Serum amyloid P component (SAP) belongs to a highly conserved superfamily of calcium-dependent ligand binding and lectin (carbohydrate binding) proteins. Due to the pentameric structure these proteins are also called pentraxins. SAP is 25kDa in size. C-reactive protein (CRP) and SAP are well-charactarized short pentraxins, which are produced in the liver in response to inflammatory mediators. Pentraxin 3 (PTX3) is the prototypic long pentraxin. Both SAP and CRP are evolutionary conserved in all vertebrates and found in distant invertebrates such as the horseshoe crab (Limulus polyphemus). They share 51% residue sequence identity and 66% homology.
SAP is highly resistant to proteolysis, especially when it forms complexes with calcium-dependent ligands. SAP avidly binds to macromolecular ligands, such as nucleosomal DNA, glycosaminoglycans and amyloid fibrils. When aggregated it can bind C1q and activate the classical complement pathway.
Human SAP is not an acute phase reactant following acute stimuli, this in contrast to mouse SAP and human CRP. In mouse, SAP levels increase significantly 24 hours after challenge with lipopolysaccharide.
SAP is a normal plasma constituent that is present in cerebrospinal fluid (CSF), in the pathognomonic lesions of Alzheimer’s disease (AD), cerebrovascular and intracerebral Aβ amyloid plaques and neurofibrillary tangles. This is a result of its binding to amyloid fibrils and to paired helical filaments, respectively. The protein itself may also be directly neurocytotoxic. SAP contributes significantly to the pathogenesis of amyloidosis. The mechanism of its participation is not yet known and importantly may differ between species.
The normal human serum level of SAP is 30-40 µg/ml. One study described that the mean SAP concentration in serum of women (24 µg/ml) is significantly lower than in men (32 µg/ml). The concentration does not alter significantly after major surgery and is only slightly increased during chronic active diseases. Significantly low levels of SAP are only seen in patients with severe hepatocellular impairment which reduces plasma protein synthesis. In cerebrospinal fluid (CSF), the mean level of SAP in healthy individuals is 8.5 ng/ml.
The human SAP ELISA kit is to be used for the in vitro quantitative determination of human SAP in serum, plasma, urine and CSF samples.
The human SAP ELISA is a ready-to-use solid-phase enzyme-linked immunosorbent assay based on the sandwich principle with a working time of 3½ hours.
The efficient format of 2 plates with twelve disposable 8-well strips allows free choice of batch size for the assay.
Samples and standards are incubated in microtiter wells coated with antibodies recognizing human SAP.
Biotinylated tracer antibody will bind to the captured human SAP.
Streptavidin-peroxidase conjugate will bind to the biotinylated tracer antibody.
Streptavidin-peroxidase conjugate will react with the substrate, tetramethylbenzidine (TMB).
The enzyme reaction is stopped by the addition of oxalic acid.
The absorbance at 450 nm is measured with a spectrophotometer. A standard curve is obtained by plotting the absorbance (linear) versus the corresponding concentrations of the human SAP standards (log).
The human SAP concentration of samples, which are run concurrently with the standards, can be determined from the standard curve.