Myelin, protein content

Peripheral myelin protein 22 (PMP22) has a molecular weight of 22 kDa and represents 2-5% of the total myelin protein content. Despite its name, PMP22 is not specific to the PNS inasmuch as it is expressed, albeit at low levels, in other... 

The lipid, water, and protein contents of certain tissues vary markedly as a function of age. For example, the adipose tissues of neonates contain about 55% water and 35% lipids, whereas the corresponding figures for the adult are about 25% and 70%, respectively (Friis-Hansen, 1971). The proportion of water in skin falls as a function of age, due to an increase in collagen. The water contents of liver, brain, and kidneys decrease from birth to adulthood by 5-15%. The decrease in water contents of liver and kidneys is primarily due to an increase in protein, whereas this change in the brain is due to an increase in myelin. The overall composition of muscle in terms of lipid and water does not seem to vary with age (Dickerson Widdowson, 1960). 

MBP constitutes about 30% of the protein content of myelin. In fact, the MBPs constitute a family of proteins comprising many isoforms (reviewed in Campagnoni... 

Membranes differ in their protein content. Myelin, a membrane that serves as an insulator around certain nerve fibers, has a low content of protein (18%). Relatively pure lipids are well suited for insulation. In contrast, the plasma membranes or exterior membranes of most other cells are much more active. They contain many pumps, channels, receptors, and enzymes. The protein content of these plasma membranes is typically 50%. Energy-transduction membranes, such as the internal membranes of mitochondria and chloroplasts, have the highest content of protein, typically 75%. 

Myelin is a major component of neural tissue lipids and contains unique proteins such as myelin basic protein. Changes in cellular lipid composition can reflect demy-elination, where these changes may consist of a decrease in myelin or protein/lipid with an increase in water content accompanied by the appearance of cholesterol esters (Norton 1984). Myelin basic protein content or synthesis may be determined to quantify myelination in the brain or reactive synthesis secondary to a toxic insult (Conti et al. 1996 Hamano et al. 1996). 

In a review on membrane proteins, Guidotti (1972) has classified membranes into three types on the basis of their protein content. The first class is the simple, inert membrane represented by myelin. It consists primarily of lipid with little protein, acts as a permeability barrier and insulator, and has only three known enzymatic activities (Beck et al., 1968 Olafson et al., 1969 Kurihara and Tsukada, 1967 Gammer et al., 1976 Yandrasitz et al., 1976). The large second class of membranes which have a protein-to-lipid ratio of about 1 1 (w w) are typified by most mammalian plasma membranes. They have many enzymatic activities and sophisticated transport systems associated with them, in addition to the permeability factor. The third class of membranes has bacterial and inner mitochondrial membranes as its models. These membranes have proportionately larger amounts of protein than lipid and have added functions such as oxidative phosphorylation and nucleic acid synthesis. In general, the specialization and enzyme function of the membrane increases in proportion to its protein content. Table 4 gives the amino acid composition of some isolated membrane proteins. Total membrane protein (intrinsic + extrinsic) often has an amino acid composition which falls into the range of other nonmembrane, "soluble" proteins (Vanderkooi and Capaldi, 1972). 

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

Membranes, which are the subject of this section, can be relatively thick (0.1 mm) if made chemically (see their use in the PEM fuel cell, (Section 13.7.3). Biological membranes are very much thinner (50-100 A), of the same (3-5 nm) range as that of passive oxides (Section 12.5). Of what do biological membranes consist Figure 14.6 shows the essential constituents. They are lipids and proteins. How much there is of one and how much of the other varies widely. Thus, in a myelin membrane the lipid content is 80% while at the other end of the range, in mitochondria, there is an inner membrane containing only about 20% lipid. There are many kinds of lipids (as well as very many kinds of proteins), but those in membranes are usually phospholipids and are represented in Fig. 14.7. The structure often contains an H atom and this allows... 

Myelin is modified plasma membrane. Myelin of the PNS resembles that of the CNS with respect to lipid composition. There is an enrichment in such specialized lipids as cerebroside and ethanolamine plasmalogen, and the high content of cholesterol plays an important role in control of membrane fluidity. The protein composition of PNS myelin is, however, distinct from that of CNS myelin. A single protein, P0, accounts for half of all protein of PNS myelin. Of the other proteins present, most are expressed in the CNS as well as the PNS but in quantitatively different amounts. Prominent among these proteins are myelin basic proteins and myelin-associated glycoprotein. 

As the plasma membrane of the glial cell is converted into myelin, the lipid composition of the brain changes (Table 48.2). The lipid-to-protein ratio is greatly increased, as is the content of sphingolipids. The myelin is a tightly packed structure. 

The myelin basic protein and the Folch-Lees apoprotein show preferential interactions with negatively charged lipids such as sulphatides at the air-water interface (London et al., 1973). Coincidentally, we found the sulphatide content altered in the CNS of animals injected with these purified proteins (Table 1). 

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