Characteristics of the amyloid A fibril-degrading activity of human serum.

Radial diffusion in agarose gel containing amyloid A (AA) fibrils was used to study the serum enzyme capable of degrading AA fibrils in vitro. This degradative activity was unaffected by soya bean trypsin inhibitor, tosyl-lysine chloromethyl ketone, and gold thiomalate but was inhibited by bovine pancreatic trypsin inhibitor, phenylmethylsulphonylfluoride, diisopropyl fluorophosphate, alpha 1-antitrypsin, and alpha 2-macroglobulin, indicating that the enzyme involved is a serine protease. Agarose gel electrophoresis showed the enzyme to be an acidic protein with the same electrophoretic mobility as albumin. The molecular weight, measured by gel filtration, was approximately 50,000. The optimum pH of this enzyme was 7.3, and it was fairly heat-resistant. The results suggest that the AA-fibril-degrading activity in human serum is due neither to elastase nor to cathepsin G. It has many characteristics in common with the enzymes unlike elastase that are involved in the complete degradation of serum AA protein.