1) Alkaline phosphatase activity of leukocytes is enhanced by radiation with 50,000 r. This disturbance accentuates the inherent aging process of white blood cells and may be explained by changes in the cell envelope.

2) X-radiation diminishes the endogenous oxygen uptake of leukocyte-platelet suspensions by approximately 20 per cent. This response to radiation is demonstrable at exposures of as little as 5,000 r. The decreasing effect is diminished when substrates such as sodium succinate or α-glycerophosphate are added, within a wide range of their concentration. With increasing substrate concentration the decrease due to radiation approaches that of the endogenous respiration and even exceeds it in some of the experiments.

3) In pure blood platelets a similar decreasing x-radiation effect occurs for endogenous respiration as well as succinic dehydrogenase activity; α-glycerophosphate dehydrogenase activity, on the other hand, is enhanced.

4) The oxygen uptake in leukocyte-platelet suspensions due only to leukocytes can be calculated. While the percentage radiation decrease of pure leukocytes is unchanged for endogenous and succinate activity, the decrease for α-glycerophosphate as substrate reaches considerably higher levels (68 per cent compared with 8.2 per cent in leukocyte-platelet suspensions). Thus α-glycerophosphate dehydrogenase activity seems to be most sensitive to x-radiation. It was shown in one of our previous studies that α-glycerophosphate dehydrogenase is one of the most important respiratory enzymes in leukocytes.

5) The glycolytic system in leukocytes remains intact following exposure to radiation with 50,000 r.

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