Mechanism of DNA substrate recognition by the mammalian DNA repair enzyme, Polynucleotide Kinase

Mammalian polynucleotide kinase (mPNK) is a critical DNA repair enzyme whose 5'-kinase and 3'-phoshatase activities function with poorly understood but striking specificity to restore 5'-phosphate/3'-hydroxyl termini at sites of DNA damage. Here we integrated site-directed mutagenesis and small-angle X-ray scattering (SAXS) combined with advanced computational approaches to characterize the conformational variability and DNA-binding properties of mPNK. The flexible attachment of the FHA domain to the catalytic segment, elucidated by SAXS, enables the interactions of mPNK with diverse DNA substrates and protein partners required for effective orchestration of DNA end repair. Point mutations surrounding the kinase active site identified two substrate recognition surfaces positioned to contact distinct regions on either side of the phosphorylated 5'-hydroxyl. DNA substrates bind across the kinase active site cleft to position the double-stranded portion upstream of the 5'-hydroxyl on one side, and the 3'-overhang on the opposite side. The bipartite DNA-binding surface of the mPNK kinase domain explains its preference for recessed 5'-termini, structures that would be encountered in the course of DNA strand break repair.

Fluorescence studies of substrate binding to human recombinant deoxycytidine kinase

Deoxycytidine kinase (dCK), is responsible for the phosphorylation of deoxynucleosides to the corresponding monophosphates using ATP or UTP as phosphate donors. Steady-state intrinsic fluorescence measurements were used to study interaction of dCK with substrates in the absence and presence of phosphate donors. Enzyme fluorescence quenching by its substrates exhibited unimodal quenching when excited at 295 nm. Binding of substrates induced conformational changes in the protein, suggesting that dCK can assume different conformational states with different substrates and may account for the observed differences in their specificity. dCK bound the substrates more tightly in the presence of phosphate donors and UTP is the preferred phosphate donor. Among the substrates tested, the antitumour drugs gemcitabine and cladribine were bound very tightly by dCK, yielding Kd values of 0.75 and 0.8 microM, respectively, in the presence of UTP.

Physical properties of human polynucleotide kinase: hydrodynamic and spectroscopic studies

Human polynucleotide kinase (hPNK) is a putative DNA repair enzyme in the base excision repair pathway required for processing and rejoining strand-break termini. This study represents the first systematic examination of the physical properties of this enzyme. The protein was produced in Escherichia coli as a His-tagged protein, and the purified recombinant protein exhibited both the kinase and the phosphatase activities. The predicted relative molecular mass (M(r)) of the 521 amino acid polypeptide encoded by the sequenced cDNA for PNK and the additional 21 amino acids of the His tag is 59,538. The M(r) determined by low-speed sedimentation equilibrium under nondenaturing conditions was 59,600 +/- 1000, indicating that the protein exists as a monomer, in contrast to T4 phage PNK, which exists as a homotetramer. The size and shape of hPNK in solution were determined by analytical ultracentrifugation studies. The protein was found to have an intrinsic sedimentation coefficient, s(0)(20,w), of 3.54 S and a Stokes radius, R(s), of 37.5 A. These hydrodynamic data, together with the M(r) of 59 600, suggest that hPNK is a moderately asymmetric protein with an axial ratio of 5.51. Analysis of the secondary structure of hPNK on the basis of circular dichroism spectra, which revealed the presence of two negative dichroic bands located at 218 and 209 nm, with ellipticity values of -7200 +/- 300 and -7800 +/- 300 deg x cm(2) x d(mol(-1), respectively, indicated the presence of approximately 50% beta-structure and 25% alpha-helix. Binding of ATP to the protein induced an increase in beta-structure and perturbed tryptophan, tyrosine, and phenylalanine signals observed by aromatic CD and UV difference spectroscopy.

