Gamma-irradiated solid lysozyme was separated by cation exchange chromatography with a phosphate buffer into a number of components. The chemical and physical properties of the major fraction, P3-I, were examined in detail. This material elutes very close to the native enzyme, is stable, and is chromatographically homogeneous. It has 94% of the native enzyme activity. Absorption spectra, heat and pH denaturation studies, sedimentation velocity, and disc gel electrophoresis all showed that P3-I differed from native lysozyme. Data indicated that P3-I had an altered shape and exhibited both reversible and irreversible conformational damage. Experiments labeling the irradiated lysozyme with HTS showed that P3-I had suffered comparable chemical damage by this criterion as the more heavily damaged fractions. These results indicate that in P3-I, and therefore in the whole irradiated enzyme, most of the molecules have suffered some radiation damage. The results are in direct conflict with classical target theory where a "hit" or ionization constitutes an inactivation.

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