TYROSINE TURNOVER AND OXIDATION IN THE OVINE FETUS IN UTERO

Abstract

The tyrosine turnover in chronically catheterized ovine fetuses in utero at 120–130 days of gestation was determined from plateau-specific activity following the continuous infusion of L-[U-14C] or L-[2,3,5,6 3H] tyrosine. The irreversible loss of tyrosine in the fetus was determined to be 5.790 mmol∙day−1∙kg−1. Since values based on irreversible loss have been reported to result in an overestimation of fetal utilization of substrates, dual labelling experiments were undertaken in which 3H-tyrosine was infused into the ewe and, 14C-tyrosine into the fetus simultaneously. Under these conditions a two-compartment kinetic model was used to determine net placental transfer, endogenous production, and net utilization of tyrosine by the fetus, which were respectively; 5.153, 1.287 and 6.536 mmol∙day−1∙kg−1. In addition, from the ratio of the plateau-specific activity of blood 14CO2 and, 14C-tyrosine it was estimated that the fraction of net utilization of tyrosine used for oxidation was 5.2%. The use of these kinetic parameters in estimating protein synthesis in the ovine fetus is discussed. Key words: Sheep, fetus, tyrosine, turnover, oxidation