Functional production and reconstitution of the human equilibrative nucleoside transporter (hENT1) in Saccharomyces cerevisiae. Interaction of inhibitors of nucleoside transport with recombinant hENT1 and a glycosylation-defective derivative (hENT1/N48Q)

We have produced recombinant human equilibrative nucleoside transporter (hENT1) in the yeast Saccharomyces cerevisiae and have compared the binding of inhibitors of equilibrative nucleoside transport with the wild-type transporter and a N-glycosylation-defective mutant transporter. Equilibrium binding of 3H-labelled nitrobenzylmercaptopurine ribonucleoside {6-[(4-nitrobenzyl)thio]-9-beta-d-ribofuranosyl purine; NBMPR} to hENT1-producing yeast revealed a single class of high-affinity sites that were shown to be in membrane fractions by (1) equilibrium binding (means+/-S.D.) of [3H]NBMPR to intact yeast (Kd 1.2+/-0.2 nM; Bmax 5.0+/-0.5 pmol/mg of protein) and membranes (Kd 0.7+/-0.2 nM; Bmax 6.5+/-1 pmol/mg of protein), and (2) reconstitution of hENT1-mediated [3H]thymidine transport into proteoliposomes that was potently inhibited by NBMPR. Dilazep and dipyridamole inhibited NBMPR binding to hENT1 with IC50 values of 130+/-10 and 380+/-20 nM respectively. The role of N-linked glycosylation in the interaction of NBMPR with hENT1 was examined by the quantification of binding of [3H]NBMPR to yeast producing either wild-type hENT1 or a glycosylation-defective mutant (hENT1/N48Q) in which Asn-48 was converted into Gln. The Kd for binding of NBMPR to hENT1/N48Q was 10. 5+/-1.6 nM, indicating that the replacement of an Asn residue with Gln decreased the affinity of hENT1 for NBMPR. The decreased affinity of hENT1/N48Q for NBMPR was due to an increased rate of dissociation (koff) and a decreased rate of association (kon) of specifically bound [3H]NBMPR because the values for hENT1-producing and hENT1/N48Q-producing yeast were respectively 0.14+/-0.02 and 0. 36+/-0.05 min-1 for koff, and (1.2+/-0.1)x10(8) and (0.40+/-0. 04)x10(8) M-1.min-1 for kon. These results indicated that the conservative conversion of an Asn residue into Gln at position 48 of hENT1 and/or the loss of N-linked glycosylation capability altered the binding characteristics of the transporter for NBMPR, dilazep and dipyridamole.

Demonstration of equilibrative nucleoside transporters (hENT1 and hENT2) in nuclear envelopes of cultured human choriocarcinoma (BeWo) cells by functional reconstitution in proteoliposomes

The equilibrative nucleoside transporters (ENTs) are a newly recognized family of membrane proteins of which hENT1 is the nitrobenzylmercaptopurine ribonucleoside (NBMPR)-sensitive (es) and hENT2 the NBMPR-insensitive (ei) transporter of human cells. BeWo cells exhibit large numbers (>10(7)/cell) of NBMPR-binding sites and high es and ei nucleoside transport activities relative to other cell types. In this work, we have demonstrated that proliferating BeWo cells possess (i) mRNA encoding hENT1 and hENT2 and (ii) hENT1-specific immunoepitopes. We examined NBMPR binding and its inhibition of uridine transport in various BeWo membrane fractions and proteoliposomes derived therefrom to determine if NBMPR binding to intracellular membranes represented interaction with functional es transporters. Unfractionated membranes and fractions enriched 5-fold in plasma membranes relative to postnuclear supernatants exhibited high NBMPR binding activity. Intact nuclei and nuclear envelopes also exhibited abundant quantities of NBMPR-binding sites with affinities similar to those of enriched plasma membranes (Kd = 0.4-0.9 nM). When proteoliposomes were made from octyl glucoside-solubilized membranes, high affinity NBMPR-binding sites were not only observed in crude membrane preparations and plasma membrane-enriched fractions but also in nuclear envelope fractions. Proteoliposomes prepared from either unfractionated membranes or nuclear envelopes exhibited both hENT1-mediated (82-85%) and hENT2-mediated (15-18%) transport of [3H]uridine. These results provided evidence for the presence of functional es and ei transporters in nuclear membranes and endoplasmic reticulum, suggesting that hENT1 and hENT2 may function in the translocation of nucleosides between the cytosol and the luminal compartments of one or both of these membrane types.

