[Cysteine--an essential amino acid? Biochemical thoughts about the position of cysteine in the metabolism of the premature neonate (author's transl)].

Liver cystathionase activity is absent in the fetal organism. This makes cysteine an essential amino acid for the prematurely born infant. The existing enzyme activity is shifting serine into DNA-synthesis. The placental transfer of cysteine from the mother to the fetus is decreased in comparison to other amino acids. A low fetal cysteine level is giving the shift to DNA-synthesis a further stimulation.