chloramine-T Method Research Articles

Application of oxidative coupling reactions for the estimation of ritodrine hydrochloride in bulk sample and dosage forms

Three simple and sensitive visible spectrophotometric methods (M 1, M 2 and M 3) have been described for the estimation of ritodrine hydrochloride (RTH) in pure state and in unit dosage forms. These are based on the oxidative coupling reaction of RTH, with 3-methyl benzothiazolione hydrazone (MBTH) in the presence of cerium 1V (Ce IV) (Method M 1), N, N-dimethylamino-paraphenylenediamine (DMPD) in the presence of chloramine-T (CAT) (Method M 2) and 4-aminophenazone (4-AP) in the presence of [Fe(CN 6)] 3− (Method M 3). The variable parameters in all these methods have been optimised and the chemical reactions involved are presented. The results obtained in the three methods are statistically validated and recoveries range from 99.7 to 101.3%.

Feasibility of iodine-125 labeled anti-human hemoglobin antibody in the detection of bleeding sites from the large bowel-A preliminary study.

A monoclonal anti-human hemoglobin antibody that cross-reacts with mouse hemoglobin was labeled with Iodine-125 through the Chloramine-T method. The labeled antibody was used in an attempt to recognize bleeding sites from the large bowel in a mouse model, through a non-invasive enema-like study. In vitro experiments after double column chromatography of the labeled antibody and 10% trichloroacetic acid conjugation revealed that about 80% of the radioactivity was incorporated into protein. Inhibition assay containing cold (non-radiolabeled) antibody showed that Iodine-125 radiolabeled antibody preserved its immunoreactivity. Autoradiographs exquisitely demonstrated accumulation of isotope in the corresponding intestinal bleeding areas. These findings suggest that this method can be useful for scintigraphic localization of bleeding sites in the large bowel.

131I-Lym-1 in mice implanted with human Burkitt's lymphoma (Raji) tumors: loss of tumor specificity due to radiolysis.

Preliminary evaluations of 125I-labeled Lym-1, an anti-lymphoma mouse IgG2a monoclonal antibody, demonstrated favorable tumor uptake in mice bearing human Burkitt's lymphoma (Raji) tumors. In this study, the pharmacokinetics of 125I- and 131I-Lym-1, and the dosimetry, efficacy, and toxicity of 131I-Lym-1 in Raji-tumored mice were evaluated. Lym-1 was radioiodinated by the chloramine-T method and analyzed for monomeric fraction and immunoreactivity (antigen cell binding, relative to unmodified Lym-1). Nude mice bearing Raji tumors (20-500 mm3) received 1.5 MBq (40 microCi) 125I-Lym-1, or 1.5, 7.4, 14.8, or 18.5 MBq (40, 200, 400, or 500 microCi) 131I-Lym-1. Pharmacokinetic data (total body and blood clearance and biodistribution) were used to estimate radiation dosimetry. Mini-thermoluminescent dosimetry (TLD) was also used to measure radiation dosimetry directly for 7 days after injection of 131I-Lym-1. Tumor size, survival, body weight, and blood counts were monitored for 60 days to evaluate therapeutic efficacy and toxicity of 131I-Lym-1. At the time of injection, the mean quality assurance (QA) values for 125I-Lym-1 were 100% monomer and 100% relative immunoreactivity; the corresponding values for 131I-Lym-1 were 73% and 66%, indicating that radiolysis had occurred during the interval between radiolabeling and injection. 125I-Lym-1 exhibited high and sustained concentration in tumors relative to normal organs, whereas 131I-Lym-1 did not. Assuming identical pharmacokinetic behavior to 125I-Lym-1, 131I-Lym-1 would deliver radiation doses of 3.45, 0.83, 1.03, 0.34, and 0.56 Gy per MBq injected (12.8, 3.1, 3.8, 1.3, and 2.1 rad/microCi), to tumor, liver, lungs, total body, and marrow, respectively. When the actual pharmacokinetic data for 131I-Lym-1 (1.5 MBq) were used to estimate dosimetry, corresponding values of 0.51, 0.72, 0.49, 0.31, and 0.41 Gy/MBq (1.9, 2.7, 1.8, 1.1, and 1.5 rad/microCi) were obtained. Similar values were obtained for mice receiving 7.4 or 14.8 MBq of 131I-Lym-1. Similarly, TLD data indicated little preferential radiation dosimetry to tumor. Response rates (cure + CR + PR) for mice receiving 0, 7.4, 14.8, and 18.5 MBq of 131I-Lym-1 were 8%, 7%, 21%, and 45%, respectively. The LD50/30 dose of 131I-Lym-1 was 12.7 MBq (343 microCi). 125I-Lym-1 exhibited high and sustained concentration in Raji tumors in mice, indicating excellent therapeutic potential for 131I-Lym-1. However, in vitro QA results for 131I-Lym-1 indicated that radiolysis had occurred, and 131I-Lym-1 demonstrated little accumulation in tumor, or preferential radiation dosimetry to tumor in the same model.

