Sites and regulation of biosynthesis of SAA

Serum amyloid A (SAA) is a 12.5 Kdalton protein usually isolated in association with the HDL3 subclass of serum lipoproteins (1). SAA is one of the plasma proteins which increase during inflammation or tissue injury (the positive acute phase reactants). Among these positive acute phase proteins SAA and C-reactive protein (GRP) are quantitatively the most impressive (100–1000 fold) (2). Hence the regulation of SAA synthesis has been a subject of general interest as well as a subject of importance to students of the biochemical basis of amyloidosis. The increase in SAA is detectable within four hours and usually peaks approximately 18 hours following initiation of tissue injury (3). SAA and the β-pleated sheet fibril protein amyloid A (AA) share considerable amino acid homology except that the 28 carboxy-terminal amino acids present in SAA are not represented in AA (4). Hence it is likely that SAA is the precursor of AA. The existing data suggest that cleavage by serine proteases, perhaps at the surface of monocytes and macrophages, may be responsible for the conversion of SAA to AA (5). Delineation of the exact mechanism of this potential precursor-product relationship awaits determination of the amino acid sequence of the peptides resulting from cleavage of purified SAA by specific neutral proteases plus studies of amyloid generation in tissue culture.