Cytoplasm, phosphatides

It can be seen from Figure 1 that the choline-containing phospholipids, phosphatidylcholine and sphingomyelin are localized predominantly in the outer monolayer of the plasma membrane. The aminophospholipids, conprising phosphatidylethanolamine and phosphatidylserine, by contrast, are enriched in the cytoplasmic leaflet of the membrane (Bretcher, 1972b Rothman and Lenard, 1977 Op den Kamp, 1979). The transmembrane distribution of the minor membrane lipid components has been determined by reaction with lipid-specific antibodies (Gascard et al, 1991) and lipid hydrolases (Biitikofer et al, 1990). Such studies have shown that phosphatidic acid, phosphatidylinositol and phosphatidylinositol-4,5-fc -phosphate all resemble phosphatidylethanolamine in that about 80% of the phospholipids are localized in the cytoplasmic leaflet of the membrane. 

Phospholipid turnover also takes place in an asymmetric manner. The enzymes responsible for phospholipid turnover in response to receptor-mediated phospholipase c activation are active from the cytoplasmic surface of the membrane. Likewise, diacylglycerol kinases converting the product of phospholipase c back into the key intermediate of phospholipid biosynthesis, phosphatidic acid, are also located on the cytoplasmic smface of the membrane (Sanjuan et al., 2001). 

Achve translocation of phospholipids aaoss the plasma membrane has been demonstrated both from the inner to the outer leaflet and from the outer to the inner leaflet. The translocation processes specifically transport phosphatidyserine and phosphatidylethanolamine from the cytoplasmic to the outer surface of the membrane while choline phosphatides are transported from the outer to the cytoplasmic surface. The rate of translocahon, in general, is greater for the amino phospholipids compared with the choline phospholipids. 

As in the cAMP system, multiple mechanisms damp or terminate signaling by this pathway. IP3 is inactivated by dephosphorylation diacylglycerol is either phosphorylated to yield phosphatidic acid, which is then converted back into phospholipids, or it is deacylated to yield arachidonic acid Ca2+ is actively removed from the cytoplasm by Ca2+ pumps. 

Cell nuclei contain appreciable amounts of phos phatides. About 12% of the dry nuclei of rat liver is composed of phosphatides the corresponding value for dry liver tissue is 8. In hepatoma and adenocarcinoma of the rat, 8% and 7% are given for the phosphatide content of dry nuclei and dry tissue, respectively (183). Other authors state that phosphatide content of rat liver nuclei varies between 7.5 and 10.8% (42). Since nuclei make up but a minor part of wet tissue, the greater part of the phosphatides is located in cytoplasm. 

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. 

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