TYROSINE RESIDUES IN UNIRRADIATED AND GAMMA-IRRADIATED DIHYDROOROTATEDEHYDROGENASE - FLUOROMETRIC AND 2ND-DERIVATIVE ABSORPTION SPECTROPHOTOMETRIC STUDIES

The effect of gamma-radiation on tyrosine residues of dihydroorotate d ehydrogenase under different conditions was investigated by means of f luorescence spectroscopy and second-derivative spectrophotometry. No c hange in the fluorescence spectral distribution was observed when unir radiated and irradiated enzyme were denatured in guanidine hydrochlori de. However, decreases in fluorescence intensity in irradiated samples indicates a radiation-induced modification of tyrosine residues. The fluorescence intensity at 340 nm decreased exponentially with radiatio n dose in aerated medium but non-exponentially under argon and nitrous oxide-saturated conditions. The percentage loss of tyrosine fluoresce nce under different conditions was determined. The number of tyrosine residues left intact following irradiation at a dose for 50% inactivat ion under different conditions was measured by second-derivative absor ption spectrophotometry. The results obtained from both these methods show that the hydroxyl radical is less efficient in inducing radiation damage of tyrosine in aerated conditions compared with that under deo xygenated conditions. This lower efficiency of the hydroxyl radical in aerated medium has been attributed to the protective effect of oxygen and/or the superoxide radical anion.