Vasoactive intestinal peptide receptor scintigraphy in patients with pancreatic adenocarcinomas or neuroendocrine tumours.

Human adenocarcinomas of the gastroenteropancreatic system overexpress vasoactive intestinal peptide (VIP) receptors and therefore represent logical diagnostic targets for receptor scintigraphy. Using iodine-123 labelled VIP, the newly employed diagnostic procedure termed VIP receptor scintigraphy (VIP-RS) appears to detect tumour tissue, especially pancreatic metastatic tumours, in almost all cases. So far, however, only a single centre has demonstrated convincing positive results. The aim of this study was to compare the sensitivity and specificity of VIP-RS with those of computer tomography (CT) and transabdominal ultrasound in patients with extensive pancreatic metastatic adenocarcinomas and neuroendocrine tumours. VIP was radiolabelled with carrier-free 123I using the chloramine T-method and preparative high-performance liquid chromatography for purification. Patients with metastatic pancreatic (n=12) and colorectal (n=3) carcinomas (adenocarcinoma: n=13, neuroendocrine tumour: n=2) were studied by VIP-RS, CT, ultrasound and, in one case, also by radioligand receptor autoradiography. Carrier-free radioiodinated VIP of maximum specific radioactivity maintained a high biological activity as determined by cAMP formation in receptor-expressing tumour cell lines. Intravenous injection of 123I-VIP did not cause any side-effects. Biodistribution, determined over 24 h, was high in the lungs and low in abdominal organs. Although all patients had extensive metastatic disease as evidenced by CT and ultrasound, VIP-RS was unable to detect either primaries or metastases in these patients. Only in two patients could a significant uptake of radiolabel be detected in organs directly infiltrated by the primary. To exclude false-negative findings, tumour tissue in one patient with a large primary, undetectable by VIP-RS, was analysed by radioligand receptor autoradiography and shown to be receptor positive. Moreover, in vitro receptor determinations showed that pancreatic carcinomas usually have fewer VIP receptors than the normal tissues to which they metastasize, like the liver. It is concluded that VIP can be radioactively labelled with maximum specific radioactivity while maintaining biological activity. Intravenous administration leads to a biodistribution almost identical to that reported previously. However, in contrast to these reports, very low sensitivity and specificity were observed for the detection of pancreatic cancers. In retrospect, these findings are not surprising since VIP receptor expression was observed to be higher in normal tissues than in neoplastic ones.

Radioiodination of human interferon-α2 interferes with binding of C-terminal specific antibodies

Radioimmunoassays based on reactivity between a monoclonal antibody (mAb) and human 125I-interferon (IFN)-α2 are frequently exploited in interferon research. In general, epitopes of antibodies specific for human IFN-α2 are located on the two immunodominant structures formed in the N- and C-terminal domains, respectively. We found that labelling of IFN-α2 with Na 125I by the chloramine-T method did not affect the binding of antibodies recognising the N-terminal region 30–53. In contrast, radioiodination of IFN was associated with a dramatic decrease in IFN reactivity with mAbs specific for the C-terminus (residues ∼120–145∼). We suggest that steric hindrance araising from the incorporation of 125I into the tyrosine residues at positions 123, 130 and 136 may be responsible for the change in immunoreactivity. The adverse effect of radioiodination of IFN-α2 on the binding potency of C-terminal specific mAbs must be taken into consideration in experiments based on the interaction of such antibodies (i.e. NK2) with the labelled antigen.

