Male mice of the RF strain, 6-8 weeks old, were given a single exposure to whole-body x-irradiation. Lactate dehydrogenase isozymes in plasma and in various organs were separated by means of disc-gel electrophoresis. An increase of M-LDH fractions was observed in the plasma of mice a few hours after irradiation with sublethal or lethal dose. This was also proved by the experiments of substrate inhibition by pyruvate. This alteration in the composition of plasma LDH isozymes occurred coincidentally with the changes in total plasma LDH activity. LDH isozyme patterns in organs showed no remarkable change at 6 hours after exposure to 1000 R, though radiosensitive thymus and spleen exhibited a marked loss of LDH activity after exposure. In the split-dose study, no noticeable change in plasma LDH isozyme patterns was found after the second exposure. From these results it is assumed that the radiosensitive cells containing the preponderance of M-LDH fractions may be possible sources of the increased amounts of plasma LDH after exposure.

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