Sterols membrane potentials

Nevertheless, it was clearly demonstrated that bras-sinosteroids, like the other steroids investigated here, interact with the plasmalemma thereby showing short-term effects on the membrane potential and/or medium acidification. In some cases these effects correlate with stomata 1 movement and solute uptake into leaves or conducting tissue. The results are compatible with an effect of these substances on the membrane-embedded moiety of the plasmalemma H -ATPase yielding a modified proton pump rate. They support, therefore, the "Annulus Hypothesis" according to which sterol effects are caused by direct lipid-protein binding. 

The first committed step in the synthesis of triterpenoid saponins involves the cyclisation of 2,3-oxidosqualene to give one of a number of different potential products [9]. Most plant triterpenoid saponins are derived from oleanane or dammarane skeletons, although lupanes are also common [9]. This cycHsation event forms a branchpoint with the sterol biosynthetic pathway, in which 2,3-oxidosqualene is cyclised to lanosterol (in animals and fungi) or to cycloartenol (in plants) (Fig. 2). Sterols are important membrane constituents and also serve as precursors for hormone biosynthesis. 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

The excitation spectrum of di-8-ANEPPS is altered when it lines up (symmetrically or asymmetrically) with the membrane dipoles causing electronic redistributions within the probe molecule (see e.g. Fig. 5a). This promotes red or blue shifts in the excitation spectrum depending on the magnitude and direction of the dipole moment of the ambient environment that the probe finds itself in as shown in Fig. 5b. Preparation of membranes with sterols etc (ie that possess quite different dipole-moments to PC) promote changes in the membrane dipole potential, and significant variations of the intensity and position of the excitation maximum are observed. The excitation spectrum of di-8-ANEPPS in phosphatidylcholine (PC) membranes for example is significantly altered when 15mol% of either 6-ketocholestanol (KC) or phloretin are added to such membranes. In the case of phloretin the difference spectmm has a minimum at 450 nm and a maximum at 520 nm (Fig. 5b). In the case of KC, however, the difference spectrum has a maximum at 450 nm and a minimum at 520 nm, which is the opposite effect to that of phloretin. 

Brockman, H. Dipole potential of lipid membranes, Chem. Phys. Lipids, 73, 57,1994. Bush, F.S., Adams, R.G., and Levin, I.W. Structural reorganizations in lipid bilayer systems effect of hydration and sterol addition on Raman spectra of dipalmitoylphosphatidylcholine multilayers. Biochemistry, 19, 4429, 1980. 

The enzymes responsible for the conversion of lanosterol to cholesterol, as were those for the conversion of farnesyl pyrophosphate to squalene and lanosterol, are all integral membrane-bound proteins of the endoplasmic reticulum. Many have resisted solubiUzation, some have been partially purified, and several have been obtained as pure proteins. As a consequence, much of the enzymological and mechanistic studies have been done on impure systems and one would anticipate a more detailed and improved understanding of these events as more highly purified enzymes become available. Many approaches have been taken to establish the biosynthetic route that sterols follow to cholesterol. Some examples are synthesis of potential intermediates, the use of inhibitors, both of sterol transformations and of the electron transfer systems, and by isotope dilution experiments. There is good evidence that the enzymes involved in these transformations do not have strict substrate specificity. As a result, many compounds that have been found to be converted to intermediates or to cholesterol may not be true intermediates. In addition, there is structural similarity between many of the intermediates so that alternate pathways and metabolites are possible. For example, it has been shown that side-chain saturation can be either the first or the last reaction in the sequence. Fig. 21 shows a most probable series of intermediates for this biosynthetic pathway. 

Sterols are under continuous investigation because of their role as precursors of hormones and their function in the central nervous system and membranes. Even though other methods are available for cholesterol determination, GC measurements are still fairly common when high sensitivity is required. Many other sterols are abundant in nature determination of sterol profiles by GC is often of chemotaxo-nomic value in investigating bacteria, plants, marine organisms, etc. In addition, the products of sterol oxidation have received some attention for their cytotoxicity, mutagenicity and carcinogenic potential [301,302]. 

Sterols are also potentially excellent biomarker compounds due to their stability and the diversity of their structures. They are present in all eukaryotes, and in marine material such as sediments, detection of 25 sterols or more is common. They share with phospholipids a structural function in membranes where, because of their unique hydrophobic and steric properties, they act as specific internal regulators of membrane fluidity and influence various membrane functions and membrane associated enzymes [72]. 

Finally it is clear that, unlike long-chain fatty acids which can appear in portal blood in limited amounts[32], cholesterol is absorbed exclusively into the lymphatic circulation[32,33]. A small amount of unesterified cholesterol can appear in lymph and is largely associated with chylomicron and lipoprotein "coat , or limiting membrane. However, this coat has a limited capability for significant transport, and the mass transport cholesterol (like that of triglycerides) requires esterification and incorporation into the lipoprotein core. Thus, the transfer of significant amounts of cholesterol from intestinal lumen to l3onph is associated with extensive esterification (70-90%) with fatty acids, and this occurs in the mucosal epithelial cells[11]. There are two intestinal enzymes potentially important in the esterification of unesterified cholesterol cholesterol esterase, or sterol ester hydrolase (EC. 

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