Development of mammalian sulfur metabolism: absence of cystathionase in human fetal tissues.

Abstract

Extract: Cystathionase activity was absent from human fetal liver and brain as early as 6 weeks of gestation. Hepatic methionine-activating enzyme (26 ± 3 nmoles/mg protein/hr) and hepatic cystathioninesynthase (21 ± 4 nmoles/mg protein/hr) were present (cf. 86 ±16 and 98 ± 19 nmoles/mg protein/hr, respectively, in mature human liver). All three activities were absent from the placenta. Human fetal liver contained higher concentrations of cystathionine (14 ± 2 $mUmoles/100 g wet weight) than mature human liver (0) and human fetal brain (4.0 ± 0.6 $mUmoles/100 g wet weight). Methionine-activating enzyme of human fetal brain, but not liver, showed a tendency to increase with development (coefficient of correlation was 0.62; 0.01 cysteine and liver > kidney > brain. Incorporation of 35S-l-methionine and 35S-l-cysteine incubated with minced liver from four human fetuses showed more active incorporation of methionine (11,836–15,045 dpm/mg protein) than cysteine (7,044–9,856 dpm/mg protein). Speculation: These studies suggest that cysteine is an essential amino acid in human fetuses and in infants for some time after birth, especially if they were born prematurely.