Cytotoxic effects of low levels of 3H-, 14C-, and 35S-labeled amino acids.

Tissue injury by radiolabeled amino acids may severely affect experimental results. In this report, 3H-, 14C-, and/or 35S-labeled proline, serine, lysine, and methionine at concentrations of 1 and 5 microCi/ml were shown to cause severe injury in organ cultures of embryonic rat lungs. This injury was evident by 6 h and was amplified by 4 days of culture. This injury was characterized with light and electron microscopy, with morphometric analysis of growth, and with quantitation of the total protein and DNA/lung. After 6 h with 5 microCi/ml of 14C- or 35S-amino-acid there were more signs of cell degeneration, and by 24 h the labeled lungs were smaller, showed more signs of cell degeneration and death, and contained 30 to 60% less new protein and DNA than control lungs. After 24 h with 5 microCi/ml of 14C- or 35S-amino-acid the total protein and DNA/lung began to decrease. This toxicity was directly proportional to the amount of intracellular decay of each isotope. With 14C- and 35S-amino-acids, lung growth slowed with approximately 100 disintegrations/cell/day (d/c/d), growth stopped with approximately 200 d/c/d, and atrophy occurred with approximately 300 d/c/d. Cell proliferation, cell differentiation, and bronchial branching continued through 4 days even though atrophy occurred with greater than 200 d/c/d. With 3H-amino-acids, growth slowed with approximately 200 d/c/d and stopped with approximately 400 d/c/d. However, no toxicity was evident with less than 60 d/c/d of 14C or 35S, or with less than 90 d/c/d of 3H. These data suggest that the amounts of intracellular decay of these weak beta-emitting isotopes should be strictly limited. Increasing amounts of tissue injury occurred with 14C or 35S at greater than 10,000 dpm/micrograms of DNA, and with 3H at greater than 20,000 dpm/micrograms of DNA.