In vitro production of peroxynitrite by haemocytes from marine bivalves: C-ELISA determination of 3-nitrotyrosine level in plasma proteins from Mytilus galloprovincialis and Crassostrea gigas

Background Peroxynitrite is increasingly proposed as a contributor to defence system in marine bivalve. It can be formed by combinaison of superoxide and nitric oxide, and can react with tyrosine residues of proteins giving rise to 3-nitrotyrosine. Results The present article describes a competitive ELISA for the measurement of 3-nitrotyrosine contents of plasma proteins from marine bivalves by means of a monoclonal anti 3-nitrotyrosine antibody mouse IgG. Conclusions This assay is sensitive enough to determine the amounts of 3-nitrotyrosine in plasma proteins from one animal only. Using the C-ELISA, we have shown that the phagocytosis of zymosan particles increased the 3-nitrotyrosine levels of plasma proteins from mussel M. galloprovincialis and oyster C. gigas 5.8 and 7.5 times respectively.


Background
Bivalves, unlike vertebrates, do not have humoral antigen specific active compounds such as antibodies and their self-defence systems are based on non-specific defensive compounds and phagocytosis by haemocytes [1,2].
In the presence of superoxide anions, nitric oxide generates peroxynitrite, a strong oxidant which kills bacteria [13] and parasitic protozoa [10,14,15]. Moreover, peroxynitrite is a nitrating agent, that converts tyrosine in 3nitrotyrosine [16]. Such nitration has been observed in proteins from human polymorphonuclear cells [17] and 3-nitrotyrosine has been used as a marker to assess peroxynitrite involvement in pathological processes such as adult respiratory distress syndrome [18], rheumatoid arthritis [19] and celiac disease [15].
To determine levels of protein-associated 3-nitrotyrosine in human plasma or serum, Khan et al. [20] developed a competitive enzyme-linked immuno-assay (C-ELISA) for 3-nitrotyrosine using a polyclonal anti-3-nitrotyrosine rabbit IgG raised against nitrated KLH. In the present study, we slightly modified this C-ELISA assay to investigate 3-nitrotyrosine levels in plasma proteins from mussel M. galloprovincialis and oyster C. gigas before and after zymosan phagocytosis.

ELISA standard curve construction
We developed a competitive ELISA to quantify 3-nitrotyrosine residues in plasma proteins from marine bivalves . A standard curve was constructed by determining the binding inhibition of the anti-3-nitrotyrosine antibody to a synthetic antigen (BSA NT ) immobilised on coated microtitration plates in the presence of serial dilutions of the free antigen BSA NT in solution. Figure 1 shows that the curve obtained was linear from 0.2 to 0.003 µM BSA NT / assay. In contrast, unmodified BSA showed no significant binding inhibition of the 3-nitrotyrosine antibody and no significant cross-reaction of KLH, an invertebrate protein model was observed.

Figure 1
Comparison of standard curves for the inhibition of anti-3-nitrotyrosine antibody binding by various proteins in the C-ELISA. The curves show competition for the anti-3-nitrotyrosine antibody between the immobilized BSA NT and competing free proteins in solution: BSA NT (n = 3); BSA (n = 2); KLH (n = 2). The percentage inhibition of maximum antibody binding (absorbance at 490 nm in the absence of competition) is plotted against the competing free protein concentration.

Effect of stimulation of mussel hemocytes with PMA on 3nitrotyrosine levels in plasma proteins:in vivo
The 3-nitrotyrosine concentration in proteins from M. galloprovincialis plasma samples was quantified by C-ELISA and expressed as BSA NT equivalents using the standard curve of Fig. 1. As shown in table 1, marked individual variations were observed and a mean concentration (n = 20) of 0.037 ± 0.025 µM BSA NT equivalents was estimated. After 1 h incubation of mussel hemocytes with PMA, the in vitro level of 3-nitrotyrosine in plasma increased to a mean value (n = 20) of 0.118 ± 0.024 µM BSA NT equivalents (3.2-fold enhancement). To confirm that the increase of BSA NT equivalents was dependent on NO production, we incubated mussel hemocytes with L-NIO, a NO-synthase inhibitor. When these hemocytes were stimulated with PMA, mean 3-nitrotyrosine concentrations of 0.082 ± 0.024 µM BSA NT equivalents were obtained (Table 1). They corresponded to 69% inhibition when compared to hemocytes untreated with NOsynthase inhibitor.

Measurement of 3-nitrotyrosine content in plasma proteins from marine bivalve before and after in vitro phagocytosis of zymosan particles:
As shown in table 2, the phagocytosis of zymosan particles by M. galloprovincialis haemocytes promoted a marked increase in the 3-nitrotyrosine concentration in mussel plasma, reaching mean concentrations (n = 20) of 0.214 ± 0.023 µM BSA NT equivalents (5.8-fold enhancement). The addition of DPI, a strong inhibitor of both NO-synthase and NADPH-oxidase, before phagocytosis lowered the 3-nitrotyrosine contents of plasma proteins to a mean concentration (n = 20) of 0.117 ± 0.036 µM BSA NT equivalents (54.6 % inhibition).
As with mussel (M. galloprovincialis), the concentration of 3-nitrotyrosylated proteins in plasma samples from oysters (C. gigas) was quantified by ELISA and expressed as BSA NT equivalents using the standard curve of Fig. 1. Substantial individual variations were observed and a mean concentration (n = 18) of 0.023 ± 0.017 µM BSA NT equivalents was estimated (Table 2). In vitro Phagocytosis of zymosan particles by hemocytes, increased the mean 3-nitrotyrosine content of plasma to 0.176 ± 0.022 µM BSA NT equivalents (7.5-fold enhancement).
As in the mussel experiments, the increase in the 3-nitrotyrosine level was strongly inhibited by incubation of haemolymph with DPI (72 % inhibition), before the addition of zymosan particles (Table 2).

