precursors. Furthermore, if the site of formation is inside the cell, this site need not necessarily involve the whole cell but only part thereof. In the latter case, it is the specific activity of the immediate precursor in this part of the cell, not that of the whole cell, that is to be considered when calculating the rate of renewal. As is seen from the above considerations and from the more detailed discussion by Chaikoff (32), difficulties are involved in attempts to arrive at an exact measure of the rate of phosphatide turnover. 

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. 

A comparison of turnover rate of phosphatides and of other compounds, not only in different organs but even in different parts of cells, is a problem of great importance, solved so far only in a special case the rate of renewal of c3rtoplasm phosphatides has been compared with the rate of renewal of nuclear phosphatides, as described on page 169. 

The enhanced phosphatide turnover in intestinal mucosa of the rat which follows feeding of fat may be interpreted as indicating the role of phosphatide turnover in fat absorption. 

Patterson and his associates found that choline deficiency resulted in decreased phosphatide turnover as well as in decreased concentration of phosphatides in liver (137) and kidneys (138) of rats. The fatty liver and... 

Further evidence in support of the decreased rate of phosphatide turnover in choline deficiency is provided by studies (27) in which choline containing was fed to both normal and choline-deficient animals. Incorporation of new choline (tagged by the presence of was found to be retarded in the choline-deficient rat. 

The specific activity of phosphatides extracted from rabbit and rat kidney is lower than that of phosphatides from small intestine and liver at early intervals after administration of radioactive phosphate. After the lapse of six hours the same result was obtained in experiments with dogs, but after eighteen hours the specific activity of the kidney phosphatide remained lower than that of liver and about equal to that of small intestine. At 98 hours the specific activities of the phosphatides in all three tissues wore roughly the same (32). Acidosis induced by ingestion of ammonium chloride increases turnover of kidney phosphatides, according to Weiss-berger (181). Administration of phlorizin did not influence turnover rate of phospatides in kidneys of the rat (182). 

The turnover rate of phosphatides is found to be increased in muscle of rats maintained on a diet deficient in fat (91). In the fasting mouse, all phosphatide fractions, with the exception of -cephalin, which remained constant, showed a large increase in rate of renewal and reached a maximum on the second day of fasting (93). 

If the phosphatide molecules of carcinomatous tissue were replaced at a high rate by labeled molecules, and such molecules were given off by the tumor to the circulation shortly after the administration of labeled phosphate, the presence of carcinomatous tissue could possibly be diagnosed by determination of the activity of plasma phosphatides. The facts, however, that the phosphatide turnover of neoplastic tissue is much slower than turnover in liver and intestinal mucosa and that phosphatide molecules in the circulation were to a very large extent built up in the liver frustrate this possibility. 

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