Physical mechanism for inactivation of metallo-enzymes by characteristic X-rays: analysis of the data of Jawad and Watt; and reply

The authors summarize mean numbers of events and interactions of various types calculated to occur per single dihydro-orotic dehydrogenase enzyme when irradiated in solution with D37 99 Gy of 8.04 keV X-rays under the experimental conditions of Jawad and Watt (1986). Jawad and Watt concluded that the enzyme inactivation was predominantly due to direct interaction of an X-ray photon with a non-metal atom of the enzyme, but the authors' analysis shows that this is not possible by five orders of magnitude. Of possible mechanisms, the one which remains feasible in this experimental system is the indirect action of radiolysis products from the solution. Jawad and Younis reply, acknowledging the error, but maintain that there cannot be a dominant contribution from iron in solid enzyme. (author)