Evidence for nuclear synthesis of lactic dehydrogenase in rat liver.

Rats were given radioactive r.-leucine by intravenous injection. At times ranging from 30 set to 4 hours after injection, livers were frozen in situ, lyophilized, and separated into nuclear and cytoplasmic fractions in nonaqueous solvents. In each of the fractions thus prepared, the following were determined: (a) concentration of free labeled amino acid, (b) specific activity of the total protein, and (c) specific radioactivity of the lactic dehydrogenase. The injected amino acid entered both the cytoplasm and nucleus very rapidly, reaching a maximum concentration at about 1 min after injection. The level then diminished. Throughout the experimental period, the concentration of free labeled leucine was the same in the nucleus as in the cytoplasm. The curves obtained by plotting specific activity of total protein against time after injection of the labeled amino acid were similar for the cytoplasm and nucleus. After an initial lag of less than 1 min, both curves rose rapidly for 20 min, then leveled off. The similarity between the two curves is interpreted to indicate that protein synthesis occurs in the nucleus as well as in the cytoplasm, and that the rate of protein synthesis is roughly comparable at these two sites. Such an interpretation depends upon the assumption that movement of proteins between the cytoplasm and nucleus is, in general, a slow process, an assumption for which there is considerable evidence in the literature. The curves obtained by plotting specific radioactivity of lactic dehydrogenase against time after injection of the labeled amino acid were again very similar for the cytoplasm and nucleus. As for total protein, both curves rose rapidly for about 20 mm, following an initial short lag, then leveled off. The similarity between these two curves suggests that lactic dehydrogenase is synthesized in the nucleus as well as in the cytoplasm, and that the rate of synthesis is about the same at the two loci. This interpretation depends upon the assumption that movement of the enzyme between the cytoplasm and nucleus occurs slowly.