Functional nucleoside transporters are required for gemcitabine influx and manifestation of toxicity in cancer cell lines

Gemcitabine (2',2'-difluorodeoxycytidine) is a novel pyrimidine nucleoside drug with clinical efficacy in several common epithelial cancers. We have proposed that gemcitabine requires nucleoside transporter (NT) proteins to permeate the plasma membrane and to exhibit pharmacological activity. In humans, there are seven reported distinct NT activities varying in substrate specificity, sodium dependence, and sensitivity to inhibition by nitrobenzylthioinosine (NBMPR) and dipyridamole. To determine which NTs are required for gemcitabine-dependent growth inhibition, cultures from a panel of 12 cell lines with defined plasma membrane NT activities were incubated with different concentrations of gemcitabine. Cell proliferation was assessed by the sulforhodamine B assay and cell enumeration to identify the concentrations of gemcitabine that inhibited cell replication by 50% (IC50s). NT activity was a prerequisite for growth inhibition in vitro because: (a) the nucleoside transport-deficient cells were highly resistant to gemcitabine; and (b) treatment of cells that exhibited only equilibrative NT activity with NBMPR or dipyridamole increased resistance to gemcitabine by 39- to 1800-fold. These data suggested that the type of NT activities possessed by a cell may be an important determinant of its sensitivity to gemcitabine and that NT deficiency may confer significant gemcitabine resistance. We analyzed the uptake kinetics of [3H]gemcitabine by each of five human NT activities in cell lines that exhibited a single NT activity in isolation; transient transfection of the cDNAs encoding the human concentrative NT proteins (hCNT1 and hCNT2) was used to study the cit and cif activities, respectively. The efficiency of gemcitabine uptake varied markedly among the cell lines with single NTs: es approximately = cit > ei > cib >>> cif. The transportability of [3H]gemcitabine was demonstrated by reconstitution of the human es NT in proteoliposomes, confirming that gemcitabine permeation is a protein-mediated process.

Influence of caltropin on the caldesmon induced polymerization of G-actin

The effect of caltropin (CaT) on the caldesmon (CaD)-G-actin interaction was monitored by viscosity measurements, bioassays measuring the release of inorganic phosphate (Pi) following G-actin polymerization and fluorescence studies using acrylodan labelled G-actin. CaD induced polymerization of G-actin into filaments in the absence of salt was accompanied by an increase in relative viscosity. This effect of CaD was essentially abolished by CaT in the presence of Ca2+. In bioassays the rate of Pi release was reduced significantly in the presence of Ca2+/CaT. Acrylodan labelled G-actin when excited at 375 nm exhibited an emission maximum at 478 nm. Polymerization of G-actin resulted in shifting the emission maximum to 465 nm. When CaD was added to G-actin containing Ca2+/CaT, the rate of G-actin polymerization was reduced considerably, suggesting that CaT interferes in the CaD-G-actin interaction.

Interaction between caltropin and the C-terminal region of smooth muscle caldesmon

Caltropin (CaT) binds caldesmon (CaD) in a Ca(2+)-dependent manner with an affinity higher than that of calmodulin (CaM). Photo-crosslinking between CaT and a benzophenone-labeled C-terminal CaD fragment (27K) results in a 35-kDa protein that corresponds to the 1:1 adduct between CaT and 27K. In the absence of Ca2+, no crosslinking is obtained. This result is similar to that obtained with CaM and 27K. The apparent affinity of CaM for GS17C, a CaM-binding peptide of CaD, is weakened by CaT, suggesting CaT competes with CaM for the peptide. In contrast to CaM, CaT does not induce changes in the tryptophan fluorescence of GS17C. Thus although the two Ca(2+)-binding proteins behave similarly, there are differences in their interactions with CaD.

Effect of caltropin on caldesmon-actin interaction

The binding of chicken gizzard caldesmon to actin was studied both in the presence and the absence of caltropin using Airfuge centrifugation experiments, disulfide cross-linking studies, and the fluorescent probe acrylodan (6-acryloyl-2-(dimethylamino)naphthalene). In co-sedimentation studies most of the caldesmon pelleted along with actin. However, when caldesmon in the presence of caltropin was mixed with actin, caldesmon did not pellet along with actin following high speed centrifugation, suggesting that caltropin has significantly weakened its binding to actin. The caltropin effect was noticed even when tropomyosin was included in the reaction mixture. Acrylodan-labeled caldesmon, when excited at 375 nm, had an emission maximum at 515 +/- 2 nm. The addition of actin produced a nearly 70% increase in fluorescent intensity, accompanied by a blue shift in the emission maximum (i.e. lambda em (max) = 505 +/- 2 nm), suggesting that the probe now occupies a more nonpolar environment. Titration of labeled caldesmon with actin indicated a strong affinity (K alpha = approximately 6 x 10(7) M-1). When actin was titrated with labeled caldesmon in the presence of caltropin in a 0.2 mM Ca2+ medium, its affinity for caldesmon was lowered (K alpha = approximately 2 x 10(7) M-1). Caltropin, which is very effective in reversing caldesmon's inhibition of the actin-activated myosin ATPase (Mani, R. S., McCubbin, W. D., and Kay, C. M. (1992) Biochemistry 31, 11896-11901), is shown in the present study to have a pronounced effect on its binding to actin, suggesting a major role for caltropin in regulating caldesmon in smooth muscle.

