Spectroscopic studies on Tb3+ binding to S-100a protein

Direct binding assay and fluorescence studies revealed that S-100a protein binds 2 mol of Tb3+/mol of protein at pH 6.6. The protein binds Tb3+ much more tightly than Ca2+, and the upper limit of the observed Kd value for Tb3+ is 3.5 x 10(-6) M. The Tb3+-binding site on the protein must be close to a tyrosine residue, as indicated by fluorescence excitation and emission spectra, where energy transfer from tyrosine is noted. Addition of Tb3+ resulted in a conformational change in the protein, as revealed by u.v.-difference spectroscopy and c.d. studies. Far-u.v. c.d. studies indicated the helical content to decrease from approx. 39% to 35% in the presence of Tb3+. From u.v.-difference-spectroscopy results the single tryptophan and the tyrosine chromophores in S-100a protein are blue-shifted (i.e. exposed to the solvent) in the presence of Tb3+ and the observed conformational changes are similar to those induced by Ca2+, suggesting that Tb3+ can be employed as a Ca2+ analogue in spectral studies with S-100a protein.

Fluorescence studies on the Ca2+ and Zn2+ binding properties of the alpha-subunit of bovine brain S-100a protein

The single cysteine on the alpha-subunit of bovine brain S-100a protein has been modified with the thiol specific probe, Acrylodan. When the labelled apoprotein was excited at 380 nm the fluorescence emission maximum was centered at 484 +/- 2 nm, suggesting that the probe is in a fairly hydrophobic environment. Addition of Ca2+ to the protein caused the emission maximum to undergo a red shift to 504 +/- 2 nm, implying that the fluorophore is now more exposed to the solvent. Zn2+, when added to the protein, induced only a small perturbation and the emission maximum shifted to 481 +/- 2 nm. Ca2+ was able to perturb the fluorophore in the presence of Zn2+. 2-p-Toluidinylnaphthalene-6-sulfonate (TNS)-labelled alpha-subunit when excited at 345 nm exhibited very little fluorescence in the absence of Ca2+. Addition of Ca2+ resulted in an increase in TNS fluorescence accompanied by a blue shift of the emission maximum to 445 +/- 1 nm indicating that the probe in the presence of Ca2+ moves to a hydrophobic domain. The fact that Ca2+ and Zn2+ can perturb the labelled sulfhydryl group in the presence of each other clearly demonstrates that the binding sites for the two metal ions must be different on the alpha-subunit as well as on the S-100a protein.

Preparation and evaluation of 99mTc(V)-DMSA complex: studies in medullary carcinoma of thyroid

Consequent to the promising results reported with 99mTc(V)-DMSA for imaging certain types of soft tissue tumors, we have developed methods to prepare this radiopharmaceutical in three ways: from freshly prepared reagents, through the use of a two component kit and use of the standard renal DMSA kit by a modified recipe. The 99mTc(V)-DMSA complex has been subjected to paper electrophoretic and chromatographic procedures and also biodistribution studies. The distinctly different behaviour of this new product compared to that of the well known renal DMSA complex has been clearly established. Scintiimaging in a preliminary clinical trial in patients with medullary carcinoma of the thyroid has been encouraging.

Ca2+ and Zn2+-binding properties of nitrated S-100b protein from bovine brain

The single tyrosine residue in S-100b protein was nitrated by treatment with tetranitromethane in 0.1 M-Tris/HCl buffer, pH 8.0, containing 2 mM-EDTA. The nitrated protein did not differ significantly in secondary structure from its native unmodified counterpart, as revealed by far-u.v. c.d. measurements. The effect of Ca2+ on the modified protein was different from that on the native protein, e.g. addition of Ca2+ resulted in a loss of helical content from 55 to 47% with the native protein whereas Ca2+ had no significant effect on the gross conformation of the nitrated derivative. Near-u.v. c.d. studies also indicated a very minimal effect on the tyrosine residue and this was also reflected in the u.v.-absorption difference spectrum. Polyacrylamide-gel electrophoresis in the absence of SDS showed the nitrated S-100b to move faster in the presence of EDTA compared with the calcium-bound state, suggesting that the modified protein does bind Ca2+ although it does not undergo a major conformational change in response to Ca2+ addition. In contradistinction, Zn2+ binding was not influenced by nitration, as demonstrated by aromatic c.d. and u.v.-difference spectroscopy. It is clear from this study that the single tyrosine residue in S-100b is critical to sense the Ca2+-induced conformational changes in the protein.

