Methods to quantitate human haptoglobin by complexation with hemoglobin

Determination of Haptoglobin in Bovine Serum using Polyclonal and Monoclonal Anti-human Haptoglobin Antibodies

Two ELISA procedures to determine haptoglobin (Hp) in bovine serum were developed. Equine haemoglobin was used as the solid phase. Self-developed goat polyclonal antibody (variant I) and monoclonal antibody (variant II) raised against human Hp were used. The results were compared with the guaiacol method. High correlation was found (r = 0.96 and r = 0.90, respectively) based on the results of 548 bovine serum samples, of which 357 were from clinically healthy cows and 191 from cows and calves monitored during treatment for the most common diseases. The Hp detection limit of ELISA using polyclonal Ab was 0.1 mg/l and using MoAb 0.21 mg/l. The addition of 2% PEG 6000 at the antibody-binding steps enabled major shortening of the incubation periods. The relatively short time, low cost of reagents, and high correlation with the reference method support the use of these ELISA variants in bovine diagnostics.

Peroxidase activity of hemoglobin-haptoglobin complexes: covalent aggregation and oxidative stress in plasma and macrophages

As a hemoprotein, hemoglobin (Hb) can, in the presence of H(2)O(2), act as a peroxidase. In red blood cells, this activity is regulated by the reducing environment. For stroma-free Hb this regulation is lost, and the potential for Hb to become a peroxidase is high and further increased by inflammatory cells generating superoxide. The latter can be converted into H(2)O(2) and feed Hb peroxidase activity. Haptoglobins (Hp) bind with extracellular Hb and reportedly weaken Hb peroxidase activity. Here we demonstrate that: (i) Hb peroxidase activity is retained upon binding with Hp; (ii) in the presence of H(2)O(2), Hb-Hp peroxidase complexes undergo covalent cross-linking; (iii) peroxidase activity of Hb-Hp complexes and aggregates consumes reductants such as ascorbate and nitric oxide; (iv) cross-linked Hb-Hp aggregates are taken up by macrophages at rates exceeding those for noncovalently cross-linked Hb-Hp complexes; (v) the engulfed Hb-Hp aggregates activate superoxide production and induce intracellular oxidative stress (deplete endogenous glutathione and stimulate lipid peroxidation); (vi) Hb-Hp aggregates cause cytotoxicity to macrophages; and (vii) Hb-Hp aggregates are present in septic plasma. Overall, our data suggest that under conditions of severe inflammation and oxidative stress, peroxidase activity of Hb-Hp covalent aggregates may cause macrophage dysfunction and microvascular vasoconstriction, which are commonly seen in severe sepsis and hemolytic diseases.

Assignment of the binding site for haptoglobin on apolipoprotein A-I

Haptoglobin (Hpt) was previously found to bind the high density lipoprotein (HDL) apolipoprotein A-I (ApoA-I) and able to inhibit the ApoA-I-dependent activity of the enzyme lecithin:cholesterol acyltransferase (LCAT), which plays a major role in the reverse cholesterol transport. The ApoA-I structure was analyzed to detect the site bound by Hpt. ApoA-I was treated by cyanogen bromide or hydroxylamine; the resulting fragments, separated by electrophoresis or gel filtration, were tested by Western blotting or enzyme-linked immunosorbent assay for their ability to bind Hpt. The ApoA-I sequence from Glu113 to Asn184 harbored the binding site for Hpt. Biotinylated peptides were synthesized overlapping such a sequence, and their Hpt binding activity was determined by avidin-linked peroxidase. The highest activity was exhibited by the peptide P2a, containing the ApoA-I sequence from Leu141 to Ala164. Such a sequence contains an ApoA-I domain required for binding cells, promoting cholesterol efflux, and stimulating LCAT. The peptide P2a effectively prevented both binding of Hpt to HDL-coated plastic wells and Hpt-dependent inhibition of LCAT, measured by anti-Hpt antibodies and cholesterol esterification activity, respectively. The enzyme activity was not influenced, in the absence of Hpt, by P2a. Differently from ApoA-I or HDL, the peptide did not compete with hemoglobin for Hpt binding in enzyme-linked immunosorbent assay experiments. The results suggest that Hpt might mask the ApoA-I domain required for LCAT stimulation, thus impairing the HDL function. Synthetic peptides, able to displace Hpt from ApoA-I without altering its property of binding hemoglobin, might be used for treatment of diseases associated with defective LCAT function.

