[Mechanisms of amyloidosis and the proteins involved]

INTRODUCTION

Recent data in amyloid research have shed light on the amyloid substance and have broadened our knowledge on the mechanism of amyloid deposition.

CURRENT KNOWLEDGE AND KEY POINTS

Despite uniform physical properties relating to the presence of beta-pleates, amyloid deposits are chemically heterogeneous and have different origins; additional types will probably be described in the future. Immunohistochemical techniques using specific antisera for each of the major protein present in fibrils could help greatly to subclassify these disorders. In most circumstances, a circulating precursor protein may result from overproduction of either intact or aberrant molecule, a reduction in its degradation or excretion, or genetic abnormalities associated with variant proteins. The cleavage of protein precursor molecules of the protein component of amyloid fibrils characterizes amyloidogenesis, though it is not necessary for some amyloidosis forms. This review summarizes advances in the understanding of the nature of amyloid substances, the mechanism of amyloid deposition and the principal pathogenic hypothesis.

FUTURE PROSPECTS AND PROJECTS

SAP component is common in all amyloidosis and may be the target for future therapy.

Quantification and mapping of antigenic determinants of serum amyloid A (SAA) protein utilizing sequence-specific immunoglobulins and Eu3+ as a specific probe for time-resolved fluorometric immunoassay

Serum amyloid A (SAA) protein, the most prominent amongst acute-phase proteins, is the specific precursor protein of secondary reactive amyloidosis. The fact that SAA once released into the circulation as a 'free' protein rapidly associates with lipoproteins of the high-density range indicates a specific role in lipoprotein metabolism. In this study a new sensitive assay for quantification of human SAA protein in biological specimens using affinity-purified polyclonal antibodies and Eu3+ as a specific probe for time-resolved fluorometric immunoassay is presented. Both purified SAA and SAA-rich high-density lipoprotein particles served as reliable standards in the indirect and the direct sandwich dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA). The detection limit of the DELFIA technique presented was 4-10 ng after sample dilution of 1/2500. The intra-assay coefficient of variation averaged 4.3% whereas the inter-assay coefficient of variation averaged 6.2%. Comparison with the nephelometric assay, a widely and commonly used assay for SAA quantification in plasma, revealed correlation coefficients of 0.9428. In addition to polyclonal anti-human SAA antibodies sequence-specific antibodies raised against synthetic peptides corresponding to region; 1-17, 14-30, 27-44, 40-63, 59-72, 68-84, 79-94, and 89-104 of the human SAA amino acid sequence were studied. Sequence-specific antibodies raised against epitopes 27-44, 59-72, 68-84, and 89-104 recognize human SAA protein in the DELFIA assay whereas antibodies raised against epitopes 1-17, 14-30, 40-63 and 79-94 failed to recognize the corresponding epitopes. Results obtained from these studies indicate that the N-terminal domain (1-30) as well as epitopes 40-63 and 79-94 of human SAA are apparently masked by the environment of the lipoprotein particle. From our studies it is proposed that the epitopes 31-39, 64-78, and 95-104 may be responsible for the interaction of SAA-rich high density lipoprotein particles with peripheral cells.

Serum amyloid A (SAA): an acute phase protein and apolipoprotein

Serum amyloid A (SAA) proteins comprise a family of apolipoproteins coded for by at least three genes with allelic variation and a high degree of homology between species. The synthesis of certain members of the family is greatly increased in inflammation. However, SAA is not often used as an acute-phase marker despite being at least as sensitive as C-reactive protein. SAA proteins can be considered as apolipoproteins since they associate with plasma lipoproteins mainly within the high density range, perhaps through amphipathic alpha-helical structure. It is not known why certain subjects expressing SAA develop secondary systemic amyloidosis. There is still no specific function attributed to SAA; however, a popular hypothesis suggests that SAA may modulate metabolism of high density lipoproteins (HDL). This may impede the protective function of HDL against the development of atherosclerosis. The potential significance of the association between SAA and lipoproteins needs further evaluation.

