Serum amyloid A (SAA)

SAA, known as serum amyloid A (SAA), is the precursor substance for amyloid A (AA), which is produced primarily by hepatocytes under the regulation of inflammatory cytokines, and circulates at low levels in the blood.SAA plays a role in the immune system, and under inflammatory conditions, levels of SAA proteins in blood and other tissues are elevated and signal antimicrobial cells to migrate to sites of infection.SAA also functions as an apolipoprotein for the HDL complex. SAA plays a role in the immune system, and under inflammatory conditions, levels of SAA protein in the blood and other tissues are elevated and signal antimicrobial cells to migrate to the site of infection.SAA also acts as an apo-protein for the HDL complex. Elevated levels of SAA ultimately affect secondary amyloidosis, the extracellular aggregation of amyloid fibers formed from circulating precursors, which can occur in a variety of tissues and organs. The most common amyloidosis is secondary to chronic inflammatory diseases.

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Molecular structure and characteristics of SAA

Serum amyloid A (SAA) is a member of the SAA family and has the characteristics of the SAA family. There are 4 SAA genes on human chromosome 11, and according to their expression in vivo, SAA can be categorized into 2 types, acute SAA (A-SAA) and constitutive SAA (C-SAA).SAA contains binding sites for lipids and calcium ions, and its secondary structure consists of α-helix and β-folding. It is a globular protein molecule. the number of amino acids in the polypeptide chains of A-SAA and C-SAA are 104 and 112, respectively. this is due to the fact that C-SAA has an extra octapeptide sequence and this octapeptide sequence can form a glycosylation site, yielding two relative masses of C-SAA, 14 kDa and 19 kDa.

Figure 1 Molecular structure of SAA

Biological functions of SAA

Serum amyloid A (SSA) rises rapidly by about 1000-fold in 4-6 h during the acute phase of inflammation or infection and decreases markedly during the recovery phase when the antigen is cleared.The sensitivity and specificity of SAA are higher than that of C-reactive protein (CRP). Specifically, after the body is stimulated, the expression of A-SAA is elevated by cytokines (IL-1, IL-6, TNFα) and becomes the main form of SAA. And the production of A-SAA was co-regulated by IL-1, IL-6 and TNFα, and cytokines alone had little effect on the expression of A-SAA. When SAA produced in the liver enters the circulation, it rapidly binds to HDL3 in the body and is degraded only after HDL dissociates from ASSA. Inflammation as well as infection slows down the degradation of A-SAA in vivo, and the combination of both increased synthesis and slowed degradation causes a sustained elevation of A-SAA in the circulation.

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