Identification of a Binding Site on the Type II Activin Receptor for Activin and Inhibin

Type II activin receptors (ActRII and ActRIIB) are single-transmembrane domain serine/threonine kinase receptors that bind activin to initiate the signaling and cellular responses triggered by this hormone. Inhibin also binds type II activin receptors and antagonizes many activin effects. Here we describe alanine scanning mutagenesis of the ActRII extracellular domain. We identify a cluster of three hydrophobic residues (Phe(42), Trp(60), and Phe(83)) that, when individually mutated to alanine in the context of the full-length receptor, cause the disruption of activin and inhibin binding to ActRII. Each of the alanine-substituted ActRII mutants retaining activin binding maintains the ability to form cross-linked complexes with activin and supports activin cross-linking to the type I activin receptor ALK4. Unlike wild-type ActRII, the three mutants unable to bind activin do not cause an increase in activin signaling when transiently expressed in a corticotroph cell line. Together, our results implicate these residues in forming a critical binding surface on ActRII required for functional interactions with both activin and inhibin. This first identification of a transforming growth factor-beta family member binding site may provide a general basis for characterizing binding sites for other members of the superfamily.

Identification of Two Major Sites in the Type I Interleukin-1 Receptor Cytoplasmic Region Responsible for Coupling to Pro-inflammatory Signaling Pathways

Type I interleukin-1 receptor is the prototype for a family of proteins, which play a central role in early responses to injury and infection. The similarity of function across the family is reflected in similarity in signaling: all members tested couple to activation of NFkappaB and stress kinases. The coupling to these pathways is mediated by a 200-residue intracellular domain (the Toll/interleukin-1 receptor domain), in which sequence conservation is primarily confined to three short motifs (boxes 1, 2, and 3) located at amino acid residue positions 10 (box 1), 60 (box 2), and 170 (box 3). We have analyzed the contribution of these motifs to function by alanine scanning mutagenesis of the human interleukin-1 receptor type I. Mutant receptors were tested for expression, ligand binding, activation of receptor-associated kinase(s), NFkappaB, stress kinases, and transcription. Mutations in all three motifs led to low cell surface expression. Mutants in box 3 were, however, wild type for signaling, whereas mutants in boxes 1 and 2 were defective. We conclude that the conserved motifs box 1 and box 2 mediate the coupling of molecules in the family to inflammation signaling pathways.

Preparation of mono-125I-labelled gastrin-17 for radioimmunoassay measurements

In the present work the efficiency of two oxidising agents (chloramine-T and iodogen) for human gastrin-17 labelling is investigated, whilst the purification of ion-exchange chromatography is compared with that of reversed-phase HPLC. In the Iodogen method the extent of radioactive iodine incorporation can be as high as 95-98 % in contrast to 60-65 % attained using the chloramine-T method. Reversed-phase HPLC purification proved to be superior to ion-exchange chromatography on the basis of a more distinct separation of mono-iodinated gastrin underlain by specific activity value (73 to 75 TBq/mmol) near identical with the theoretical one.

Pharmacokinetic Disposition of Polyethylene Glycol-Modified Salmon Calcitonins in Rats.

This study first reports the pharmacokinetic disposition of polyethylene glycol (PEG)-modified salmon calcitonin (sCT) based on the number of attached PEG molecules. PEG-modified sCT was prepared by covalent linkage with succinimidyl carbonate monomethoxy polyethylene glycol. Mono- and di-PEG-sCTs were separated by size exclusion and reverse phase HPLC, and radioiodinated by the chloramine-T method with Na125I. 125I-mono-PEG sCT, 125I-di-PEG-sCT and unmodified 125I-sCT were administered to rats by i.v. injection. Serial blood samples, urine and various tissue samples were taken for the determination of radioactivity. Di-PEG-sCT exhibited significantly reduced systemic clearance (2.3 vs. 11.1 ml/min/kg) and steady-state volume of distribution (229.9 vs. 603.1 ml/kg), while mono-PEG-sCT showed a prolonged elimination half-life (189.1 min vs. 59.8 min) compared with unmodified sCT. The extent of urinary excretion of the PEG-modified sCTs was higher than for the unmodified sCT, but all these chemicals were excreted in urine in small quantities (< or = 0.6%). There was a tendency toward reduced accumulation of PEGylated sCTs in tissues, with its reduction being inversely proportional to the molecular size. Accumulation of the total radioactivity of the unmodified and PEG-modified sCTs was highest in the liver, followed by kidneys, lungs, spleen, heart and thyroid. When expressed per tissue gram weight, however, the highest radioactivity was found in the kidneys. PEGylated sCTs may have greater therapeutic potential via reduced systemic clearance and prolonged elimination half-life over unmodified sCT.

