Elastase-type proteases on the surface of human blood monocytes: Possible role in amyloid formation

A group of DFP-inhibitable serine proteases that are associated with the cell surface of human peripheral blood monocytes and degrade the amyloid precursor protein SAA has been partially characterized. Enzymes that resemble elastases in being inhibitable by Ac-Ala-Ala-Pro-Val-CH₂Cl but differ from the secreted macrophage elastase in m.w. can be recovered from SDS gels in the region ranging from 58 to 72 x 10³ daltons. These enzymes degrade SAA to a product similar in m.w. and antigenic properties to the amyloid A protein. When intact cells are labeled with ³H Ac-Ala-Ala-Pro-Val-CH₂Cl at 4°C for 3 min, a major 58 x 10³ and two minor 48 and 65 x 10³ dalton bands are seen. Another group of enzymes that digests SAA completely through a transient AA-like intermediate can be recovered from the 40 to 58 x 10³ dalton region of the gels. These enzymes are only minimally inhibited by Ac-Ala-Ala-Pro-Val-CH₂Cl. We suggest that both types of enzymes may be involved in the proteolytic processes that lead to amyloid formation in secondary amyloidosis.