Haptoglobin Phenotypes Differ in Their Ability To Inhibit Heme Transfer from Hemoglobin to LDL†

LDL oxidation plays a pivotal role in atherosclerosis. Excellular hemoglobin (Hb) is a trigger of LDL oxidation. By virtue of its ability to bind hemoglobin, haptoglobin (Hp) serves as an antioxidant. Oxidation of LDL by hemoglobin was analyzed to occur by heme displacement from methemoglobin lodged in LDL. The LDL-associated heme is disintegrated, and iron inserted this way in LDL triggers formation of lipid peroxides. The genetic polymorphism of haptoglobin was found to be a risk factor in the pathogenesis of atherosclerosis. Individuals with Hp2-2 have more vascular incidences as compared to those with Hp1-1. In the current study, oxidation of LDL by metHb was carried out at physiological pH without addition of external peroxides. Hb-derived oxidation of lipids and protein was found to be practically inhibited by Hp1-1 but only partially by Hp2-2. Heme transfer from metHb to LDL was almost completely omitted by Hp1-1 and only partially by Hp2-2. We concluded that partial heme transfer from the Hb-Hp2-2 complex to LDL is the reason for oxidation of LDL lipids as well as protein. These findings provide a molecular basis for Hp2-2 atherogenic properties.

A threshold for central T cell tolerance to an inducible serum protein

We report an inducible system of self Ag expression that examines the relationship between serum protein levels and central T cell tolerance. This transgenic approach is based on tetracycline-regulated expression of a secreted form of hen egg lysozyme, tagged with a murine hemoglobin (Hb) epitope. In the absence of the tetracycline-regulated transactivator, serum levels of the chimeric protein are extremely low (< or = 0.1 ng/ml) and the mice show partial tolerance to both Hb(64-76) and lysozyme epitopes. In the presence of the transactivator, expression increases to 1.5 ng/ml and the mice are completely tolerant. Partial tolerance was further investigated by crossing these mice to strains expressing transgenic TCRs. At the lowest Ag levels, 3.L2tg T cells (specific for Hb(64-76)/I-E(k)) escape the thymus and approximately 10% of CD4(+) splenocytes express the 3.L2 TCR. In contrast, 3A9 T cells (specific for hen egg lysozyme(46-61)/I-A(k)) are completely eliminated by negative selection. These data define a tolerogenic threshold for negative selection of Ag-specific T cells by circulating self proteins that are 100-fold more sensitive than previously demonstrated. They suggest that partial tolerance at extremely low levels of self Ag exposure is the result of a restricted repertoire of responding T cells, rather than a simple reduction in precursor frequency; tolerogenic thresholds are T cell specific.

Structure-function analysis of the antioxidant properties of haptoglobin

Haptoglobin serves as an antioxidant by virtue of its ability to prevent hemoglobin-driven oxidative tissue damage. It was recently demonstrated that an allelic polymorphism in the haptoglobin gene is predictive of the risk for numerous microvascular and macrovascular diabetic complications. Because these complications are attributed in large part to an increase in oxidative stress, a study was conducted to determine whether the different protein products of the 2 haptoglobin alleles differed in the antioxidant protection they provided. A statistically significant difference was found in the antioxidant capacity of purified haptoglobin protein produced from the 2 different alleles, consistent with the hypothesis that differences in genetically determined antioxidant status may explain differential susceptibility to diabetic vascular complications. These differences may be amplified in the vessel wall because of differences in the sieving capacity of the haptoglobin types. Therefore, an attempt was made to identify the minimal haptoglobin sequences necessary to inhibit oxidation by hemoglobin in vitro, and 2 independent haptoglobin peptides that function in this fashion as efficiently as native haptoglobin were identified. Identification of the biochemical basis for differences among haptoglobin types may lead to the rational development of new pharmacologic agents, such as the mini-haptoglobin described here, to avert the development of diabetic vascular complications.

