Whole body and tissue fractional protein synthesis in the ovine fetus in utero.

Whole-body and tissue fractional protein synthesis rates were determined in chronically-catheterized ovine fetuses at 120-130 d of gestation following an 8 h continuous infusion of L-[U-14C]- or L-[2,3,5,6-3H]tyrosine. From the net utilization of tyrosine by the fetus, corrected for apparent oxidation, and tyrosine concentration in the fetal carcass protein, whole-body protein synthesis was estimated to be 63 g/d per kg. Following 8 h of infusion of labelled tyrosine the ewes were killed and fetal tissues were removed for the determination of tyrosine specific activity. The fractional rate of protein synthesis (ks) was calculated from the specific activity ratio, protein bound:intracellular free tyrosine. Tissue ks values for the liver, kidney, lungs, brain, skeletal muscle and small intestine were 78, 45, 65, 37, 26 and 93%/d respectively. The absolute rate of synthesis was calculated by multiplying the tissue protein content by ks. Muscles, gastrointestinal tract, liver and lungs contributed approximately 20.5, 20.5, 14.4 and 9.4% respectively to whole-body protein synthesis. The efficiency of protein synthesis as expressed by the RNA activity was higher in liver, lung and brain followed by kidney, skeletal and cardiac muscle.