Calcium-dependent regulation of the caldesmon-heavy meromyosin interaction by caltropin

The binding of chicken gizzard caldesmon to smooth muscle heavy meromyosin (HMM) was studied using caldesmon-Sepharose 4B affinity chromatography, far-ultraviolet circular dichroism (CD), and the fluorescent probe acrylodan. When HMM was applied to a caldesmon-Sepharose column in the presence of 40 mM NaCl, most of the protein was retained on the column, and HMM could be eluted by increasing the NaCl level to 0.5 M; this interaction was not Ca(2+)-dependent. Far-UV CD studies indicated an interaction between caldesmon and HMM since the experimentally observed ellipticity values at 222 and 207 nm deviated from the theoretical values for the complex, and this interaction was also not Ca(2+)-sensitive. Addition of HMM to a caldesmon-caltropin complex induced a conformational change suggesting the formation of a ternary complex for which Ca2+ was essential. Acrylodan-labeled caldesmon, when excited at 375 nm, had an emission maximum at 515 +/- 2 nm. Addition of HMM resulted in a nearly 20% decrease in fluorescence intensity with little or no shift in the emission maximum. Titration of HMM with labeled caldesmon indicated a strong affinity for HMM [K(a) was on the order of (4.5 +/- 0.5) x 10(7) M-1], and this interaction was observed both in the presence and in the absence of calcium. When HMM was titrated with labeled caldesmon in the presence of caltropin in a 0.2 mM Ca2+ medium, its affinity for caldesmon was lowered nearly 3-fold [K(a) approximately (1.50 +/- 0.5) x 10(7) M-1].(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-dependent regulation of caldesmon by an 11-kDa smooth muscle calcium-binding protein, caltropin

Caldesmon from chicken gizzard muscle has been examined for its ability to interact with caltropin using affinity chromatography and the fluorescent probe acrylodan. The action of caltropin on the inhibitory effect of caldesmon on actomyosin ATPase was also studied. Like calmodulin, caltropin could release the inhibitory effect of caldesmon in the presence of Ca2+. Complete reversal was obtained when 1 mol of caltropin was added per mol of caldesmon. When caldesmon was applied to caltropin-Sepharose in the presence of Ca2+, most of the caldesmon was bound to the column and could be eluted with EGTA, indicating that there is a direct interaction between caldesmon and caltropin. Acrylodan-labeled caldesmon, when excited at 375 nm, had an emission maximum at 504 nm. Addition of caltropin in the presence of Ca2+ resulted in a nearly 50% increase in fluorescence intensity, and this was accompanied by a blue shift in the emission maximum (i.e., lambda em,max 492 nm), suggesting that the probe now occupies a more nonpolar environment. Titration of caltropin with labeled caldesmon indicated a strong affinity for this protein (Kd was in the order of 8 x 10(-8)-2 x 10(-7) M). However, when caltropin was added to labeled caldesmon in the presence of EGTA, there was no indication of any interaction. Caltropin was at least as potent as calmodulin, if not better, in reversing the inhibitory effect of caldesmon in the presence of calcium, making it a potential Ca2+ factor in regulating caldesmon in smooth muscle.

Purification and characterization of a novel 12,000-Da calcium binding protein from smooth muscle

A new low molecular weight calcium binding protein, designated 12-kDa CaBP, has been isolated from chicken gizzard using a phenyl-Sepharose affinity column followed by ion-exchange and gel filtration chromatographies. The isolated protein was homogeneous and has a molecular weight of 12,000 based on sodium dodecyl sulfate-gel electrophoresis. The amino acid composition of this protein is similar to but distinct from other known low molecular weight Ca2+ binding proteins. Ca2+ binding assays using Arsenazo III (Sigma) indicated that the protein binds 1 mol of Ca2+/mol of protein. The 12-kDa CaBP underwent a conformational change upon binding Ca2+, as revealed by uv difference spectroscopy and circular dichroism studies in the aromatic and far-ultraviolet range. Addition of Ca2+ to the 12-kDa CaBP labeled with 2-p-toluidinylnaphthalene-6-sulfonate (TNS) resulted in a sevenfold increase in fluorescence intensity, accompanied by a blue shift of the emission maximum from 463 to 445 nm. Hence, the probe in the presence of Ca2+ moves to a more nonpolar microenvironment. Like calmodulin and other related Ca2+ binding proteins, this protein also exposes a hydrophobic site upon binding calcium. Fluorescence titration with Ca2+ using TNS-labeled protein revealed the presence of a single high affinity calcium binding site (kd approximately 1 x 10(-6) M).

