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

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.