The cultured rat embryo undergoing organogenesis (9.5-11.5 days of gestation) together with its associated yolk sac synthesize purine nucleotides via the de novo synthetic pathway. Although both the embryo and its yolk sac contain significant levels of the purine base salvage enzymes adenine and hypoxanthine phosphoribosyltransferase, the culture medium that consists largely of rat serum contains high activity levels of purine catabolic enzymes. Short-term pulse-chase experiments with adenine and guanine, carried out under virtually serum-free conditions, confirmed that purine base salvage mechanisms were active and that there was no significant net transfer of purines between the embryo and its yolk sac. The 3-carbon atom of serine is the major source of one-carbon units for purine ring synthesis with a significant contribution from the 2-ring carbon atom of tryphtophan. No one-carbon units are derived from glycine, histidine or choline. Most of the embryonic amino acid requirements are supplied by yolk sac mediated proteolysis of the culture medium protein. A high level of embryonic GTP is reflected in a very low ATP/GTP ratio and is presumably related to the relatively undifferentiated state of many of the cells. The reason for the high level of GTP is however purely conjectional.
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