Scanning Electron Microscopy of Purified Elastin with and Without Enzymatic Digestion
The biochemical and ultrastructural composition of the elastic fiber is not yet fully understood. Several authors suggested on the basis of transmission electron microscopic (TEM) investigations that the elastic fibers consist of an amorphous-looking central core being composed of elastin surrounded by a network of microfibrils. Scanning electron microscopic (SEM) studies on bovine ligamentum nuchae elastin were first published by Gotte, Mammi and Pezzin (1972).
SEM studies were performed before and after enzyme treatment of elastin purified by different methods from ligamentum nuchae and from human aorta.
As revealed by SEM, purified elastin has a cylindrical shape about one μm in diameter. These cylindrical fibers form a random network. Some of the fibers run parallel forming larger bundles.
Collagen fibers of 70 nm diameter and of 64 nm periodicity were still present in elastin preparations obtained by sodium hydroxide extraction. These collagen fibers have intricate sterical interactions with the elastin. This has not been shown previously by TEM.
The size of the subunits measured at a 100,000 fold enlargement with SEM suggest that the one urn cylindrical elastin fibers are built up of bundles of fibrils not larger than 100 nm in diameter. The uneveness of the surface of the 100 nm elastin fibers is caused by a substructure of 1.12 nm diameter which corresponds well with the earlier suggestions by Partridge (1963). The special arrangement, coiling and branching of the polypeptides, of the subunit and of the elastin fiber suggests the existence of triple helical structure at each of the three different levels of organization (Gray, Sandberg, Foster, 1973). The diameters of the structural units at the different levels are about 1, 10 and 100 nanometers, respectively.
After trypsin digestion, the purified elastin reveals an irregular undulating surface structure in SEM micrographs. It is supposed that the trypsin digests some protein- or glycoprotein-like coat of the elastin while the elastin proper is resistant to this enzyme.
After elastase digestion (15,30, 60 minutes, 2 and 4 hours) the elastin fibers break up revealing as a result a rodlike structure of 40 nm diameter. The nature of the rodlike structure is ambiguous. They represent either the glycoprotein containing backbone of the elastic fibers or the fragments of elastin containing strong crosslinkages resistant to the digestion by elastase.
It is supposed that the elastase binds to specific binding sites on the surface of elastic fibers and the elastolysis starts at these places.
KeywordsCollagen Fiber Elastic Fiber Transmission Electron Microscopic Micrograph Trypsin Digestion Elastin Fiber
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