Haptoglobin inhibits lecithin-cholesterol acyltransferase in human ovarian follicular fluid

The activity of the enzyme lecithin-cholesterol acyltransferase (LCAT; E.C. 2.3.1.43) is involved in the removal of cholesterol excess from peripheral cells. This activity is stimulated by the HDL (high density lipoprotein) apolipoprotein A1 (ApoA1). Haptoglobin (Hpt) was previously found to be associated with ApoA1 in ovarian follicular fluid. LCAT activity was analyzed in follicular fluids, collected from an IVF program, containing different amounts of Hpt or Hpt/ApoA1 ratio. Addition of purified Hpt to follicular fluid caused a decrease in the enzyme activity, which was measured as the rate of synthesis of cholesteryl esters. In the fractions of fluid proteins, as obtained by gel filtration chromatography, Hpt and HDL were titrated by ELISA while the LCAT activity was assayed by using radioactive cholesterol and purified HDL. When isolated LCAT was incubated with fractions containing different Hpt/ApoA1 ratios, the enzyme activity was found negatively correlated with the Hpt/ApoA1 ratio (P < 0.01). LCAT kinetic parameters were measured in two fractions with the same amount of ApoA1 (5 microg/ml) but different amounts of Hpt (0.69 or 3.77 microg/ml): the V(max) did not change while the K(m) values were 24.1 or 78.6 microM in the presence of the low or high Hpt level, respectively. The analysis of fluids associated with cytoplasmically mature MII oocytes, in a cross-sectional study, confirmed that a negative correlation exists between the Hpt/ApoA1 ratio and the LCAT activity (P < 0.01). The results suggest that Hpt inhibits the reverse transport of cholesterol by preventing ApoA1 stimulation of the LCAT activity.

Quantitation of human haptoglobin: comparative ELISA studies using adsorption and capture methods

Three ELISA methods for the quantitation of haptoglobin (Hp) in plasma and albumin are described: a polystyrene direct adsorption method and capture methods with antibody and hemoglobin. Hp aggregates generated by 60 degrees C heating showed as much as a hundred-fold higher response by polystyrene adsorption compared to the two capture methods, while unheated Hp showed comparable responses by the three methods.

The study of self-tolerance using murine haemoglobin as a model self antigen

T cell tolerance to self proteins involves both thymic and peripheral mechanisms. We have used allotypic differences in murine haemoglobin (Hb) to study the development of tolerance to the abundantly expressed self-protein. In Hb beta s/H-2k mice, the response to Hb beta d is directed against Hb beta d (64-76) presented by I-Ek molecules. Using T cell hybridomas and clones specific for this epitope, we have demonstrated that Hb(64-76)/I-Ek complexes and present on antigen-presenting cells in all lymphoid organs including dendritic cells, B cells and macrophages. In the thymus, the presence of these complexes results in negative selection of transgenic T cells with high levels of Hb(64-76)/I-Ek-specific receptor. However, cells with intermediate levels of specific receptor escape negative selection and can be found in the periphery. Under normal circumstances these cells remain tolerant, but can be activated by mechanisms which increase the number of Hb(64-76)/I-Ek complexes.

Development of a sensitive and specific time resolved fluorimetric immunoassay for the bovine acute phase protein haptoglobin (Hp)

Haptoglobin (Hp) is recognised as a major acute phase protein in the bovidae and its presence in serum is used as an indicator of inflammation. A mouse monoclonal antibody (1D9) specific for bovine Hp was labelled with a lanthanide (Eu) chelate and used to develop a competitive immunoassay. This competitive immunoassay allowed direct measurement of Hp in serum and was validated for intra- and interassay coefficients of variation (below 8%). Cross-reactivity with other serum proteins was measured (less than 0.1%) and limits of detection for Hp in serum were established for adult male (0.344 microgram/ml) and adult female cattle (1.589 micrograms/ml). The immunoassay was compared with an established haptoglobin-haemoglobin binding assay.

Bovine haptoglobin: single radial immunodiffusion assay of its polymeric forms and dramatic rise in acute-phase sera

Using purified bovine haptoglobin (Hp) and specific antisera, a single radial immunodiffusion (SRID) assay method has been developed to measure the serum Hp level in cattle. Bovine Hp is a highly polymerized protein showing heterogeneous molecular forms in serum. After treatment with cysteine or glutathione, Hp was partially reduced to a homogeneous form, suitable for SRID assay. This method gives values comparable to those obtained by hemoglobin-binding capacity assay, and has the advantage of being simple and convenient. Although serum Hp was not detectable in healthy cattle, it was found more than 50-fold after invasive surgery, indicating that Hp is a characteristic acute-phase protein in cattle.