Rebekah A. Taube

Radiotoxicity of 125I in mammalian cells.

The radiotoxicity of 125I in Chinese hamster V79 lung fibroblasts has been studied following extracellular (Na125I), cytoplasmic [125I]iododihydrorhodamine (125I-DR), and nuclear (125IUdR) localization of the radionuclide. Exposure of the cells for 18 h to Na125I (less than or equal to 7.4 MBq/ml) had no effect on survival. A similar exposure to 125I-DR produced a survival curve with a distinct shoulder and with a mean lethal dose (D37) of 4.62 Gy to the nucleus. While this value compares well with the 5.80 Gy X-ray D37 dose, it is in contrast to the survival curve obtained with DNA-bound 125IUdR which is of the high LET type and has a D37 of 0.80 Gy to the nucleus. Furthermore, when the uptake of 125I into DNA is reduced by the addition of nonradioactive IUdR or TdR to the medium and the survival fraction is determined as a function of 125I contained in the DNA, a corresponding increase in survival is observed. This work demonstrates the relative inefficiency of the Auger electron emitter 125I when located in the cytoplasm or outside the cell. It indicates that a high dose deposited within the cytoplasm contributes minimally to radiation-induced cell death and that radiotoxicity depends not upon the specific activity of IUdR but upon the absolute amount of 125I that is associated with nuclear DNA.

Membrane transport of nucleosides in rabbit polymorphonuclear leukocytes

The membrane transport of nucleosides in rabbit polymorphonuclear leukocytes has been examined by use of a rapid sampling technique. Both purine and pyrimidine nucleosides are transported by a single saturable system as indicated by the identity of their Kms and Kis against a spectrum of nucleosides. The specificity of the carrier was examined in detail. Adenosine (Km = 0.010 mM, vmax approx. 10 pmoles/106 cells per 45 sec) has the highest affinity for the system. Its fate after uptake is deamination and subsequent conversion to nucleotide. The most critical structural requirements for binding include the pyrimidine base moiety and a 3′-OH configuration on pentose, but other groups make significant contributions to binding. From an analysis of the substrate specificity it is argued that changes in the conformation of the carrier active site are induced by the substrate.

Strand Breaks after the Decay of Iodine-125 in Proximity to Plasmid pBR322 DNA

To elucidate the kinetics of DNA strand breaks caused by low-energy Auger electron emitters in proximity to DNA molecules, we synthesized (125)I-labeled 2-iodoacridine (2-(125)IA), which intercalates with DNA, and 4-iodoacridine (4-(125)IA), which does not. Supercoiled DNA from pBR322 plasmid, labeled with 3H, was purified and incubated with 2-(125)IA or 4-(125)IA in aqueous solution. Reaction mixtures were stored at 4 degrees C to accumulate radiation dose from the decay of (125)I, and DNA was resolved by gel electrophoresis into supercoiled (DNA-I), nicked-circular (DNA-II) and linear (DNA-III) forms, representing undamaged DNA, single-strand breaks (SSBs) and double-strand breaks (DSBs), respectively. Gamma irradiation from an external (137)Cs source led to an exponential decrease in DNA-I with a D0 value of 10.8 +/- 0.3 Gy. Under identical conditions, the D0 values for 2-(125)IA and 4-(125)IA were 22.4 +/- 0.6 x 10(11) disintegrations and 4.7 +/- 0.4 x 10(11) disintegrations, respectively. External gamma irradiation and 4-(125)IA produced SSB/DSB ratios of 26.5 +/- 2.1 and 15.9 +/- 2, respectively, while that for 2-(125)IA was 0.6. The average number of DSBs from each decay of (125)I was 0.67 for 2-(125)IA and 0.27 for 4-(125)IA. The results indicate that the decay of (125)I bound to a DNA-intercalating compound produces DSBs 2.5-fold more efficiently than (125)I bound to a nonintercalating compound and support the theoretical expectations that predict a DSB yield that is highly dependent on the proximity of the Auger electron emitter to DNA.

99mTc generators — II. Physicochemical factors in the radiolytic reduction of pertechnetate

The effect of pulsed and continuous irradiation on aqueous solutions of pertechnetate was studied using radical scavengers, aerobic and anaerobic conditions, and changes in concentration and pH. It was determined that TcO4− in homogeneous aqueous solutions is reduced to a lower oxidation state by external γ-irradiation and that this reduction is strongly dependent on pH. At neutral pH, TcO4− is reduced by eaq−, and the bimolecular rate constant of the reaction is 1.9 × 1010M−1s−1. Reduction by H radicals could not be demonstrated. At pH 2.0, reduction by H radicals is efficient, even more so than that by eaq− at neutral pH. Systems composed of various 99Mo−99mTc generator components were irradiated under aerobic and anaerobic conditions to investigate the influence of these components on reduction. It was found that presence of water in the generator system is essential for the reduction of TcO4− and that alumina and/or molybdenum enhance the process.

Efficient radiolabeling of mammalian cells using 111In-tagged liposomes

When indium-111 oxine labeled neutral liposomes were incubated with Chinese hamster V79 cells in the presence of 100 mM calcium, the cell-associated radioactivity increased approximately 75-fold over that observed in the absence of calcium. This is considerably higher (approximately 20 times) than the cellular uptake obtained when these cells are incubated in the presence of 111In-oxine alone. The highest uptake of radioactivity occurred when no bovine serum albumin was present in the medium, while as little as 0.001% of the protein greatly reduced the cell-liposome association. These efficient cell labeling conditions were not found to affect the survival of the cells.