Isolation, characterization and metal-ion-binding properties of the alpha-subunit from S-100a protein

The brain-specific S-100 protein is a mixture of two predominant components, S-100a and S-100b, with subunit compositions of alpha beta and beta beta respectively. In the present study, the alpha-subunit, isolated from S-100a by using anion-exchange chromatography in the presence of 8 M-urea, was homogeneous by the criteria of SDS/polyacrylamide-gel, urea/SDS/polyacrylamide-gel and non-SDS/polyacrylamide-gel electrophoresis. The alpha-subunit underwent a conformational change upon binding Ca2+ and Zn2+ at pH 7.5, as revealed by u.v. difference spectroscopy, c.d. and fluorescence measurements. Far-u.v. c.d. studies indicated the apparent alpha-helical content to fall when the protein bound either Ca2+ or Zn2+. Addition of Ca2+ to the alpha-subunit resulted in exposing to the solvent the single tryptophan residue and one or more tyrosine and phenylalanine residues. Zn2+ induced only a small conformational change, and among the aromatic chromophores only tyrosine residues were affected to a small extent. Ca2+ was able to bind to the alpha-subunit in the presence of Zn2+, and the two metal-ion-binding sites appeared to be different. When the apoprotein was excited at 280 nm, the fluorescence emission maximum was located at 337 nm. In the presence of Ca2+, the emission maximum occurred at 340 nm and was accompanied by a nearly 25% increase in fluorescence intensity. Fluorescence titration with Ca2+ at pH 7.5 revealed only one class of binding site, with a Kd value of 1.26 X 10(-4) M. The effect of K+ on the protein was slightly antagonistic to that of Ca2+, as indicated by u.v. difference spectroscopy and fluorescence titration.

Mild radioiodination of insulin with dichloramine-T

Iodination of insulin using N-dichloro-p-toluene sulphonamide(dichloramine-T) has been standardised. Dichloramine-T, a water insoluble derivative of chloramine-T showed excellent properties as an iodination reagent, for the preparation of radiolabeled insulin for use in radioimmunoassay. Iodination using dichloramine-T could be done at as low as 0.5-2 micrograms of the reagent, and at this concentration the molar ratio worked out to 1:3 to 1:12 (protein:dichloramine-T). Iodination yields of greater than 90% were obtained at pH 5-7. Evaluation of the iodinated tracer for suitability in radioimmunoassay was carried out by estimating radiochemical purity, immunological purity, non specific binding and stability on storage.

Hydrodynamic properties of bovine brain S-100 proteins

The size and shape of S-100a and S-100b proteins in solution have been examined by gel filtration and ultracentrifugation in the presence and absence of Ca2+. S-100a and S-100b proteins, in the absence of Ca2+, have an intrinsic sedimentation coefficient, so20,w of 2.20 and 2.15 S, respectively and in 1 mM Ca2+ their so20,w values were decreased to 2.05 and 1.95 S, respectively, indicating an unfolding of the protein molecules. The Stokes radii of S-100a and S-100b (-Ca2+) were 23.4 A and 24.0 A and they decreased to 22.2 A and 22.3 A in the presence of Ca2+. The Ca2+ effect on S-100b greater than S-100a was in agreement with our earlier CD observations. Among the monovalent cations tested (K+, Na+ and Li+) K+ had the maximum effect on the Stokes radii and so20,w values of S-100 proteins. Since certain functions of the nervous system are accompanied by local changes in ionic concentrations of Ca2+, Na+ and K+, it is conceivable that these respective conformational changes induced in S-100 proteins by these metals may be related to their function in the brain.

Separation of bound and free ligand by ethacridine (Rivanol) in thyroxin radioimmunoassay

In this simple and low-cost radioimmunoassay (RIA) procedure for thyroxin (T4), ethacridine (6,9-diamino-2-ethoxyacridine lactate; Rivanol) is used to separate antibody-bound ligand and free ligand. This acridine dye precipitates free T4 not bound to antibody. At pH 8.6, it precipitates more than 90% of 125I-labeled T4. Precipitation is rapid and is constant over the concentration range of 1 to 3 g of ethacridine per liter; time of addition and temperature are not critical. The inherent yellow color of the dye gives it the advantage of a color-coded reagent. Analysis of 65 serum samples by the proposed method and by a procedure involving polyethylene glycol gave similar results: correlation coefficient, 0.988; slope, 0.981; y-intercept, -1.42. Within- and between-assay variations (CV) were less than 5 and 6%, respectively.

Separation of bound and free ligand by ethacridine (Rivanol) in thyroxin radioimmunoassay

In this simple and low-cost radioimmunoassay (RIA) procedure for thyroxin (T4), ethacridine (6,9-diamino-2-ethoxyacridine lactate; Rivanol) is used to separate antibody-bound ligand and free ligand. This acridine dye precipitates free T4 not bound to antibody. At pH 8.6, it precipitates more than 90% of 125I-labeled T4. Precipitation is rapid and is constant over the concentration range of 1 to 3 g of ethacridine per liter; time of addition and temperature are not critical. The inherent yellow color of the dye gives it the advantage of a color-coded reagent. Analysis of 65 serum samples by the proposed method and by a procedure involving polyethylene glycol gave similar results: correlation coefficient, 0.988; slope, 0.981; y-intercept, -1.42. Within- and between-assay variations (CV) were less than 5 and 6%, respectively.