High yield direct 76Br-bromination of monoclonal antibodies using chloramine-T

Monoclonal antibody (MAb) A33 was labeled with the positron emitter 76Br ( T 1/2=16.2 h). Direct labeling was done using the conventional chloramine-T method. After optimization of the labeling conditions, a maximum yield (mean ± max error) of 77 ± 2% was obtained at pH 6.8. In vitro binding of 76Br-A33 to SW1222 colonic cancer cells showed that the immunoreactivity was retained. Also, the MAbs 38S1 and 3S193 and the peptide hEGF were 76Br-labeled, resulting in labeling yields (mean ± max error) of 75 ± 3%, 63 ± 4%, and 73 ± 0.1%, respectively. We conclude that antibodies and peptides can be labeled conveniently with 76Br for the purpose of whole-body tumour imaging by positron emission tomography.

Disposition of radioactivity after injection of liver-targeted proteins labeled with 111In or 125I. Effect of labeling on distribution and excretion of radioactivity in rats

The effect of radiolabeling liver-specific proteins on the in vivo disposition of radioactivity was investigated. The suitability of 111In and 125I as radiolabels for protein disposition studies in vivo was examined. Galactosylated and cationized bovine serum albumin were labeled with either 125I by the chloramine-T method or 111In, using 1-(4-isothiocyanatobenzyl)ethylenediaminetetraacetic acid (SCN-BZ-EDTA) or diethylenetriaminepentaacetic acid (DTPA) as bifunctional chelating agents (BCAs) and administered intravenously to rats. 125I radioactivity disappeared rapidly from the liver with subsequent excretion in the urine and bile, mainly in the TCA soluble fraction. 111In-associated radioactivity, on the other hand, remained in the hepatic tissue in considerably higher amounts during the experiment and was excreted in the bile and urine to a lower extent when compared with 125I. When the effect of BCA on excretion of 111In radioactivity was compared, no significant differences were observed in the urinary clearances. However, biliary excretion was significantly higher for 111In-SCN-BZ-EDTA-bound radioactivity. In conclusion, when compared with 125I, 111In labeling seems to more accurately characterize the in vivo distribution of liver-targeted proteins after their iv administration in rats and allows a more accurate pharmacokinetic evaluation to be performed.

A novel immunoscintigraphy technique using metabolizable linker with angiotensin II treatment.

Immunoscintigraphy is a tumour imaging technique that can have specificity, but high background radioactivity makes it difficult to obtain tumour imaging soon after the injection of radioconjugate. The aim of this study is to see whether clear tumour images can be obtained soon after injection of a radiolabelled reagent using a new linker with antibody fragments (Fab), in conditions of induced hypertension in mice. Fab fragments of a murine monoclonal antibody against human osteosarcoma were labelled with radioiodinated 3'-iodohippuryl N-epsilon-maleoyl-L-lysine (HML) and were injected intravenously to tumour-bearing mice. Angiotensin II was administered for 4 h before and for 1 h after the injection of radiolabelled Fab. Kidney uptake of 125I-labelled-HML-Fab was much lower than that of 125I-labelled-Fab radioiodinated by the chloramine-T method, and the radioactivity of tumour was increased approximately two-fold by angiotensin II treatment at 3 h after injection, indicating high tumour-to-normal tissue ratios. A clear tumour image was obtained with 131I-labelled-HML-Fab at 3 h post-injection. The use of HML as a radiolabelling reagent, combined with angiotensin II treatment, efficiently improved tumour targeting and enabled the imaging of tumours. These results suggest the feasibility of PET scan using antibody fragment labelled with 18F-fluorine substitute for radioiodine.

