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The turnover of DNA and histones in the livers and brains of mice has been determined. These mice had been exposed to constant levels of tritiated water from conception until they were 8 months old. At this point, exposure to tritium was discontinued, and the tritium remaining in DNA and histones was measured at various intervals afterward. The half-lives calculated for these components (with 95% confidence limits given in parentheses) were 117 (85-188) days for liver histone, 318 (241-466) days for liver DNA, It is generally agreed that histone synthesis is closely coupled to DNA synthesis in somatic cells and that the turnover of histones within a tissue is very slow compared to that of other nuclear or cytoplasmic proteins (1, 2). Whether this slow tissue turnover is solely due to cell turnover within the tissue or is in part due to histone turnover within intact cells is at present controversial. Piha et al (3) measured the rate of loss of label from brain and liver histones of mice given '4C-labeled lysine in embryo and concluded that all of the observed histone turnover could be accounted for by cell turnover. On the other hand, Hempel et al (4) measured the rate of incorporation of 3H-labeled methionine into the histones of the adult cat kidney and concluded that histone turnover was rapid and almost entirely due to turnover within the living cell.At the heart ofthis controversy is the magnitude ofcell turnover in these tissues. Piha et aL (3) estimated cell turnover from DNA turnover reported by others. Hempel et al. (4) considered cell turnover in the adult cat kidney to be negligible because very few labeled cells were seen in autoradiographs of these tissues taken after injection of tritiated thymidine. However, values for DNA turnover reported in the literature are not reliable. They are based on relatively few measurements and vary by orders of magnitude. The finding of negligible cell proliferation in adult cat kidneys conflicts with findings of significant incorporation ofDNA precursors in adult mouse kidneys (5) and suggests that the autoradiographic findings might be artifacts.The question ofwhether there is histone turnover within the living cell has important implications relating to the stability of the nucleosome, the basic unit of organization of DNA and histone within the chromosome. Complete lack ofturnover implies that the nucleosome is rigid and inert, whereas significant turnover suggests a flexible, dynamic structure. To help resolve this question, we have determined the rate ofboth histone and DNA turnover in mouse brain and liver. These measurements were made on mice that had been exposed to constant levels of tritiated water from conception until 8 months old. They are based on the rate of loss of tritium from DNA and histones after discontinuing exposure of these mice to tritiated water. MATERIALS AND METHODSThe procedures used here have been described in detail elsewhere (6). Mice were obtained from litters born to parents maintained on tritiated drinking water (3.0 ,AC...

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The distribution of tritium among the amino acids of serum proteins in mice chronically exposed to tritiated water was determined by ion exchange chromatography of the protein hydrolysate. The specific activity of nonexchangeable tritium in these amino acids relative to the specific activity of tritium in the tissue water of mice ranged from 0.04 for phenylalanine and threonine to 1.0 for glycine and alanine. Since tritium from tissue water can enter the nonexchangeable positions of amino acids only as the result of metabolic processing, the relative specific activity of tritium in each amino acid is an indicator of the extent of such processing. The tritium content of tyrosine and all the amino acids required in the diet for survival is quite low, except for histidine, and can be entirely accounted for by transamination or, in the case of methionine, by transmethylation. The tritium content of the other amino acids is too high to result from such minor processing and must reflect primarily the fraction synthesized de novo. The implications of these findings with respect to the radiobiological consequences of a diet containing tritiated proteins are discussed.

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