title: Lys and Arg in UBI: a specific site for a stable Tc-99m complex?
abstract: The aim of this study was to help establish if ubiquicidin peptide 29-41 fragment (UBI) contains a specific site for 99mTc labeling by a new direct method under alkaline conditions. Since this peptide does not have cysteine residues, it is possible that neighboring arginine and lysine in the peptide amino acid sequence (Thr-Gly-Arg-Ala-Lys-Arg-Arg-Met-Gln-Tyr-Asn-Arg-Arg) could be a specific coordination site to form a stable 99mTc-UBI complex. Following direct labeling, the in vitro stability of 99mTc-UBI was compared to UBI radiolabeled by one indirect method using HYNIC/tricine and HYNIC/tricine/EDDA. Radiochemical purity of 99mTc-UBI averaged 97% compared to 88% for 99mTc-HYNIC-UBI/tricine and 98% for 99mTc-HYNIC-UBI/tricine/EDDA. Both 99mTc-HYNIC-UBI (tricine or EDDA) and 99mTc-UBI showed stability in human serum and solutions of cysteine. 99mTc-UBI radiochemical purity 24 h after dilution in 0.9% NaCl was greater than 90% at pH 9 and greater than 95% at pH 6.5. Under one set of experimental conditions, in vitro binding to bacteria of 99mTc-UBI was 35% and identical to that of 99mTc-HYNIC-UBI/tricine and 99mTc-HYNIC-UBI/tricine/EDDA at 32% and 31% respectively. The biodistribution of 99mTc-UBI in mice showed a rapid renal clearance. To help identify the site(s) of 99mTc binding following direct labeling, molecular mechanics and quantum-mechanical calculations were performed which showed that the amine groups of Arg(7) and Lys are the most probable site. The calculations show that these groups can form a square pyramid with two water molecules for the Tc cation (dxysp(3)). It will be necessary to isolate and characterize the 99Tc(V)(O)-UBI.(H2O)n complex to confirm these results.
authors.affiliation: Universidad Autónoma el Estado de México, Mexico.
authors.forename: Laura
authors.initials: L
authors.lastname: Meléndez-Alafort
authors.forename: Flor de María
authors.initials: Fde M
authors.lastname: Ramírez
authors.forename: Guillermina
authors.initials: G
authors.lastname: Ferro-Flores
authors.forename: Consuelo
authors.initials: C
authors.lastname: Arteaga de Murphy
authors.forename: Martha
authors.initials: M
authors.lastname: Pedraza-López
authors.forename: Donald J
authors.initials: DJ
authors.lastname: Hnatowich
mesh_terms: D000818:Animals; D001120:Arginine / Q000737:chemistry*; D001665:Binding Sites; D003198:Computer Simulation; D004355:Drug Stability; D006834:Hydrazines / Q000737:chemistry; D007553:Isotope Labeling / Q000379:methods*; D008239:Lysine / Q000737:chemistry*; D008297:Male; D008657:Metabolic Clearance Rate; D051379:Mice; D008807:Mice, Inbred BALB C; D008958:Models, Molecular; D009539:Nicotinic Acids / Q000737:chemistry; D009928:Organ Specificity; D011485:Protein Binding; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000737:chemistry / Q000493:pharmacokinetics; D012269:Ribosomal Proteins / Q000097:blood / Q000737:chemistry* / Q000493:pharmacokinetics*; D013210:Staphylococcus / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013667:Technetium / Q000097:blood / Q000737:chemistry* / Q000493:pharmacokinetics*; D014018:Tissue Distribution
chemical_list: C120392:6-hydrazinopyridine-3-carboxylic acid; D006834:Hydrazines; D009539:Nicotinic Acids; D019275:Radiopharmaceuticals; D012269:Ribosomal Proteins; C040469:ribosomal protein S30; D013667:Technetium; D001120:Arginine; D008239:Lysine