Absence of Cystathionase in Human Fetal Liver: Is Cystine Essential?

Abstract

Cystathionase activity is not measurable in the livers of 24 human fetuses and 3 premature infants, and the concentration of cystathionine in the liver is higher than that of the brain. The placenta does not subserve the trans-sulfuration function. Cystine (or cysteine) thus may be an essential amino acid in the immature human.