X-RAY INACTIVATION OF PHOSPHOFRUCTOKINASE - RELATIVE RADIATION SENSITIVITY OF CATALYTIC AND ALLOSTERIC PROPERTIES

Rabbit muscle phosphofructokinase has been irradiated in solution with X-rays. The dose-response curve for the loss of catalytic activity was biphasic, indicating that the enzyme consists of two fractions of widely differing sensitivity to radiation. Sucrose gradient studies indicated that the resistant fraction was associated with high molecular weight aggregates of the enzyme. The sensitive fraction was found to be inactivated with a G value of 0.055. Measurements of the disappearance of SH groups upon irradiation and of the protective effect of p-chloromercuribenzoate blocking prior to irradiation indicated that destruction of SH groups plays an important role in the radiation-induced loss of the catalytic activity. The allosteric activity, as measured by the ability of AMP to reverse the ATP inhibition of the enzyme, was relatively resistant to radiation inactivation. Loss of allosteric activity did not occur until a considerable threshold dose had been accumulated. The shape of the dose-response curve could be explained by the radiation-induced changes in kinetic parameters. It was shown that the radiation-induced decrease in allosteric activity was due to a reduced sensitivity to ATP inhibition in the irradiated enzyme. © 1969.