Labeling of Histones and Other Nuclear Proteins with l-Lysine-U-C¹⁴ in Tissues of Tumor-bearing Rats^*

One hour after injection of 15 µc. of l-lysine-U-C¹⁴ into rats bearing tumors, the specific activity of the histones and other nuclear proteins of the Jensen and Walker tumors was greater than that of any other tissue, including brain, heart, intestine, kidney, liver, lung, muscle, pancreas, spleen, testis, and thymus.

The specific activity of the histones of the Jensen and Walker tumors was greater than that of proteins of any other cellular fraction of the tumors including proteins of the microsomes, mitochondria, cytoplasmic sap, HCl-precipitable nuclear proteins, and NaCl-soluble nuclear proteins.

In other tissues one or more cytoplasmic fractions contained proteins of higher specific activity than did the tumor. In the pancreas proteins of the zymogen granules had the highest specific activity of any of the proteins studied. The specific activity of proteins of the microsomal fraction was highest in liver, testis, and lung 1 hour after injection of the tracer. In kidney, brain, skeletal muscle, heart, intestine, thymus, and spleen the specific activity of proteins of the cytoplasmic sap was greatest 1 hour after injection of the tracer.

The kinetics of labeling of the various intracellular proteins were studied over the 1st hour after injection of the tracer in tissues which accumulated the greatest amount of isotope in protein. It was found that in each tissue at 15 minutes after injection of the tracer the specific activity of microsomal proteins was greatest; in the tumor the specific activity of histones was slightly less than that of the microsomal proteins at 15 minutes and somewhat greater at 30 minutes. In all other tissues the specific activity of the microsomal protein was 2–9 times that of the histones at 15 minutes after injection of the tracer.

At 1 hour the histones of both of the tumors contained 20 per cent of the total isotope incorporated into tumor proteins, as compared with 2–11 per cent in tissues where the nuclear fraction was reasonably uncontaminated. The tumors were the only tissues in which labeling of histones was significantly in excess of that predictable on the basis of uniform distribution of the isotope throughout the protein of the tissue.

These studies indicate that labeling of nuclear proteins and primarily histones is an important part of the utilization of the isotope of l-lysine-U-C¹⁴ in transplantable rat tumors and a relatively minor function for nontumor tissues.