The Mechanism of X-ray Inactivation of Phosphoglyceraldehyde Dehydrogenase

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Abstract
The x-ray inactivation of crystalline rabbit-muscle phosphoglyceraldehyde dehydrogenase (GAPDH) in solution has been correlated with the concurrent radiochemical destruction of enzyme SH-groups. The inactivation was found to be directly proportional to the disappearance of enzyme SH-groups and was complete when three SH-groups had been destroyed. Exactly the same relationship between enzyme inactivation and disappearance of free SH-groups was found when an increasing number of SH-groups were blocked by the addition of p-chloromercuribenzoate. The data strongly indicate that the x-ray inactivation of GAPDH can be entirely accounted for by destruction of its SH-groups. The G-value for the x-ray destruction of the enzyme SH-groups was found to be 0·23, indicating that the major part of the radiation energy is dissipated in processes not involving the protein SH-groups. Blocking of enzyme SH-groups by p-chloromercuribenzoate prior to the irradiation provided partial protection of the enzyme. The data are consistent with the view that, contrary to current assumptions, all SH-groups of GAPDH are of importance for the enzyme activity.