Membranes, natural lipoprotein

Penetration of membranes. Every living cell is surrounded by a semi-permeable membrane which is about four molecules in thickness. Such a membrane, usually lipoprotein in nature, is strongly charged, and its interior is almost inaccessible to ions. The difficulties in the way of the penetration of ions are (a) their relatively greater size, due to hydration, and (b) their charge, which is either similar to the portion of protein surface which they approach (resulting in repulsion) or is opposite (resulting in fixation). 

Carotenoids are also present in animals, including humans, where they are selectively absorbed from diet (Furr and Clark 1997). Because of their hydrophobic nature, carotenoids are located either in the lipid bilayer portion of membranes or form complexes with specific proteins, usually associated with membranes. In animals and humans, dietary carotenoids are transported in blood plasma as complexes with lipoproteins (Krinsky et al. 1958, Tso 1981) and accumulate in various organs and tissues (Parker 1989, Kaplan et al. 1990, Tanumihardjo et al. 1990, Schmitz et al. 1991, Khachik et al. 1998, Hata et al. 2000). The highest concentration of carotenoids can be found in the eye retina of primates. In the retina of the human eye, where two dipolar carotenoids, lutein and zeaxan-thin, selectively accumulate from blood plasma, this concentration can reach as high as 0.1-1.0mM (Snodderly et al. 1984, Landrum et al. 1999). It has been shown that in the retina, carotenoids are associated with lipid bilayer membranes (Sommerburg et al. 1999, Rapp et al. 2000) although, some macular carotenoids may be connected to specific membrane-bound proteins (Bernstein et al. 1997, Bhosale et al. 2004). 

However, some researchers have preferred to develop hydrophobic sensitizers and, indeed, it needs to be recognized that some degree of hydrophobicity is advantageous to facilitate the penetration of biological membranes. In these cases a delivery vehicle is required, and this must be biocompatible, and provide a suspension which is stable up to the point at which the natural transport system (e.g., albumin or lipoprotein or both) takes over. 

It is commonly known that lipids, carbohydrates, and glycolipids are present in the Golgi apparatus (27). The determination of the components that react with the ZIO mixture was carried out by removing each component from tissues before incubation in the ZIO mixture. After lipid extraction by acetone (14), chloroform-methanol (15), or propylene oxide (27), no osmium-zinc precipitates could be detected in structures that normally reacted with ZIO. Blumcke et al. (15) summarized the nature of the lipids that react with the ZIO mixture as follows lipids and lipoproteins of cell membranes, neutral fat droplets (41), and lipid globules of type II pneumocytes and alveolar macrophages were, however, not as electron dense as the normally reactive lamellae containing highly unsaturated fatty acids. 

In preparation for the next part of this course on the nervous system, we have to consider a very important site of action in and on the cell membrane. It is believed that most drugs act at a specific site called a receptor located within a target cell or in/on its membranes. Receptors are proteins, glycoproteins, lipoproteins, or nucleoproteins in their biochemical makeup. These receptors serve a natural purpose in that they form complexes with natural chemicals and these complexes trigger responses within a cell. A receptor, then, is a chemical entity which interacts with an... 

The main precursors of plasma HDL are most likely disk-shaped bilayers composed of PL and protein and secreted by the liver and intestine. HDL are also derived from the surplus surface material removed from TG-rich lipoproteins during lipolysis. HDL are involved in the net transfer of cholesterol from peripheral tissues to the liver, where it can be eliminated or recirculated. This process is initiated by the uptake of FC from cell membranes into the HDL. The nature of this uptake is not known but may involve binding of HDL to the membrane. 

Lipids are a complex group of substances, which include the long-chain fatty acids and their derivatives, sterols and steroids, carotenoids, and other related isoprenoids. It is evident that the term lipid denotes a wide range of compounds that appear to have little obvious interrelation. However, although these compounds possess widely different structures, they are derived in part from similar biological precursors and exhibit similar physical and chemical characteristics. Furthermore, most lipids occur naturally in close association with protein, either in membranes as insoluble lipid-protein complexes or as soluble lipoproteins of the plasma. 

Effective concentrations of camosine vary fix)m 0.5 to 50 mM depending on the model used but in all cases are within the range of its physiological concentrations. In protection of blood plasma lipoproteins Ko.s for suppression of their oxidation was 3.5 mM, Ko,5 for neutralization of hydroxyl radicals was about 1.5 mM, and for protection of muscle membrane lipids against peroxidation — 20 mM [52]. It was important to compare efficiency of camosine with that of other natural antioxidants (see Table 5). 

Due to its lipophilic nature, vitamin E accumulates in cellular membranes, fat deposits and other circulating lipoproteins. 

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