Abstract
Cystathionase activity was not measurable in the liver or brain of 24 human fetuses and 4 prematures. Methionie-activating enzyme and cystathionine systhase were present. None of these enzymes of transsulfuration was present in the placenta. Thus cyst(e)ine may be an essential amino acid in the immature human until sometime after birth. A single full term infant who died at 7 hr had 7% of normal cystathionase activity in the liver. Amino acid analyes of simultaneously obtained maternal and fetal blood, amniotic fluid, placenta, fetal liver, and fetal brain were performed. No consistent trends were noted during the period studied. However, the fetal-maternal ratio of all plasma amino acid concentrations at time of abortion was high (as at term). The highest ratio, by far, was that of custine. In some cases, cystine was not measurable in the blood of the mother. Furthermore, in contrast to the mature human, cystathionine, the substrate for cystathonase, was higher in fetal liver than in fetal brain. In spite of this accumulation of cystathionine in fetal liver, there was none measurable in fetal or maternal blood nor in amniotic fluid. These studies suggest that the human fetus is entirely dependent on a maternal source of cyst(e)ine and that prematures and perhaps evn full term newly born infants are dependent upon dietary sourches of cyst(e)ine. Human milk is a high cyst(e)ine, low protein formula, whereas cow's milk is a low cyst(e)ine, high protein formula. Thus, these studies suggest that premature infants fed a high protein cow's milk formula retain more nitrogen and grow faster than infants fed cow's milk formula containing lower amounts of protein, closer to that found in human milk, because the amount of cyst(e)ine, rather than total nitrogen, may be a limiting factor for protein synthesis. Theses results may also afford an enzymatic explanation for the transient hypermethioninemia seen in infants on high protein diets.
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Gaull, G., Sturman, J. & Raiha, N. 19. Studies of sulfur amino acids in the immature human: Is cyst(e)ine essential?. Pediatr Res 5, 86–87 (1971). https://doi.org/10.1203/00006450-197102000-00024
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DOI: https://doi.org/10.1203/00006450-197102000-00024