Radiation inactivation of glutamate dehydrogenase hexamer: lack of energy transfer between subunits

The effects of ionizing radiation on glutamate dehydrogenase and on fluorescein isothiocyanate-tagged glutamate dehydrogenase were analyzed by target theory. Enzymatic activity, fluorescence, and the survival of the 56,000-dalton monomer subunit were determined on frozen samples irradiated at -1250C and on lyophilized samples irradiated at either -1350C or +300C. The effects of temperature were the same for all three parameters. Enzymatic activity was lost after small doses of high-energy electrons, whereas fluorescence and monomer subunits survived much larger doses of radiation. Target analysis revealed that the functional unit size for enzymatic activity was the hexamer, confirming both the earlier radiation study and conventional biochemical analyses. Target sizes obtained from fluorescence and subunit structure measurements were close to that of the monomer. These results indicate that the primary ionization caused by electron bombardment results in damage to a single polypeptide strand and that there is no massive transfer of radiation energy to other units in the hexamer