A direct method for investigating the transfer of endogenous nitrogen from maternal body protein to fetal protein

Maternal body protein was labelled with [15N]glycine during the last third of pregnancy in the rat; the label was found in fetal protein, demonstrating that amino acids derived from maternal protein had crossed the placental membrane. The enrichment of fetal carcass protein was greater than that of maternal skeletal muscle, expressed as atoms per cent excess but similar to that of her liver. Enrichment of both fetal body and maternal liver declined during the period studied, while that of maternal skeletal muscle remained constant. The capture of label by fetal protein increased gradually at a rate which was the same as its rate of decline in the maternal liver, suggesting that the latter was a major source of the fetal label. The average [15N] content of the maternal liver was 2.3 per cent of that injected, while the total conceptus content was more than twice this value, 5.6 per cent, indicating that more than half was contributed from other maternal sources as well as her liver. The results were not influenced by maternal food restriction of two-thirds.