Radiation-Induced Inactivation of Dihydroorotate Dehydrogenase in Dilute Aqueous Solution

• Published in: Radiation research 1992-10, Vol.132 (1), p.7-12
• Main Authors: Saha, A., Mandal, P. C., Bhattacharyya, S. N.
• Format: Article
• Language: English
• Subjects: Aqueous solutions; Atoms; Biological and medical sciences; Cobalt Radioisotopes; Dehydrogenases; Dose-Response Relationship, Radiation; Enzymes; Free Radicals; Gamma Rays; Irradiation; Medical sciences; Oxidoreductases - radiation effects; Oxidoreductases Acting on CH-CH Group Donors; Phosphates; Radiation dosage; Radiochemistry; Radiolysis; Radiotherapy. Instrumental treatment. Physiotherapy. Reeducation. Rehabilitation, orthophony, crenotherapy. Diet therapy and various other treatments (general aspects); Solutions; Superoxides; Technology. Biomaterials. Equipments. Material. Instrumentation; Water
• ISSN: 0033-7587; 1938-5404
• DOI: 10.2307/3578327

The inactivation of dihydroorotate dehydrogenase by γ irradiation in dilute aqueous solution has been investigated. The activity of the enzyme decreased exponentially as a function of the absorbed dose under aerated and nitrous oxide-saturated conditions. The contributions of the individual radical species derived from water radiolysis were estimated from the inactivation results observed under aerated, argon-saturated, and nitrous oxide-saturated conditions. The hydrogen atom and hydroxyl radical were found to be important in enzyme inactivation. The effect of selected inorganic radical anions such as ${\rm Br}_{2}^{\bullet -},\ {\rm I}_{2}^{\bullet -}$, and $({\rm SCN})_{2}^{\bullet -}$ on the enzyme activity was also studied, and the results implicate the possible involvement of cysteine and tyrosine residues in the catalytic activity of dihydroorotate dehydrogenase. Changes in the kinetic parameters (Michaelis-Menten constant, K m, and maximal velocity, V max) due to irradiation under the conditions investigated suggest that radiation-induced inactivation is due to modification of the substrate binding sites and that of the active site residues in the enzyme. Evidence for the reduction of iron-sulfur centers in the enzyme during the inactivation process has been put forward from the difference spectrum of the irradiated dihydroorotate dehydrogenase. It has also been shown by electrophoretic studies that radiation-induced inactivation was not due to any fragmentation of the protein structure or the formation of any intermolecular crosslinking.