Accumulation and lethal effect of tritium (tritiated water) inRhodopseudomonas spheroides

Summary

Nonsulfur purple photosynthetic bacteria,Rhodopseudomonas spheroides cells were cultured in medium containing tritiated water (THO) under the light-anaerobic and dark-aerobic conditions. The experimental R value defined as specific activity ratio of organic bound³H to THO in medium was 0.49 and 0.48 for the light-anaerobically grown cells and the dark-aerobically grown cells, respectively. From the relation of R value to number of weight doubling of the cells (n), ratio of experimental R to theoretical R, i.e., (2ⁿ−1)/2ⁿ derived by assuming no isotope effect, was 0.51 and 0.49 on an average for the light-anaerobically grown cells and the dark-aerobically grown cells, respectively.³H-incorporation from THO-medium into the light-anaerobic nongrowing cells was affected by the light intensity and suppressed by adding HgCl₂, KCN, and 2,4-dinitrophenol as well as³H-labelling in the dark-aerobic nongrowing cells was affected by oxygen tension and suppressed by adding these metabolic inhibitors. From the fractionation of the lyophilized cells by modified Schneider method, the distribution of exchangeable³H in cold acid-soluble and ether-ethanol-soluble fractions and nonexchangeable³H bound to small molecules and macromolecules was 7.4/25.3/67.3 in the growing cells cultured anaerobically in the THO-medium up to late exponential phase in the light. The distribution in the nongrowing cells incubated anaerobically with the THO-medium for 18 h in the light of 300 and 3,000 lux was 82.1/8.4/9.5 and 58.2/19.2/22.6, respectively. These distributions of³H were changed with growth phase and/or incubation time. On the biological effect of³H-THO for the cells stocked at −196° C to accumulate³H-decays, the dark-aerobic nongrowing cells labelled with THO were rather radiosensitive than the dark-aerobically and light-anaerobically grown cells cultured in the THO-medium. The killing efficiencies, i.e., the probability that a single disintegration would be lethal, ranged from 1/200 to 1/275 for the above three kinds of cells labelled with THO. The killing efficiencies forR. spheroides labelled with THO were similar to that for radiosensitive strain CB13 and wild strain Hfr ofEscherichia coli labelled with³H-thymidine and stored at −196° C.