Conformation and dynamics of bovine brain S-100a protein determined by fluorescence spectroscopy

We have used time-resolved laser fluorescence spectroscopy to investigate the intensity and anisotropy decays of the single tryptophan residue in bovine brain S-100a (alpha beta) protein. The steady-state and acrylamide quenching results indicated that the Trp 90 of the alpha-subunit was partially buried in a relatively nonpolar environment at pH 7.5. Both Ca2+ and pH 8.5 slightly enhanced the exposure of the residue to the solvent, but the residue remained partially buried in the calcium complex at both pH values. The best representation of the intensity decays was a linear combination of three exponential terms, regardless of solvent condition and temperature. The three lifetimes (tau i) were in the range of 0.4-5 ns and insensitive to emission wavelength, but their fractional amplitudes (alpha i) shifted in favor of the shortest component (alpha 1) when the decays were measured at the blue end of the emission spectrum. These results suggest that an excited-state interaction between the indole ring and the side chain of an adjacent residue may be responsible for the observed shortest lifetime. In the presence of Ca2+, the three lifetimes remained relatively unaltered, but the values of alpha 1 decreased by a factor of 2.3 at pH 7.2 and a factor of 1.8 at pH 8.2. This Ca(2+)-induced decrease may be attributed to disruption of the putative excited-state interaction resulting from reorientations of the alpha-helical segments flanking a Ca(2+)-binding loop (residues 62-73). At both pH 7.2 and 8.4, the anisotropy decays of the apoprotein followed a biexponential decay law.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectral [corrected] studies on the cadmium-ion-binding properties of bovine brain S-100b protein

The effect of Cd2+ binding on bovine brain S-100b protein was studied using c.d. u.v. difference spectroscopy and fluorescence measurements. At pH 7.5, S-100b protein binds two Cd2+ ions per monomer with a Kd value of 3 x 10(-5) M. Addition of Cd2+ resulted in perturbing the single tyrosine residue (Tyr17) in the protein as indicated by u.v. difference spectroscopy and aromatic c.d. measurements. In the presence of Cd2+, the tyrosine residue moves to a more non-polar environment, since a red shift was observed in the u.v. difference spectrum. When the protein was excited at 278 nm, the tyrosine fluorescence emission maximum was centred at 306 nm. Cd2+ addition resulted in an increase in intrinsic fluorescence intensity. Fluorescence titration with Cd2+ indicated the protein binds Cd2+ with a Kd value of 3 x 10(-5) M. 2-p-Toluidinylnaphthalene-6-sulphonate-labelled protein, when excited at 345 nm, had a fluorescence emission maximum at 440 nm. Addition of Cd2+ to labelled protein resulted in a 5-fold increase in fluorescence intensity accompanied by a 5 nm blue shift in the emission maximum, suggesting that the probe, in the presence of Cd2+, moves to a hydrophobic domain. U.v. difference spectroscopic studies indicated a unique Cd2(+)-binding site on the protein, since Cd2+ addition yielded a large positive absorption band in the 240 nm region that is not found with either Ca2+ or Zn2- ions. Similar absorption bands have been observed in Cd-protein complexes such as Cd-metallothionein [Vasak, Kagi & Hill (1981) Biochemistry 20, 2852-2856] and also in model complexes of Cd2+ with 2-mercaptoethanol. This absorption band is believed to arise as a result of charge-transfer transitions between the thiolate and Cd2+. Of the two Cd2- -binding sites on the beta-chain, one must be located at the N-terminal end near the single tyrosine residue, since Cd2- and Zn2+ produced similar effects on the intrinsic protein fluorescence. The other Cd2+ site which is unique to Cd2+ must be Cys84, located at the C-terminal end.

