Factors controlling the selection of tissue-specific metabolic pathways in early embryonic cells

Rana pipiens embryos from the mid-blastula to the early gastrula stage were dissociated into cell cultures, and incubated with ¹⁴C-labeled tryptophan. The uptake of the tryptophan by the cells, its incorporation into protein and its metabolism by enzymes of the serotonin and kynurenine pathways were measured as a function of time, tryptophan concentration, and embryonic stage. It was found that the intracellular concentration of tryptophan was a constant fraction of the extracellular level except for a brief period around stage , during which the cells accumulated the amino acid to a higher concentration than in the external medium. The dominant metabolic pathway of tryptophan was a function of the intracellular concentration; at the lowest levels reported here most of the tryptophan was metabolized via the kynurenine pathway; at the highest levels most was metabolized via the serotonin pathway.