Distribution of lactic dehydrogenase in X-irradiated tissues.

Exposure to whole-body X-irradiation in rats causes a marked increase in total lactic dehydrogenase (LDH) activity and decrease in H-LDH/M-LDH ratio in serum and tissues, the maximum effect being observed on the 8th post-irradiation day. While there is an elevation of M-LDH isoenzyme, H-LDH remains relatively constant. Studies on incorporation of DL-leucine-l-14C into heart LDH isoenzymes also revealed increased biosynthesis of M-LDH isoenzyme. This indicates that an adaptive mechanism is operative in the X-irradiated rats, whereby, in order to augment anaerobic glycolysis, synthesis of M-LDH is stimulated. Administration of the radioprotector cystamine does not alter the effects of X-irradiation on serum LDH activity.