Physical mechanism for inactivation of metallo-enzymes by characteristic X-rays

• Published in: International journal of radiation biology and related studies in physics, chemistry and medicine Vol. 50; no. 4; p. 665
• Main Authors: Jawad, H H, Watt, D E
• Format: Journal Article
• Language: English
• Published: England 1986
• Subjects: Dihydroorotate Oxidase - radiation effects; Metalloproteins - radiation effects; Oxidoreductases - radiation effects
• ISSN: 0020-7616
• DOI: 10.1080/09553008614551071

Measurements have been made of the inactivation of the metallo-enzyme dihydro-oratic dehydrogenase in solution by characteristic X-rays at energies above and below the K absorption edge of the constituent iron atom. From the dose-survival curves and knowledge of the equilibrium electron spectrum generated by the X-ray 'field', inactivation cross-sections are deduced and expressed in terms of intrinsic efficiencies for the various proposed direct and indirect mechanisms of inactivation. It is concluded that the inactivation is caused by direct X-ray interaction in an area equivalent to about 30 per cent of the mean geometrical cross-section of the molecule, and is independent of whether the target is wet or dry. The contribution from Auger electron cascades, Coulomb charges etc. initiated by the inner-shell vacancy in the metal atom is negligible--possibly due to saturation effects. It seems that the presence of the metal atom simply serves to enhance the overall interaction probability with the molecule in a manner consistent with expectations from the photon absorption coefficients. No anomalously large damage is detected. These conclusions are supported by comparison with published results for other metallo-enzymes and bromine-loaded bacteria.