Sources of amino acids for protein synthesis during early organogenesis in the rat. I. Relative contributions of free amino acids and of proteins.

Rat conceptuses on the 10th day of gestation were cultured for 27 h in whole rat serum. An addition of either [3H]leucine or [3H]leucine-labelled rat serum proteins was made once during the culture period, and the acid-soluble and acid-insoluble radioactivities of embryo and visceral yolk sac measured at harvesting. The extent of radiolabel incorporation into embryonic and yolk-sac proteins increased linearly with the duration of exposure of the conceptus to the radiolabelled leucine or radiolabelled serum proteins, indicating roughly constant rates of incorporation, per unit mass of tissue, throughout the culture period. The incorporation rates, expressed as clearances, were 0.73 and 0.78 microliter/mg tissue protein/h for embryo and yolk sac, respectively, when the source was [3H]leucine; and 1.8 and 1.3 microliters/mg tissue protein/h, for embryo and yolk sac, respectively, when the source was [3H]leucine-labelled serum proteins. It is estimated, from the known leucine and protein concentrations in serum, that protein contributed over 99 per cent of the leucine supplied to the conceptus for its protein synthesis. In parallel experiments, measurements were made on cultures conducted in the presence of an antiserum against rat visceral yolk sac (100 micrograms/ml). Antiserum profoundly inhibited incorporation of radioactivity into embryo and yolk-sac proteins, when the source was 3H-labelled protein, a result consistent with the known ability of the antiserum to inhibit pinocytosis in the yolk sac. Antiserum also decreased incorporation from [3H]leucine in the yolk sac, suggesting that a proportion of the free leucine entering the yolk sac does so by pinocytosis. The failure of antiserum to affect incorporation of [3H]leucine into the embryo probably indicates that leucine can enter the embryo without the mediation of yolk-sac pinocytosis. The primacy of protein, as a source of amino acids for the organogenesis-stage embryo, is consistent with the serious effects, in terms of embryonic death and malformation, that result from the interruption of amino acid supply when either pinocytosis or lysosomal proteolysis in the yolk sac is inhibited.