Discussion
This study describes a C-ELISA for the detection of 3-nitrotyrosine residues in proteins from marine bivalve plasma.
Concentrations of 3-nitrotyrosine residues in the plasma proteins from untreated mussels and oysters varied between animals but they always increased markedly after in vitro PMA-stimulation of haemocytes or after phagocytosis of zymosan particles. These increases were inhibited by haemolymph preincubation with DPI or L-NIO.
The low levels of endogenous 3-nitrotyrosine residues in the plasma of untreated animals could have been due to peroxynitrite, a reactive nitrogen species that can modify tyrosine residues of proteins into 3-nitrotyrosine, as shown by Beckman and Koppenol [14] when studying human blood cells. Other reactive nitrogen species such as nitric oxide and nitrite anion could also be involved [21]. The 3-nitrotyrosine levels detected in proteins from untreated mussel and oyster haemolymph could thus reflect the exposure of proteins to all the nitrating agents produced by cells and thus confirm the generation of nitric oxide by these species.    The increased 3-nitrotyrosine levels observed after PMA-stimulation of haemocytes and zymosan particle phagocytosis, and inhibition with DPI and L-NIO, confirmed our previous results [8,11,12] and showed that peroxynitrite is generated by haemocytes in response to activation of both NADPH-oxidase and NO-synthase metabolic pathways.

Conclusion
The C-ELISA method we developed is sensitive enough to determine the amounts of 3-nitrotyrosine in plasma proteins of a single animal and to measure variations in 3-nitrotyrosine contents promoted by haemocyte stimulation or zymosan particle phagocytosis.
However, this method remains semi-quantitative since the 3-nitrotyrosine antibody may not bind all 3-nitrotyrosine residues in a sample containing a mixture of proteins due to inaccessibility to some 3-nitrotyrosine residues because of the influence of adjacent aminoacids on antibody binding. We used the C-ELISA method to detect and quantify the stress of mussels and oysters exposed to environmental variations.

Experimental animals
Two year old mussels (M. galloprovincialis) and oysters (C. gigas) raised at Palavas (France), were maintained in laboratory in a running sea water system (salinity: 33??, 17°C) with continuous aeration.

Haemolymph collection
One ml haemolymph from each animal was withdrawn (from the posterior adductor muscle of mussels and from the pericardial cavity of oysters) in disposable plastic syringes (2 ml, 21G needle) and transferred to polypropylene test tubes kept on ice.

Haemocyte stimulation
Haemocytes were stimulated by the addition of 50 µl of the zymosan suspension (40 mg mL -1 , final pH 7.1) or by 10 µl of PMA (1 mg mL -1 ). In inhibition experiments, haemolymph was preincubated for 5 min with DPI, L-NIO or filtrated sea water (control). The haemolymph was centrifuged (180 g, 5 min., 4°C) and the supernatant (plasma) collected by carefull pipetting using Pasteur pipettes. The viability of cells in the presence of various added agents was tested before the experiments by trypan blue exclusion assay.

Synthesis of nitrosylated BSA (BSA NT )
3-nitrotyrosine was coupled to carboxylic groups of BSA using N-hydroxysuccinimide/N-ethyl-N'-(dimethylaminopropyl)-carbodiimide reagent at pH 8.3. After incubation overnight under stirring at 4°C, the solution was extensively dialysed against PBS. The 3-nitrotyrosine content of BSA NT was determined by absorbance at 438 nm (pH 9.0) using a molar extinction coefficient of 4300 M -1 cm -1 . It was in the 2-3 mol nitrotyrosine / mol BSA range.

C-ELISA immunoassays
Each well from the 96-well plates was coated with 50 µL BSA NT (10 µg mL -1 in PBS pH = 7.2), incubated overnight at 4°C, washed with 250 µL PBS / 0.1% Tween buffer and then blocked for 1 hr at 37°C with 5% skimmed milk to prevent nonspecific binding.
Serial dilutions of BSA NT (0.2 to 0.003 µM) in PBS (for standard curve determination) or plasma sample from individual marine bivalves were incubated v/v overnight at 4°C under stirring with monoclonal anti 3-nitrotyrosine mouse IgG (1:2000). These solutions were then poured into coated wells (50 µL / well). After 2 h incubation at 37°C, 3-nitrotyrosine antibody bound to BSA NT was labelled by the addition of anti-mouse IgG peroxidase-conjugate (1: 4000, 50 µL / well) and incubated for 90 min at 37°C. Then colour development was initiated by the addition of peroxidase substrate (orthophenylene diamine), allowed to develop for up to 15 min at room temperature and terminated by the addition of 4 N sulphuric acid. Antibody bound to BSA NT was determined from the absorbance at 490 nm and expressed as BSA NT equivalents.