Effect of growth hormone on mitochondrial protein synthesis.

L-[U-W]Leucine incorporation into the trichloroacetic acid-precipitable fraction of mitochondria after incubation with succinate as energy source was significantly higher with normal rat liver mitochondria than with hypophysectomized rat liver mitochondria. The incorporation was sensitive to inhibition by chloramphenicol, terramycin, dinitrophenol, and NaCN, but not to RNase and cycloheximide. The differences in the rate of incorporation between the two types of mitochondria were not abolished when the endogenous energy source from succinate was replaced by an external ATP-generating mixture or when cold trichloroacetic acid precipitate was extracted with hot 5% trichloroacetic acid or when high concentrations of unlabeled leucine (up to 200 mM) were added. Uptake of L-[UJ4C]leucine, in the absence of incorporation, was higher with hypophysectomized rat liver mitochondria than with normal rat liver mitochondria. However, when mitochondria were washed extensively after incubation there was no significant difference in the uptake. Growth hormone treatment of hypophysectomized rats for 5 days significantly increased both in uivo and in vitro incorporation of leucine. The in uifro stimulating effect of growth hormone became apparent as early as 15 min and was significant 30 min after a single intravenous injection of the hormone. These results suggest that growth hormone has a significant effect on liver mitochondrial protein synthesis.