Evaluation of Radiation Dose and Biological Impact of OBT Based on Intracellular Distribution

Tritium (3H), a key fuel for nuclear fusion, poses internal radiation risks primarily through low-energy β-particle emissions. Organically bound tritium (OBT) is generally considered to deliver higher radiation doses than tritiated water (HTO), yet internal dose assessments typically assume uniform intracellular distribution despite the known compartmentalization of many biomolecules. In the present study, we established a quantitative cell fractionation approach to determine the subcellular distribution of non-exchangeable OBT derived from 3H-lysine in human cultured cells. Using this method, we quantified OBT distribution among nuclear, membrane-associated, and cytoplasmic fractions in hTERT/RPE-1 cells. OBT was detected in all major cellular fractions, with approximately 27% of activity recovered in the nuclear fraction. We further compared DNA damage responses induced by tritium compounds with distinct intracellular localization patterns, including 3H-dextran, HTO, and 3H-lysine. Although OBT exposures were associated with higher estimated absorbed doses than HTO, only HTO treatment resulted in a significant increase in the DNA damage marker γH2A.X. These results suggest that intracellular source–target geometry and molecular context may influence the biological effectiveness of tritium exposure. While the dose estimates in this study rely on simplified geometrical assumptions and an in vitro experimental system, the findings highlight the potential importance of intracellular localization when interpreting the biological effects of chemically distinct forms of tritium. Such experimental observations may contribute to ongoing discussions regarding microdosimetric considerations in tritium dose assessment.