Abstract
Experimental studies on the dependence of radiosensitivity in relation to the growing phase, to the DNA metabolism inhibition, and to labeled TdR uptake in respect to DNA synthesis were performed in yeast cells. The sites of the inhibition were TMP synthesis or dCMP synthesis. Following results were obtained :
1) . In the inocorporation of labeled TdR into the cell, the time of its high rate was agree with the time of radiosensitive. And it was suggested that the initial indution of enzymes involved in TMP synthesis was the key to the radiosensitivity.
2) . The incorporation of labeled TdR into the cells was increased by irradiation. These results might show that there had been some questions in labeled TdR incorporation being used as adequate indicator of DNA synthesis. However, the incorporation was much less in yeast cells than in E. coli or in mammalian cells.
3) . Enhancement of irradiation with 5-FU was remarkable but not with BUdR in yeast cells.
4) . As administration of TMP, dUMP, AdR, GdR, and hydroxyurea in high concentration were concerned, these substances caused cell growth inhibition and their inhitions were slightly prevented by addition of dCMP in the medium. Among them, hydroxyurea inhibiton was prevented, though slightly, not only be addition of dCMP but also by addition of AdR or GdR. From these results, it was suggested that these substances were the inhibitors in the deoxyribosidation step from CDP to dCDP. These inhibitions did not change the radidosensi-tivity, excepting hydroxyurea, which enhanced it in pretreatment.
