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Rate-limiting Factors in the Uptake of Radioactive Amino Acids into Proteins of Tumor Slices^*

Joseph R. Davis and Harris Busch

( Department of Pharmacology, University of Illinois College of Medicine, Chicago 12, Ill.)

1. The specific activity of proteins of tumor slices was 100, 152, and 470 counts/min/mg following 1 hour of incubation of the slices with 250,000 counts/min of L-aspartate-U-C¹⁴, L-glutamate-U-C¹⁴, or L-alanine-U-C¹⁴, respectively; under the same conditions, the specific activity of protein of liver slices was 40, 31, and 11 counts/min/mg, respectively. Addition of 20 µmoles of glucose to the medium enhanced the specific activity of protein of tumor slices by a factor of 3–4 without significantly affecting the specific activity of proteins of the liver slice.

2. Incorporation of the isotope from L-aspartate-U-C¹⁴, L-glutamate-U-C¹⁴, and L-alanine-U-C¹⁴ into protein of tumor and liver slices progressively increased with increasing O₂ tension in the atmosphere.

3. Although glutamic and succinic acids caused a decrease in protein labeling of tumor slices incubated with L-aspartate-U-C¹⁴ and L-glutamate-U-C¹⁴, they induced a slight increase in labeling of protein with isotope of L-alanine-U-C¹⁴.

4. The specific activity of the protein of the tumors decreased linearly with a decrease in pH from 7.4 to 6.0.

5. The nuclear fraction of the tumor cell contained 55 per cent of the total isotope incorporated into protein, while the nuclear fraction of the liver cell contained only 23 per cent of the total isotope incorporated into protein. The microsomal fraction of the liver contained 33 per cent of the total isotope incorporated into protein, while the microsomal fraction of the tumor contained only 9 per cent of the total isotope incorporated into protein.

6. Labeling of tumor proteins with radioactive amino acids is greater in vitro than in vivo for tumor, and the difference is related, at least in part, to O₂ tension, pH, and availability of substrate, particularly glucose.

^* Supported in part by grants from the American Cancer Society and the Jane Coffin Childs Fund for Medical Research.

U.S. Public Health Service medical student part-time Research Fellow.

Received 2/10/58.