Isolation and characterization of a novel molecular weight 11,000 Ca2(+)-binding protein from smooth muscle

A new low molecular weight calcium-binding protein, designated as SMCaBP-11, has been isolated from chicken gizzard using a phenyl-Sepharose affinity column followed by ion-exchange and gel filtration chromatographies. The isolated protein was homogeneous by the criteria of gel electrophoresis in the absence and presence of sodium dodecyl sulfate (NaDodSO4). Molecular weight studies by both sedimentation equilibrium in 6 M guanidine hydrochloride and 15% polyacrylamide-SDS gels indicated the subunit molecular weight to be 11,000, and since a molecular weight of 21,000 was obtained in native solvents, the protein exists as a dimer in benign medium. The amino acid composition of this protein is similar but distinct from other known low molecular weight Ca2(+)-binding proteins. Ca2(+)-binding assays using Arsenazo III (Sigma) indicated the protein to bind 2 mol of Ca2+/subunit. In non-SDS gels, the protein moved faster in the presence of EDTA, suggesting that Ca2+ binding affects its mobility in a manner similar to other smooth muscle calcium-binding proteins such as calmodulin and 67-kDa calcimedin. Upon binding calcium, the protein underwent a conformational change as revealed by UV difference spectroscopy and circular dichroism studies in the aromatic and far-ultraviolet range. When the protein was excited at 280 nm, the tyrosine fluorescence emission maximum was centered at 306 nm. Ca2+ addition resulted in a nearly 15% decrease in intrinsic fluorescence intensity. Fluorescence titration with Ca2+ exhibited two classes of calcium-binding sites with Kd values of 0.2 and 80 microM, in agreement with UV difference spectral data.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and spectral studies on the Ca2+-binding properties of 67 kDa calcimedin

The 67 kDa calcimedin, isolated by using a phenyl-Sepharose affinity column followed by DEAE-cellulose and gel-filtration chromatographies, was homogeneous by the criterion of SDS/polyacrylamide-gel electrophoresis. In non-SDS gels, the protein moved faster in the presence of EDTA, suggesting that Ca2+ binding affects its mobility in a manner similar to other Ca2+-binding proteins such as calmodulin and S-100 proteins. The 67 kDa protein underwent a conformational change upon binding Ca2+, as revealed by u.v. difference spectroscopy and near-u.v. c.d. measurements. Tryptophan and tyrosine residues were perturbed upon Ca2+ binding, moving to a more non-polar environment in the presence of Ca2+. Upon excitation of the protein at 280 nm, the fluorescence emission maximum was centered around 325 nm, suggesting that the tryptophan residues are located in a fairly hydrophobic region. Ca2+ addition did not induce a significant change in the intrinsic protein fluorescence intensity at 325 nm. Addition of Ca2+ to the 67 kDa protein labelled with 2-p-toluidinylnaphthalene-6-sulphone (TNS) resulted in a 25% increase in fluorescence intensity, accompanied by a blue shift of the emission maximum from 442 to 432 nm. Hence, the probe in the presence of Ca2+ moves to a more non-polar microenvironment, like calmodulin and other Ca2+-binding proteins. Fluorescence titration with Ca2+ using TNS-labelled protein revealed one class of binding site, with a Kd value of 2 x 10(-5) M.

Tyrosine and tyrosinate fluorescence of S-100b. A time-resolved nanosecond fluorescence study. The effect of pH, Ca(II), and Zn(II)

The properties of the tyrosine and tyrosinate emissions from brain S-100b have been studied by nanosecond time-resolved fluorescence at emission wavelengths in the range 305 to 365 nm. The effect of pH on the fluorescence has been studied at pH 6.5, 7.5, and 8.5 for the Ca(II) apo and holo forms of the protein, and for the apo and holo forms in the presence and absence of Zn(II) at pH 7.5. The fluorescence decay is biexponential at pH 8.5 and triexponential at pH 6.5 and 7.5. The three components of the decay have wavelength and metal ion dependent lifetimes in the ranges 0.06 to 1.05 ns, 0.49 to 3.76 ns, and 3.60 to 14.5 ns. The observation of a long lifetime component at wavelengths characteristic of emission from tyrosinate suggests that in class A proteins this may be a useful diagnostic of the environment of tyrosine in their native structures. The time-resolved emission spectra provide evidence for efficient, subnanosecond protolysis of the excited state of the single tyrosine (Tyr17) under all conditions studied except in 6 M guanidium chloride in which the protein shows only emission from tyrosine (lambda em 305 nm), suggesting that the tyrosinate emission is a property of the tertiary structure of the native protein. The Zn(II)-dependence of the fluorescence is fully consistent with the earlier suggestion that Tyr17 is near the Zn(II) binding site and remote from the high affinity Ca(II) binding site.

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.