Degradation of protein SAA to an AA-like fragment by enzymes of monocytic origin.

On incubation in cultures of blood mononuclear leucocytes from normal individuals, protein SAA is gradually degraded and yields an intermediate fragment with the same molecular weight as that of protein AA. This protein fragment has the same antigenic properties as protein AA, as judged from double-immunodiffusion analysis. Cell fractionation studies attribute the degrading capacity to the monocytes. Evidence was also found that the proteolytically active substances was released to the medium by cells in culture. The proteolytic substance, which also degraded protein AA, could be inhibited by diisopropyfluorophosphate and phenylmethyl sulfonylfluoride. This suggested that the enzyme might be a serine protease.