Elastin fibers

Aaron, B.B. and Gosline, J.M., Elastin as a random-network elastomer A mechanical and optical analysis of single elastin fibers. Biopolymers, 20, 1247-1260, 1980. 

This coacervation process forms the basis for the self-assembly, which takes place prior to the crosslinking. The assembly of tropoelastin is based on an ordering process, in which the polypeptides are converted from a state with little order to a more structured conformation [8]. The insoluble elastic fiber is formed via the enzymatic crosslinking of tropoelastin (described in Sect. 2.1). Various models have been proposed to explain the mechanism of elasticity of the elastin fibers. 

Ross, R., Bomstein, P. Studies of the components of the elastin fiber. In Chemistry and molecular biology of the intercellular matrix, Vol. 1, pp. 641. Balazs, E. A. (ed.). London, New York Academic Press 1970... 

In developing elastic tissue, the microfibrils are the first components to appear in the extracellular matrix. They are then thought to act as a scaffold for deposition, orientation, and assembly of tropoelastin monomers. They are 10—12 nm in diameter, and lie adjacent to cells producing elastin and parallel to the long axis of the developing elastin fiber (Cleary, 1987). 

In vivo elastin fiber formation requires the coordination of a number of important processes. These include the control of intracellular transcription and translation of tropoelastin, intracellular processing of the protein, secretion of the protein into the extracellular space, delivery of tropoelastin monomers to sites of elastogenesis, alignment of the monomers with previously accreted tropoelastin through associating microfibrillar proteins, and finally, the conversion to the insoluble elastin polymer through the crosslinking action of lysyl oxidase (Fig. 2). 

Groult, V., Hornebeck, W., Ferrari, P., Tixier, J. M., Robert, L., and Jacob, M. P. (1991). Mechanisms of interaction between human skin fibroblasts and elastin Differences between elastin fibers and derived peptides. Cell Biochem. Fund. 9, 171-182. 

This review focuses upon the post-translational modification and chemical changes that occur in elastin. Outlined are the steps currently recognized as important in the assembly of pro-fibrillar elastin subunits into mature fibers. Descriptions of some of the proposed mechanisms that appear important to the process are also presented. It will be emphasized that from the standpoint of protein deterioration, elastin is a very novel protein. Under normal circumstances, the final product of elastin metabolism, the elastin fiber does not undergo degradation that is easily measured. Unlike the metabolism of many other proteins, deterioration or degradation is most evident biochemically in the initial stages of synthesis rather than as a consequence of maturation. Since the presence of crosslinks is an essential component of mature elastin, a section of this review also addresses important features of crosslink formation. 

Figure 2. Synthesis of mature elastin fibers. Some evidence suggests the possibility for proforms to elastin that appear as the first products of translation. These products are cleaved to tropoelastin (27), which appears to combine with microfibrillar protein. Although post-translational events important to the synthesis of the microfibrillar protein have not been defined, it is clear that it is a major component on which is organized or assembled the profibrillar forms of elastin. Cross-linking is catalyzed by lysyl oxidase, a copper-requiring protein (30). Recent information on the elastin proteinase(s) involved in tropoelastolysis would suggest that proteolysis may also play a role in elastin fiber...

When the metabolic turnover of elastin in arterial tissue or in lung is examined, it is extremely difficult to demonstrate active turnover. Once an elastin fiber is formed it appears to be fixed. The turnover of rat aorta elastin is best measured in years (8). Data shown in Figure 5 also suggests negliable turnover. The animal used for this study, the Japanese quail, was chosen because it fully matures at 5-6 weeks of age. Similar to the rat its elastin appears to turn over in amounts best estimated in years. 

The dermis is the largest layer of the skin. It is a region of strong and flexible connective tissue. The dermis consists of two primary layers, the papillary layer and the reticular layer. The papillary layer is the smallest layer of the dermis and is composed mainly of collagen and elastin fibers. The reticular layer is the largest layer of the dermis and is composed of mainly dense connective tissue. The layer of subcutaneous fat found directly beneath the dermis provides insulation and additional mechanical support to the skin. 

Fig. 2. Electron micrograph showing the fibrillar structure of an elastic fiber from bovine ligamentum nuchae. (Gotte and Serafini-Fracassini, 1962.) The elastin fibers were treated with dilute sodium hydroxide at 98°C for 1 hr, washed with hot water, and disintegrated ultrasonically at 40 kc. The magnification is 63,000X and the individual fibrils are 10 2 The specimen was shadowed with chromium at 20 degrees.

This indicated clearly that hydroxylapatite crystallites are associated intimately with elastin fibers. In the hope of tracing this assocnation to the molecular level the fibers were dissolved by the action of elastase. Brief treatment of elastic fibers with this enzyme yields a soluble, non-dialyzable protein (Partridge et al., 1955) and it was found that after elastase treatment followed by centrifugation to remove particulate matter the soluble protein produced still contained calcium and still displayed... 

The kinetics of the enzymatic dissolution of elastin are very complex and the reaction of elastin fibers with elastolytic enzymes is commonly characterized by a lag phase or a markedly sigmoid time course (cf. Hall and Czerkawski, 1959 Naughton et al., 1960). The length of the slow initial phase is influenced by the enzyme-substrate ratio and the source and method of preparation of the elastin used in the assay the course of the reaction with different enzymes is also influenced by the presence or absence of salts, reducing agents, and many other substances. As a result... 

The amino acid analysis of the a- and d-proteins was compared with that of the purified elastin fibers from which they were prepared. It was found that the composition of the three proteins was closely similar thus showing that the original fibers must be homogeneous as regards amino acid composition even though they show a fibrillar structure. There were... 

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