Whole body and tissue fractional protein synthesis in the ovine fetusin utero

Abstract

1. 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.2. 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.3. 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 (k₃) was calculated from the specific activity ratio, protein bound: intracellular free tyrosine. Tissue k, values for the liver, kidney, lungs, brain, skeletal muscle and small intestine were 78, 45, 65, 37, 26 and 93% /d respectively.4. The absolute rate of synthesis was calculated by multiplying the tissue protein content by k₂. Muscles, gastrointestinal tract, liver and lungs contributed approximately 20.5, 20.5, 14.4 and 9.4% respectively to whole- body protein synthesis.5. 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.