A novel immunoscintigraphy technique using metabolizable linker with angiotensin II treatment

Immunoscintigraphy is a tumour imaging technique that can have specificity, but high background radioactivity makes it difficult to obtain tumour imaging soon after the injection of radioconjugate. The aim of this study is to see whether clear tumour images can be obtained soon after injection of a radiolabelled reagent using a new linker with antibody fragments (Fab), in conditions of induced hypertension in mice. Fab fragments of a murine monoclonal antibody against human osteosarcoma were labelled with radioiodinated 3′-iodohippuryl N-ɛ-maleoyl-L-lysine (HML) and were injected intravenously to tumour-bearing mice. Angiotensin II was administered for 4 h before and for 1 h after the injection of radiolabelled Fab. Kidney uptake of 125I-labelled-HML-Fab was much lower than that of 125I-labelled-Fab radioiodinated by the chloramine-T method, and the radioactivity of tumour was increased approximately two-fold by angiotensin II treatment at 3 h after injection, indicating high tumour-to-normal tissue ratios. A clear tumour image was obtained with 131I-labelled-HML-Fab at 3 h post-injection. The use of HML as a radiolabelling reagent, combined with angiotensin II treatment, efficiently improved tumour targeting and enabled the imaging of tumours. These results suggest the feasibility of PET scan using antibody fragment labelled with 18F-fluorine substitute for radioiodine. © 1999 Cancer Research Campaign

Labeling of monoclonal antibodies with diethylenetriaminepentaacetic acid-appended radioiodinated peptides containing D-amino acids.

The optimal use of radioiodinated internalizing monoclonal antibodies (mAbs) for radioimmunotherapy necessitates the development of practical methods for increasing the level of retention of 131I in the tumor. Lysosomally trapped ("residualizing") iodine radiolabels that have been previously designed are based mostly on carbohydrate-tyramine adducts, but these methods have drawbacks of low overall yields and/or high levels of mAb aggregation. We have developed a method using thiol-reactive diethylenetriaminepentaacetic acid (DTPA)-peptide adducts wherein the peptides are assembled with one or more D-amino acids, including D-tyrosine. Two such substrates, R-Gly-D-Tyr-D-Lys[1-(p-thiocarbonylaminobenzyl)DTPA], referred to as IMP-R1, and [R-D-Ala-D-Tyr-D-Tyr-D-Lys]2(CA-DTPA), referred to as IMP-R2, wherein R is 4-(N-maleimidomethyl)cyclohexane-1-carbonyl, were synthesized by preparing functional group-protected peptides on a solid phase, selectively derivatizing the lysine side chain with 1-(p-isothiocyanatobenzyl)DTPA or DTPA dianhydride (CA-DTPA), deprotecting other functional groups, and finally derivatizing the peptide's N-terminus so it contained a maleimide group. Radioiodinations of the peptides followed by conjugations to disulfide-reduced mAbs, carried out as a one-vial procedure, resulted in 32-89% overall yields, at specific activities of 1.8-11. 1 mCi/mg, with less than 2% aggregation. Two internalizing mAbs, LL2 (anti-CD 22 B-cell lymphoma mAb) and RS7 (an anti-adenocarcinoma mAb which targets EGP-1 antigen), labeled with this procedure exhibited a 2-3-fold better cellular retention in Ramos and Calu-3 tumor cell lines, in vitro, respectively, compared to the same mAbs radioiodinated with the chloramine-T method. The rationale for the new approach, syntheses, radiochemistry and in vitro data are presented.

