The effects of thyroxine on the percentage distribution in vivo of C14 lysine in mouse pituitary thyrotropic tumors have been studied. Tumor-bearing mice received 15 µc of l-lysine-U-C14 by injection. The distribution of isotope was determined in 5 cellular fractions of tumor and in several normal tissues. The percentage distribution of labeled amino acid into histone fractions of 2 strains of responsive tumors was depressed when tumors were grown in the presence of thyroxine. There was no effect of thyroxine on the percentage distribution of histone in a nonresponsive tumor strain, when compared to the distribution in untreated control tumors. The distribution of isotope in liver, kidney, and muscle from tumor-bearing mice did not appear to be affected by the presence or absence of thyroxine. The histones of tumors and of normal tissues were analyzed for amino acid composition. No significant differences were noted among the several tumor strains and the normal tissues studied. The relationship of these results to the mechanism of thyroxine effect on these tumors is discussed.

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Aided by Contracts SA43ph2402 and PH43-63-82, Cancer Chemotherapy National Service Center, National Institutes of Health; and Institutional grant In-4, American Cancer Society.

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