Elastin solubility

Albumen has the largest number of acid and basic groups. It is the most soluble of the proteins present in a hide. The albumen is not a fibrous material, however, and therefore has no value in the leather. Keratin is the protein of the hair and the outermost surface of the hide. Unless the hair is desired for the final product it is removed by chemical and/or physical means. The elastin has Htde acid- or base-binding capacity and is the least soluble of the proteins present. The lack of reactivity of the elastin is a detriment for most leather manufacture. The presence of elastin in the leather greatly limits the softness of the leather. 

Tropoelastin is the soluble precursor of elastin and consists of alternating hydrophobic and hydrophilic peptide domains. The most common amino acids in the hydrophobic domains are Gly, Val, Ala, and Pro, which are often present in repeats of tetra-, penta-, and hexapeptides, such as Gly-Gly-Val-Pro, Gly-Val-Gly-Val-Pro, Gly-Val-Pro-Gly-Val, and Gly-Val-Gly-Val-Ala-Pro, respectively [3, 4]. The hydrophilic domains are mainly composed of lysines interspersed by alanines. 

Research into elastin, its properties, and the fiber formation was for a considerable period of time hindered due to its insolubihty. However, discovery of the soluble tropoelastin precursor made new investigations possible. The tropoelastin protein can be isolated from copper-deficient animals. However, this is a very animal-unfriendly and low yielding process [2]. Therefore, it is preferred to obtain tropoelastin from overexpression in microbial hosts such as Escherichia coli (E. coli). Most studies are thus based on tropoelastin obtained via bacterial production. 

Fig. 11.3 Purification ofSOl-lOOxELP-proteins from transgenic tobacco plants by inverse transition cycling and analysis by SDS-PAGE. 1 15 pg of total soluble leaf protein extracted in raw extract buffer 2 cleared supernatant of original 15 pg total soluble leaf protein after heat treatment (60 min, 95 °C) 3 cleared supernatant of original 300 pg leaf protein after heat treatment 4 cleared supernatant of original 300 pg leaf protein after heat treatment (60 min, 60 °C) with 2 M NaCI 5 redissolved spider silk-elastin protein pellet from original 300 pg of total soluble leaf protein after heat treatment (60 min, 60 °C) with 2 M NaCI.

Figure 3 Biosynthesis and purification of 90-kD elastin analogue analyzed by denaturing polyacrylamide gel electrophoresis (10-15% gradient, visualized by silver staining). Lanes 1-7 time course of target protein expression at 0, 30, 60, 90, 120, 150, and 180 minutes after induction. Lane 9 soluble lysate of induced E. coli expression strain BLR(DE3)pRAMl. Lanes 10-13 protein fractions obtained from immobilized metal affinity chromatography of the lysate on nickel-NTA agarose (imidazole gradient elution). Lanes 8,14 protein molecular weight standards of 50, 75, 100, and 150 kD.

In contrast to milk, where samples are primarily derived from cows, meat analysis has to be performed in samples of a widely different animal origin including cattle, lamb, swine, poultry, and fish. Muscle is a complex matrix with a pH of 5.7, composed of muscle fibers, various types of connective tissue, adipose tissue, cartilage, and bones. Sarcoplasmic proteins such as myoglobin, and glycolytic enzymes are soluble in water while the myofibrillar proteins such as myosin and actin are soluble in concentrated salt solutions (14). The connective tissue proteins, collagen and elastin, are insoluble in both solvents. 

Sandberg, L. B., Weissman, N., and Smith, D. W. The purification and partial characterization of a soluble elastin-like protein from copper-deficient porcine aorta. Biochemistry 8, 2940-2945 (1969). 

Numerous studies have been undertaken to elucidate the secondary structure of soluble elastin. These studies have been performed on elastin, elastin solubilized by oxalic acid (a-elastin) or potassium hydroxide (/, -elastin). synthetic polypeptide models of elastin, and tropoelastin. Techniques used include circular dichroism, FT-Raman, and electron microscopy. No consensus has been reached on the overall structure of elastin. 

The regulation of the trypsin-like protease Pr2 of M anisopliae was examined by Paterson et al. (1993). Three insoluble protein sources, insect cuticle, elastin and collagen, as well as two soluble proteins, gelatine and especially bovine serum albumin (BSA) were found... 

Enzymes, such as creatine kinase, have been grafted on to collagen films by using water soluble carbodiimides. Porcine intestinal collagen has been crosslinked with EDC in acetone to provide a remodelable scaffold. EDC crosslinking of collagen/elastin matrices is also used to prepare fiat scaffolds. ... 

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... 

From the foregoing it appears that elastin can be regarded as a cross-linked polymer gel and its swelling and solubility properties are consistent with this view. Elastin appears to be entirely insoluble in any reagent except those that attack the primary chain structure by hydrolysis and the breakino of covalent bonds. 

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