Phospholipid occurrence

The Fatty Acid Composition of Phospholipids Occurrence of Phospholipids in Foods... 

Cholestasis can lead to a similar occurrence of a broad-/l-band. This results from the presence of lipoprotein X [Lp(X)], which migrates slightly closer to the application point than LDL but cannot be separated from it. Lp(X) is derived from bile lipids including free cholesterol and phospholipids that acquired apolipoproteins after they were released into the blood. However, this Lp(X)-derived broad-jS-band migrates closer to the application point than LDL and can be distinguished from the broad-/l-band of familial dysbetalipoproteinemia (type III). 

LECITHIN. Lecithin and other phospholipids are of universal occurrence in liv ing organisms. They are constituents of biological membranes and are involved in permeability, oxidative phosphorylation, phagocytosis, and chemical and electrical excitation. 

Upon heating, very pure uniform phospholipids show an additional small transition below the crystalline to liquid crystalline phase transition. According to Janiak et al. [104], this is due to a structural change in the head groups. The occurrence of pretransition is mainly determined by the structure of the head groups and their environment. 

Shelf life Shelf-life issues that need to be addressed include avoidance of pre-administration leakage of the liposome-associated drug (retention loss), size stability (occurrence of fusion or aggregation) and phospholipid degradation (occurrence of peroxidation and hydrolysis). 

A number of workers have observed amino acids in lipide extracts, including those of microbial origin (11, 12, 17, 29). Recently, Macfar-lane (34) has reported that most of the phospholipide in Clostridium welchii is bound to amino acids and that some of this material occurs as the O-amino acid ester of phosphatidylglycerol. The relatively prominent occurrence of lipides in cell membranes has led to the recurrent suggestion that transport of hydrophilic substances through such membranes would be greatly facilitated by combination with hydrophobic substances. Consequently, most workers who have observed the incorporation of amino acids into lipide fractions quite naturally... 

The corresponding dimeric analogues of lipids are also important, owing to their occurrence in biological membranes. For example cardiolipins, which constitute a class of complex dimeric phospholipids that occur mainly in the... 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

The temperature dependence of the thickness of foam bilayers shows the occurrence of a first-order phase transition of melting of hydrocarbon tails of the phospholipid molecules. This melting is realised at a temperature very close to the temperature of the corresponding phase transition in fully hydrated water dispersions of phosphatidylcholines. This result is in agreement with the theoretical considerations of Nagle [436] for the decisive role of van der Waals attractions between hydrocarbon chains of phospholipid molecules for the chainmelting phase transition in bilayer systems. 

Figure 12.1. Overview of eicosanoid metabolism, a Stmetures of eicosanoid precursor fatty acids, and their occurrence in membrane phospholipids (PC is shown as an example). Araehidonie acid is the prototypic precursor eicosatrienoic and eicosapen-tanoic acid differ from it by the a lackingor an additional double bond, respectively, b Conversion of preemsor fatty acids occurs by various enzymes, notably cyclooxygenases and lipoxygenases. Isoprostanes are non-enzymahe derivatives that may form in vivo at appreciable rates one characterishc featme is that they occur as racemic mixtures. Their physiological significance is not entirely clear.

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

In liver cirrhosis, there are several changes reduction in LCAT (with cholesterol ester fall ), HDL and LDL as well as in VLDL, with a corresponding change in their distribution pattern occurrence of hypertriglycerid-aemia and atypical lipoproteins reduction in phospholipid synthesis (28), possibly with greatly impaired structure and function of the biomembranes. Hepatic extraction of bile acids is reduced with the result that they reach the peripheral circulation - even in the early stages of cirrhosis - and give rise to increased serum values. Bile acids have cholestatic and cytotoxic effects. When bile acid metabolism is markedly compromised, enteral absorption of fat-soluble vitamins is impeded, so that A, D, E and K hypovitaminoses may be observed. 

In the first case amiodarone was given after the onset of the peripheral blood film abnormalities and the only change in the bone marrow was the occurrence of the granulomata. The authors proposed that the granulomata had occurred because of phospholipid accumulation. 

IRRAS has become an important tool for studying Langmuir monolayers and LB films. Much work has been done, in particular on the acyl chain conformational order in monolayers of single chain amphiphiles and phospholipids as a function of surface pressure and on the occurrence of phase transitions . Examples of IRRAS studies on LB films can be found Reviews of the applications of IRRAS are given by Dluhy et al. and Mendelsohn et al. ). 

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