Lysine-specific cleavage of beta 2-microglobulin in amyloid deposits associated with hemodialysis

Abstract

Amyloid fibrils isolated from bone and carpal synovia of seven patients on long-term hemodialysis were further characterized biochemically. In addition, renal amyloid stones of three dialyzed patients were examined. All deposits and stones were of beta 2-microglobulin-origin (AB-amyloid) by immunohistochemical and immunochemical evaluation. Amyloid fibril extracts were dissolved in 80% formic acid and separated by high performance liquid chromatography in 60% formic acid and 20% 2-propanol. Three major retarded fractions with molecular weights of approximately 24, 12 and 7 to 10 kD were recovered. N-terminal amino acid sequence analysis documented beta 2-microglobulin (beta 2m) as the principal polypeptide in all investigated cases. In addition to proteins with intact N-termini, one fragment commencing with isoleucine in position 7 was found in osseous or synovial amyloid. In renal amyloid stones, one additional fragment was found beginning with serine in position 20. Generally, these data point to proteolytic cleavage carboxyterminal to a lysine residue and establish that not only intact beta 2m but also at least one beta 2m fragment is present in beta 2m-derived amyloid deposits of patients with long-term hemodialysis. The fragmentation pattern is consistent with the action of lysine-specific protease(s) and underscores a potentially important role of limited proteolysis in the pathogenesis of AB-amyloid deposits.