Effect of nitration on prolactin activities

Comparison of nitrotyrosine antibodies and development of immunoassays for the detection of nitrated proteins

Three monoclonal antibodies (mAb) and three polyclonal antibodies (pAb) have been characterized and compared with respect to their cross-reactivities and affinities for 3-nitrotyrosine, eight aromatic compounds with similar chemical structures, a peptide containing a single nitrotyrosine residue, and fourteen nitrated protein standards (bovine serum albumin, BSA) containing different numbers of nitrotyrosine residues per protein molecule (0.2 to 16.8). In indirect competitive immunoassays, mAb Alexis 39B6 exhibited the highest affinity for free 3-nitrotyrosine (10(6) L mol(-1)), while the pAb Oxis 24312 from sheep exhibited the highest affinities for nitrated proteins (up to 10(8) L mol(-1)). The apparent affinities determined in the indirect competitive assays were inversely correlated with the limits of detection (LOD) determined in one-sided immunoassays. With the sheep pAb minimum LOD on the order of 10 pmol L(-1) were achieved for highly nitrated proteins, corresponding to effective LOD on the order of 100 pmol L(-1) for nitrotyrosine residues. In the one-sided assays, however, the LOD for nitrated proteins increased proportionally with increasing background concentrations of native proteins in the investigated samples. Sandwich immunoassays combining pAb and mAb for selective enrichment and detection of nitrated proteins allowed to eliminate this native protein matrix effect and to achieve LOD on the order of 300 pmol L(-1) for highly nitrated proteins independent of native protein background concentrations.

Nitration modifying function of proteins, hormones and neurotransmitters

Several lines of evidence have been accumulated for occurrence of nitration in vivo. In this brief review, we summarized nitration studies on functional changes of proteins, hormones and neurotransmitters, before as well as after the discovery of peroxynitrite. Most of nitrated molecules exhibit less active properties than the parental compounds. It is still unknown whether nitration is merely a footprint of oxidative stress, an important pathway of nitric oxide metabolisms or a part of integral processes for maintaining cellular homeostasis.

Identification of a tyrosine residue in ovine placental lactogen as essential for its binding to receptors

Nitration of ovine placental lactogen (oPL) with a 10-fold molar excess of tetranitromethane over protein content resulted in the modification of 0.8 tyrosine residue. No conformational changes were observed by either fourth-derivative spectral analysis or circular dichroism. Nitration significantly decreased the binding capacity of the hormone to lactogenic and somatogenic rat liver receptors. This binding capacity was not restored by reduction of the nitro groups, thus indicating that the decrease was not due to the difference in pK between tyrosine and nitrotyrosine. The nitrotyrosine-containing peptide was isolated from a tryptic digest by HPLC and its modification extent was of 67%, which is consistent with the decrease in binding capacities (65% and 70%). Its amino acid sequence was determined and the modified tyrosine residue was identified as Tyr-46. These results provide the first evidence of the involvement of a tyrosine residue in the binding of oPL to both lactogenic and somatogenic receptors. This tyrosine appears to be a shared binding epitope between oPL and the prolactins.

Chemical modification of ovine prolactin with N-acetylimidazole

Reaction of ovine prolactin (oPRL) with a 150-fold molar excess of N-acetylimidazole over protein content resulted in the modification of 2.5 tyrosine residues and 1.2 lysine residues. Acetylation greatly decreased the in vitro binding capacity to lactogenic sites. This binding capacity was partially restored by ammonium bicarbonate treatment, which removes O-acetyl groups from tyrosine residues but not N-acetyl groups from lysine residues. The modification extent of the tyrosine residues was determined. The results suggest that acetylation of tyrosine 44 or of tyrosine 96 is likely to be responsible for the decrease in binding activity of acetylated oPRL, and that one of these residues may play a role in the interaction of oPRL with lactogenic receptors.

Thermodynamic analysis of the interaction of prolactin with its receptor in the rabbit mammary-gland microsomes

The interaction of prolactin (PRL) with its membrane receptor depends markedly on temperature. Thermodynamic parameters for this reaction have been evaluated from data for time-course kinetics and equilibrium binding at multiple temperatures between 19 and 31 degrees C. The free-energy change with temperature and the van't Hoff plot were found to be linear. These suggest that there are minimal structure changes at the PRL-receptor contact site over this temperature range. The positive signs of the entropy and enthalpy of reaction, and of the entropy of activation (delta S++) for association, indicate that the hydrophobic bonding is the most significant force involved in PRL-receptor formation. The delta S++ for dissociation was negative, and the enthalpy of activation for dissociation was about 20.3 kJ.mol-1 larger than that for association, indicating that the PRL-receptor complex is further stabilized by contributions of hydrogen bonds and van der Waals contacts after the initial interaction. The free energy of activation for dissociation, at 25 degrees C was about 2.5-fold larger than that for association. This would cause slow dissociation of PRL from its receptor.

Characterization of a biotin-conjugated ovine prolactin ligand

Ovine prolactin was biotinylated with N-hydroxysuccinimidobiotin. Biotinylation was proportional to the molar ratio of reactants. Gel electrophoresis of the biotinylated derivative revealed little or no change in migration, but isoelectric focusing showed an acidic shift when compared to oPRL. Biotinylated ovine prolactin (B-oPRL) was detected by anti-oPRL antiserum and avidin-fluorescein-isothiocyanate (FITC) on protein blots. Competitive binding assays using 125I-B-oPRL and 125I-oPRL revealed: (a) similar dissociation constants and ID50 values for binding to anti-oPRL antibodies; (b) similar dissociation constants and maximum binding values for binding to liver membrane preparations; and (c) similar dissociation curves for displacement by several pituitary hormones. In contrast, binding of biotinylated oPRL to Nb2 node cells was reduced by approximately 70% and its bioreactivity was only 10% of that of oPRL. Our results indicate that biotinylation of oPRL does not alter its binding characteristics as measured by radioimmunoassay and radioreceptor assay using hepatic lactogenic receptors, but decreases its binding and bioreactivity when measured in Nb2 lymphoma cells. Assuming that N-hydroxysuccinimidobiotin interacts with reactive free amino groups of oPRL, our results suggest that these groups are essential for binding and bioreactivity of the molecule in the Nb2 lymphoma cell system.

Characterization of a prolactin-daunomycin ligand as a probe for drug targeting

Conjugates of ovine prolactin and daunomycin were prepared for use as affinity-labelled drug carriers in cancer cells carrying the prolactin receptor. The binding affinity of the conjugates to prolactin receptors in rat liver membrane preparations and in viable granulosa cells derived from estradiol- and pregnant mare serum gonadotropin (PMSG)-treated immature female rats was less than an order of magnitude lower than prolactin. The toxicity of the conjugate in cultured granulosa cells was dependent upon the concentration of the daunomycin present in the culture. The cytotoxic effect of the ligand was abolished by the addition of free prolactin or NH4Cl to the granulosa cell cultures. These conjugates may be useful probes in drug targeting against hormone-sensitive cancer.