Physical and Functional Interaction of Murine and Xenopus Smad7 with Bone Morphogenetic Protein Receptors and Transforming Growth Factor-β Receptors

Members of the transforming growth factor-beta (TGF-beta) family transmit signals from membrane to nucleus via intracellular proteins known as Smads. A subclass of Smad proteins has recently been identified that antagonize, rather than transduce, TGF-beta family signals. Smad7, for example, binds to and inhibits signaling downstream of TGF-beta receptors. Here we report that the C-terminal MAD homology domain of murine Smad7 (mSmad7) is sufficient for both of these activities. In addition, we show that mSmad7 interacts with activated bone morphogenetic protein (BMP) type I receptors (BMPR-Is), inhibits BMPR-I-mediated Smad phosphorylation, and phenocopies the effect of known BMP antagonists when overexpressed in ventral cells of Xenopus embryos. Xenopus Smad7 (XSmad7, previously termed Smad8) and mSmad7 are nearly identical within their bioactive C-domain, but have quite distinct N-domains. We found that XSmad7, similar to mSmad7, interacted with BMP and TGF-beta type I receptors and inhibited receptor-mediated phosphorylation of downstream signal-transducing Smads. However, XSmad7 is a less efficient inhibitor of TbetaR-I-mediated responses in mammalian cells than is mSmad7. Furthermore, overexpression of XSmad7 in Xenopus embryos produces patterning defects that are not observed following overexpression of mSmad7, suggesting that mSmad7 and XSmad7 may preferentially target distinct signaling pathways. Our results are consistent with the possibility that the C-domain of antagonistic Smads is an effector domain whereas the N-domain may confer specificity for distinct signaling pathways.

Conjugate chemistry, iodination and cellular binding of mEGF-dextran-tyrosine: preclinical tests in preparation for clinical trials.

A conjugate with specific binding to the epidermal growth factor receptor, EGFR, and of interest for clinical tests was prepared using mouse epidermal growth factor, mEGF, and dextran. The mEGF was first coupled to dextran by reductive amination in which the free amino group on the N-terminal of mEGF was reacted with the aldehyde group on the reductive end of the dextran chain. The end-end coupled intermediate was further activated by the cyanopyridinium agent CDAP and tyrosines introduced to the dextran part of the conjugate. The mEGF-dextran-tyrosine conjugate was, with high efficiency, iodinated with the chloramine-T method. Approximately 25-35% of the radioactivity could be removed from the conjugate after exposure to protease K while 65-75% of the radioactivity could be removed after exposure to dextranase. Thus, the largest amount of the iodine was on the dextran part of the conjugate. The iodinated mEGF-dextran-tyrosine had EGFR specific binding since the binding to an EGFR rich human glioma cell line could be displaced by an excess of non-radioactive mEGF. The conjugate was to a large extent internalized in these cells and the administrated radioactivity was thereby retained inside the cells for at least up to 50 h.

Bromine-76 and carbon-11 labelled NNC 13-8199, metabolically stable benzodiazepine receptor agonists as radioligands for positron emission tomography.

NNC 13-8241 has recently been labelled with iodine-123 and developed as a metabolically stable benzodiazepine receptor ligand for single-photon emission computed tomography (SPECT) in monkeys and man. NNC 13-8199 is a bromo-analogue of NNC 13-8241. This partial agonist binds selectively and with subnanomolar affinity to the benzodiazepine receptors. We prepared 76Br labelled NNC 13-8199 from the trimethyltin precursor by the chloramine-T method. Carbon-11 labelled NNC 13-8199 was synthesised by N-alkylation of the nitrogen of the amide group with [11C]methyl iodide. Positron emission tomography (PET) examination with the two radioligands in monkeys demonstrated a high uptake of radioactivity in the occipital, temporal and frontal cortex. In the study with [76Br]NNC 13-8199, the monkey brain uptake continued to increase until the time of displacement with flumazenil at 215 min after injection. For both radioligands the radioactivity in the cortical brain regions was markedly reduced after displacement with flumazenil. More than 98% of the radioactivity in monkey plasma represented unchanged radioligand 40 min after injection. The low degree of metabolism indicates that NNC 13-8199 is metabolically much more stable than hitherto developed PET radioligands for imaging of benzodiazepine receptors in the primate brain. [76Br]NNC 13-8199 has potential as a radioligand in human PET studies using models where a slow metabolism is an advantage.