A rapid enzyme-linked immunosorbent assay for serum amyloid A using sequence-specific antibodies

A microtitre plate based enzyme-linked immunosorbent assay for determining the concentration of serum amyloid A (SAA) is described. The method employs easily produced sequence-specific rabbit antibodies and the preferential absorption of SAA to polystyrene, which obviates the use of capture antibodies and allows an assay time of only 3.5 h, so that the diagnostic potential of the SAA level as a rapid and reliable marker for inflammation can be fully exploited. The assay has a working concentration range of 0.1-2500 mg/L, which embraces the known biological variation of the SAA concentration. The intra-assay coefficient of variation (CV) for SAA concentrations above 10 mg/L is between 1.6 and 3.3% and the interassay CV between 3.0 and 4.2%. Recovery of SAA added to serum is from 96 to 102%.

A rapid method for measuring serum amyloid A protein by latex agglutination nephelometric immunoassay

Serum amyloid A protein (SAA) is a sensitive acute phase reactant. We developed a method for the rapid measurement of human SAA in serum by kinetic nephelometry of anti-SAA antibody-coated latex agglutination. Measurement takes less than 6 min using an automated analyser. Standardization of the assay employs SAA-enriched high density lipoprotein as the primary standard. The values determined by our new method and by conventional enzyme-immunoassay showed good agreement (r = 0.988). The normal range was 0.17-10.0 mg/L [mean(SD)]. This rapid method should prove useful in clinical laboratories.

A sensitive immunochemiluminescence assay for human serum amyloid A protein

We describe an immunochemiluminescence assay for human plasma serum amyloid A protein (SAA) in which specific rabbit polyclonal antibodies against synthetic peptides are used. The detection of the antigen-antibody reaction at 425 nm is based on a brief emission of light by a luminophor component (signal) in response to chemical energy. The working range of the assay covers plasma SAA concentrations from 5 to 100 micrograms/L. The lower detection limit is 5 micrograms/L, the within- and between-assay CVs are less than 12%. Bilirubin, cholesterol and triglyceride in final concentrations of up to 220 mumol/L, 8.1 mmol/L and 2.68 mmol/L, respectively, do not interfere with the assay. Results were correlated with those obtained by the enzyme-linked immunosorbent assay using the same antibodies (r = 0.95; p less than 0.001; n = 50). This method is inexpensive, simple and easily automated.

Isoproteins and an isoelectric focusing mutant of human apoprotein serum amyloid A

On isoelectric focusing of human plasma and subsequent immunoblotting, using antii-human serum amyloid A (SAA) antibodies, a genetic variant of SAA was detected in a family of Turkish origin. All affected members of the family were apparent heterozygotes for the mutant protein, which underwent a charge shift of about one charge unit toward the anode. The variant is likely to be a mutant of the most prominent forms of SAA (SAA1 and SAA2, or SAA1 and SAA1 des Arg). The appearance of a genetic variant of two of the six reported SAA-isoforms in human plasma supports the concept of SAA proteins being products of different genes.

Variations in apolipoproteins serum amyloid A, A-I, A-II, and C-III in severely head-injured patients

In five severely head-injured patients we determined the plasma concentrations of apolipoproteins serum amyloid A, A-I, A-II, C-III, and B, prealbumin and C-reactive protein on day 1, 5, 10 and 15 after head injury where possible. A dramatic increase in apolipoprotein serum amyloid A up to a mean plasma level of 0.764 g/l was accompanied by a considerable decrease in apolipoprotein A-I, apolipoprotein A-II and apolipoprotein C-III concentrations. The variations observed by immunological methods were confirmed by two-dimensional gel electrophoresis performed on plasma and different lipoprotein fractions. In addition to its association with high density lipoproteins, apolipoprotein serum amyloid A was also found with lipoproteins of low and very low density. Two-dimensional electrophoresis also showed the presence of several different serum amyloid A-peptides not seen in plasmas from healthy subjects. We propose that apolipoprotein serum amyloid A may be responsible for the decrease of the main HDL apolipoproteins in head-injured patients.

