'Amyloid degrading activity' of human serum, an in vitro clearing effect which does not involve degradation of amyloid fibrils.

Clearing of turbid amyloid A fibril containing agarose gels by human serum has been ascribed to 'amyloid degrading activity'. We report here that this optical phenomenon is not due to an enzymatic reaction, does not involve proteolysis of the fibril subunits and is not inhibited by sera of patients with AA amyloidosis. The extent of clearing correlates closely with the serum albumin concentration and, as previously reported by others, serum albumin itself causes clearing comparable to that of whole serum. Furthermore addition of albumin solutions to turbid aqueous suspensions of AA amyloid fibrils causes immediate clearing. Serum albumin is known to clarify turbid non-amyloid fibril containing gels and is used commercially to improve the optical properties of radial immunodiffusion plates. We therefore propose that this property of albumin, the mechanism of which is not yet understood, underlies the so called 'amyloid degrading activity' of human serum and the latter is not therefore likely to be of in vivo biological or clinical significance.