Application of [ 125I]-[Tyr 8]-substance P prepared by the chloramine-T method to receptor-binding experiments after subsequent reduction with mercaptoethanol and purification by reversed-phase liquid chromatography

Radiolabeling of [Tyr 8]-substance P ([Tyr 8]-SP) with the 125I-isotope was performed by use of the chloramine-T technique. The primary formed radiolabeled product, having been quantitatively converted to the corresponding sulfoxide yielding [ 125I]-[Tyr 8]-(Met 11 → O)-Sp completely lacked any binding to proteins rich in SP receptor populations. However, after reductive treatment with mercaptoethanol for about 2 h, a complete reconstitution of the Met 11 thioether structure was observed. The reduced peptide, consisting of [ 125I]-[Tyr 8]-(Met 11)-SP was separated from its by-products by reversed-phase high-performance liquid chromatography on octadecylsilyl silica gel with 100 m M triethyl ammonium formate buffer containing 22% acetonitrile (pH 2.2). The labeled SP derivative prepared by this two-step synthesis was obtained in 73% overall yield related to the [Tyr 8]-SP starting material and exhibited a specific activity of 1.9·10 6 Ci/M. In contrast to [ 125I]-[Tyr 8]-(Met 11 →O)-SP, satisfactory recepto-binding was now observed with the [ 125I]-[Tyr 8]-(Met 11)-SP derivative.

REEVALUATING THE EFFECTS OF TYROSINE IODINATION OF RECOMBINANT HIRUDIN ON ITS THROMBIN INHIBITION KINETICS

To reevaluate the effects of iodination of hirudin on its thrombin-inhibiting activity, we iodinated a recombinant hirudin analog, CX-397, by a chloramine-T method and isolated the eight kinds of iodinated derivatives by reverse-phase HPLC. Their structure were different combinations of non-, mono-, or di-iodinated residues at Tyr-3 and Tyr-64. Their ability to inhibit α-thrombin were determined and compared with each other. Iodination at Tyr-64 slightly increased the association rate constant ( k on ) 1.26- to 1.79-fold; this may be explained by the increase in the negative charge at C-terminal region of CX-397 caused by the electrophilic ortho substitution of Tyr-64 with iodine. On the contrary, iodination at Tyr-3 caused dual effects on the interaction between CX-397 and α-thrombin; namely, mono-iodination at Tyr-3 caused a small decrease in k off (1.29- to 1.63-fold) than non-iodinated ones, whilst di-iodination at Tyr-3 brought about 3.04- to 3.88-fold increase in k off . Since the N-terminal region of hirudin has been reported to bind with thrombin by hydrophobic interaction, the destabilizing effect of di-iodination at Tyr-3 can be explained by the increase in its polar nature, whereas the opposite effect of mono-iodination can not. These results indicate that it is not appropriate to use a mixture of variably radioiodinated hirudins as a radiotracer. We also provide a simple method to obtain well characterized radioiodinated hirudins. ○ 1997 Elsevier Science Ltd

Resistance to the Antitumor Agent Gallium Nitrate in Human Leukemic Cells Is Associated with Decreased Gallium/Iron Uptake, Increased Activity of Iron Regulatory Protein-1, and Decreased Ferritin Production

The mechanism of drug resistance to gallium nitrate is not known. Since gallium can be incorporated into ferritin, an iron storage protein that protects cells from iron toxicity, we investigated whether ferritin expression was altered in gallium-resistant (R) CCRF-CEM cells. We found that the ferritin content of R cells was decreased, while heavy chain ferritin mRNA levels and iron regulatory protein-1 (IRP-1) RNA binding activity were increased. IRP-1 protein levels were similar in gallium-sensitive (S) and R cells, indicating that R cells contain a greater proportion of IRP-1 in a high affinity mRNA binding state. 59Fe uptake and transferrin receptor expression were decreased in R cells. In both S and R cells, gallium inhibited cellular 59Fe uptake, increased ferritin mRNA and protein, and decreased IRP-1 binding activity. Gallium uptake by R cells was markedly diminished; however, the sensitivity of R cells to gallium could be restored by increasing their uptake of gallium with excess transferrin. Our results suggest that R cells have developed resistance to gallium by down-regulating their uptake of gallium. In parallel, iron uptake by R cells is also decreased, leading to changes in iron homeostasis. Furthermore, since gallium has divergent effects on iron uptake and ferritin synthesis, its action may also include a direct effect on ferritin mRNA induction and IRP-1 activity.