Direct binding enzyme-linked immunosorbent assay (ELISA) for serum amyloid A (SAA)

A solid-phase, direct binding ELISA for serum amyloid A (SAA) proteins is described, in which noncovalent interactions of SAA with other plasma constituents are disrupted to permit direct coating of the wells of flexible polyvinyl chloride microtitration plates with an amount of SAA antigen proportional to its concentration in plasma. The wells are coated overnight at 60 degrees C with plasma diluted in 3 M potassium bromide and 0.1 M sodium bicarbonate. pH 9.6. The next day, any remaining sites on the wells are blocked by incubation for 1 h at ambient temperature with a 5% solution of dry milk solids and 0.05% Tween 20 in 0.02 M phosphate buffer, pH 7.4. The wells are rinsed and incubated for 90 min at 37 degrees C with polyclonal rabbit or rat anti-human SAA antiserum. Then, the wells are rinsed and incubated with goat anti-rabbit or rat IgG antiserum to which has been conjugated horseradish peroxidase. o-phenylenediamine and hydrogen peroxide substrates are added to the wells, color is allowed to develop, and sulfuric acid is added to stop the enzyme-catalyzed reaction. The amount of SAA coated to wells is quantified by absorbance at 490 nm. Four or more serial three-fold dilutions of plasma samples are assayed simultaneously on separate plates. Each plate contains a set of wells with identical concentrations of SAA standard protein diluted in decreasing concentrations of plasma proteins corresponding to the dilution of sample. The method can detect SAA concentrations in plasma samples ranging from 1 microgram/ml to greater than 1000 micrograms/ml. The method is suited to serial monitoring of SAA concentration in patients undergoing treatment for inflammatory conditions and to the quantitative analysis of large numbers of samples.

Influence of serum amyloid A on cholesterol esterification in human plasma

Lecithin-cholesterol acyltransferase (EC 2.3.1.43, LCAT) is the enzyme responsible for the formation of the bulk of cholesteryl ester in human plasma. The LCAT-reaction takes place mainly on high-density lipoproteins and requires an apolipoprotein as activator. Besides apolipoprotein (apo) A-I several other potent activator apolipoproteins (AIV, E and CI) were identified, furthermore apo A-II was shown to be a modulator of the enzyme's reaction in the presence of apo A-I. Serum amyloid A, an apolipoprotein mainly associated with high-density lipoprotein, massively accumulates in plasma upon acute phase reactions. We therefore studied the possible influence of this acute phase reactant on cholesterol esterification in human plasma. There was a significant decrease of esterified cholesterol in plasma during acute phase reaction. We found a highly significant correlation between the unesterified part of plasma cholesterol and serum amyloid A levels (r = 0.694, P = 0.0001). Also, plasma LCAT activity was negatively correlated with serum amyloid A levels. Lipoproteins containing apo A-I and A-II (LpA-I: A-II) and lipoproteins containing apo A-I but no A-II (LpA-I) decreased significantly with the appearance in plasma of serum amyloid A. To study the influence of serum amyloid A on the LCAT reaction, artificial substrates were prepared either by a detergent dialysis procedure or by addition of apolipoprotein to a sonicated aqueous dispersion of lipid. In addition two different molar ratios of apolipoprotein/phospholipid (PC) (1:50 and 1:310) were chosen at a constant molar ratio of total cholesterol/PC of 1:20. The various substrates were incubated with purified LCAT enzyme. DMPC - or egg yolk phosphatidylcholine - cholesterol-[4-14C]cholesterol-serum amyloid A complexes per se did not stimulate LCAT activity significantly. However, apo serum amyloid A incorporated together with apo A-I by a detergent dialysis procedure lead at low concentrations of serum amyloid A to a marked increase in cholesteryl ester formation as compared to apo A-I alone but inhibited the cholesteryl ester formation at high concentrations. Thus, the low levels of esterified cholesterol in acute phase plasma are to some extent due to decreased plasma enzyme activity and in part may be due to interference of apo serum amyloid A with the natural substrate complexes of plasma HDL.

Antipeptide antibodies discriminate between different SAA proteins in human plasma

Antipeptide antibodies raised against two distinct sequences of human serum amyloid A (SAA) discriminate between different plasma isoforms of this acute phase reactant. As different SAA gene products have meanwhile been identified in human plasma, the discrimination between these different isoforms may help to clarify further the time of appearance of these different forms in plasma and their potential amyloidogenicity.