Objective:To investigate the impacts and mechanisms of the GCH1-S81A mutants of the rate-limiting enzyme GTP cyclohydrolase 1 (GCH1) at key phosphorylation sites on the radiosensitivity of esophageal cancers during the de novo synthesis of tetrahydrobiopterin (BH4). Methods:The KYSE-150 cell lines of esophageal squamous cell carcinoma with stable GCH1 overexpression at different phosphorylation levels were constructed in this study. Based on GCH1′s phosphorylation levels and radiation doses, the experimental groups were divided into the blank control group, the empty virus group, the empty virus + irradiation group, the wild-type GCH1 group, the GCH1 phosphorylation group, the GCH1 dephosphorylation group, the GCH1 dephosphorylation + irradiation group, the validation group, and the validation + irradiation group. The Western blot and the CCK-8 assay were employed to detect the infection efficiency and the survival rates of tumor cells in various groups, respectively. The flow cytometry was applied to detect the changes in the apoptosis rate, reactive oxygen species (ROS) level, and lipid peroxide level of tumor cells in various groups. The colony formation assay was used to detect the changes in the radiosensitivity of tumor cells. Besides, the Western blot was performed to detect the changes in the expression of ferroptosis-related proteins.Results:The GCH1 dephosphorylation group showed a significantly decreased expression level of phosphorylated GCH1-S81 protein in the cells at 48 h after infection ( t=9.35, 16.57, P<0.05). Compared to the empty virus + irradiation group, the GCH1 dephosphorylation + irradiation group exhibited a significantly decreased cell survival rate 24 h after 10 Gy X-ray irradiation ( t=26.97, P<0.05). The ROS levels in KYSE-150 cells at 8 h after 10 Gy X-ray irradiation, and the apoptosis rates and lipid peroxide levels at 48 h after irradiation, all showed a significant increase ( t=17.89-131.1, P<0.05), which was further aggravated following the addition of GCH1-S81A mutants ( t=157.06-97.81, P<0.05). This phenomenon could be inhibited by complementing wild-type GCH1 ( t=66.38-23.08, P<0.05). Compared to the empty virus + irradiation group, the GCH1 dephosphorylation + irradiation group manifested decreased colony formation capacity under various X-ray doses (2, 4, 6 and 8 Gy, t=7.31-8.16, P<0.05). The GCH1-S81A mutation reduced the expression level of the ferroptosis-related protein GPX4 ( t=4.55, P<0.05), which was further decreased after 10 Gy X-ray irradiation ( t=12.98, P<0.05). Conclusions:The GCH1-S81A dephosphorylated mutants can inhibit the growth of esophageal carcinoma cells KYSE-150 and enhance their radiosensitivity, which may hold promise as a novel approach to improve the efficacy of radiotherapy for esophageal cancers.