The term amyloidosis refers to the extracellular deposition of an abnormal protein material with characteristic tinctorial properties and ultrastructural features.1 Under the electron microscope, amyloid deposits consist largely of a loose network of 7.5 to 10-nm rigid, linear, nonbranching, aggregated, paired fibrils of indefinite length and with hollow cores on profile.2 Infrared spectroscopy and X-ray diffraction crystallography have revealed that amyloid fibrils are usually composed of polypeptide chains arranged perpendicularly to the long axis of the fibril with the conformation of a meridional, antiparallel, β-pleated sheet.3,4 During the course of purification of extracted amyloid fibril preparations, a further amyloid tissue protein, quite separate from the fibrillar component of amyloid deposits, was identified.5 This novel amyloid tissue protein, termed amyloid P (plasma) component (AP) because of its antigenic identity to an α-globulin in the blood of normal individuals,6 may represent up to 14% by weight of extracted amyloid material.1
Breathnach SM. The Cutaneous Amyloidoses: Pathogenesis and Therapy. Arch Dermatol. 1985;121(4):470–475. doi:10.1001/archderm.1985.01660040054011
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