EFFECT OF IONIZING RADIATION ON THE PYRIDINE NUCLEOTIDES OF THYMOCYTES

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Abstract
The pyridine nucleotides of thymocytes have been examined following irradiation of the cells in vivo or in vitro. NAD was found to constitute about 95% of the total oxidized pyridine nucleotides in thymocytes while the level of NADH2was found to be very low. There is a rapid loss of NAD from thymocytes following irradiation in vitro or in vivo which is evident with as little as 25 rads. There is no concomitant rise in the NADH2levels following irradiation. The loss of NAD occurs principally from the nucleus of the cell and is characterized by an intracellular release of NADase from the mitochondria and nucleus into the cytoplasm. Following irradiation there is a reduced capacity for thymocytes to synthesize NAD from 7-C14-nicotinamide either in vitro or in vivo. The administration or presence of nicotinamide produces an unphysiological increase in the level of NAD in whole thymus or thymocytes in suspension. The level of NAD in treated cells is higher than in untreated controls following irradiation. This effect is not produced by nicotinic acid, cysteamine, L-cysteine, or promethazine. The administration of nicotinamide does not prevent the development of pyknotic degeneration in irradiated thymocytes. There is no change in the RNA content of the thymus under the conditions which reduce the NAD content.