LEUCINE SOURCES FOR THE RAT FETUS

Fetal and maternal plasma were assayed for the concentration of free l eucine, acid-insoluble radioactivity and acid-soluble radioactivity at intervals after an intravenous bolus injection of [H-3]leucine into a naesthetized pregnant rats at 17.5 days post-conception. The concentra tions of total free leucine and of free [H-3]leucine in maternal and f etal plasma were effectively unchanged from 5 to 180 min post-injectio n. Plasma free leucine concentrations in the fetus were five times tho se in the mother. The concentration of free [H-3]leucine in fetal plas ma was similar to that in maternal plasma. Thus the specific radioacti vity of free leucine in fetal plasma is substantially lower than that in maternal plasma, indicating that a significant portion of the free leucine in plasma of the 17.5-day rat fetus comes from a source other than the free leucine in the maternal plasma. The data are consistent with a major contribution of amino acids coming from the degradation o f extraembryonic protein in the yolk sac. Other possible sources of un labelled leucine are discussed. (C) 1997 W. B. Saunders Company Ltd.